connecting rod & rod length too stroke info



Re: connecting rod info

Postby grumpyvette » August 18th, 2011, 6:20 pm

Spare the Rod and Spoil the Engine
Category: Tech Talk —

Published in National Dragster

Written by David Reher

Imagine riding an elevator that makes a 10-story round trip 7,000 times a minute, alternately stretching and compressing its occupants with every cycle. That’s exactly the kind of punishing treatment a connecting rod endures. A connecting rod must bear the compression force of thousands of pounds of cylinder pressure, withstand the tension loads produced by the piston’s inertia at TDC, and survive the bending loads that try to push the piston through the cylinder wall.

The connecting rods are vital links in every reciprocating engine. They tend to be taken for granted until they break – and when a rod lets go, it will spoil your day and ruin your engine.

In drag racing, the choice of material for connecting rods comes down to steel and aluminum. I’m not privy to the inner workings of Formula 1 racing engines, but we did experiment with titanium connecting rods in our Pro Stock engines a few years ago. While titanium has some appealing attributes, it also has some shortcomings when used as a connecting rod material. The necessity to coat the thrust surfaces, the expense of machining and tooling, and the problems with galling fasteners in titanium convinced me that aluminum was a more practical choice.

Aftermarket steel connecting rods have become popular in mid-level sportsman racing. It’s tough to beat a set of affordable steel rods in a bracket or Super-style racing engine. One of our customers has had a set of relatively inexpensive steel rods in his big-block for 11 years. The engine turns 7600 rpm and makes 1,000 horsepower, so this is not a weak motor. We’ve replaced the bolts during regular rebuilds, but the rods just go back in after every overhaul.

Steel rods have limitations, of course. They’re seldom suitable for a big-inch, go-fast engine. The chief problem is weight. A steel rod for a large displacement motor might weigh 1200 grams, versus 850 grams for a typical big-block bracket engine. Like a valve spring, a connecting rod is subject to its own mass, so a portion of the load on the bolts and cap is produced by the weight of the beam and the small end of the rod. As the rod becomes longer and heavier, the stress on the fasteners and cap increases dramatically.

Heavy steel connecting rods are also tough on pistons. As the crankshaft turns, the rod’s reciprocating motion is controlled by the piston. If it weren’t for the restraint of the piston moving up and down in its cylinder, the rod would sling around in a circle. I often see the telltale evidence of the thrust loads generated by heavy connecting rods on pistons. The pistons are more susceptible to cracks where the pin boss joins the skirt, and the skirts are also more likely to collapse when a heavyweight rod is used.

This brings us to the chief advantage of an aluminum connecting rod: weight. Aluminum weighs approximately 1/3 as much as steel, and because it is so light, connecting rod manufacturers can use thick cross-sections in their rods without incurring a weight penalty. The tensile strength of steel is approximately 200,000 psi; the tensile strength of aluminum is about 95,000 psi. Consequently an aluminum rod can equal the strength of a steel rod at two-thirds of its weight.

Aluminum rods also are a little friendlier to the crankshaft and pistons than steel rods. The aluminum seems to cushion the peak loads, and that becomes apparent in the condition of the bearings and piston pins when an engine is torn down.

The downside of aluminum is its fatigue life. Aluminum loses strength with heat and load cycles, so it has a relatively short lifespan in a highly stressed application such as a connecting rod. Steel, on the other hand, does not fatigue as long as it isn’t stressed to its yield point. Think of a steel paperclip; if the wire is bent back and forth until it reaches its yield point, the wire will break. But as long as the metal isn’t stretched to it’s yielding point, the paperclip will last a lifetime.

Aluminum loses strength when it is subjected to heat cycles. Fortunately in a drag racing engine we have the ability to control engine heat to a great extent. I’ve written previously about the importance of keeping a racing engine’s temperature under control. Now I’ll add the effects of heat cycles on aluminum rods to the list of reasons why it’s advantageous to keep an engine cool.

Aluminum is also highly notch sensitive. A stress riser produced by an exposed bolt thread, a sharp radius or a tool mark is likely to be the point where the rod fails. If a lifter breaks and its needle bearings leave dozens of tiny notches in a set of aluminum rods, it’s an excellent idea to replace the rods. Even though the rods may otherwise be in good condition, the stress risers left by the lifter bearings have compromised the aluminum’s strength. In contrast, steel has relatively little notch sensitivity – although it’s a really bad idea to run a steel connecting rod that’s been cut with a hacksaw just to see how long it will last.

It’s very unlikely that aluminum rods will fail as long as they are replaced at regular intervals. I don’t put new bolts in aluminum rods simply because we install new aluminum rods with every rebuild. If I’m using steel rods in an application where they’re not being stressed to their yield point, then I replace the bolts periodically.

Steel connecting rods are available in two different styles: a conventional “I” beam rod (similar to a factory forged rod) and an “H” beam rod (often referred to as a Carrillo-style rod). I’ve had good success with both styles, so I really don’t have a strong preference for one over the other. In an endurance racing application, the H-beam rod is more suitable for a pressure-oiled piston pin, but that’s not a consideration for a drag racing engine.

Steel connecting rods will provide good longevity at an affordable price in an engine that has a reasonable rod length and doesn’t turn extremely high rpm. As horsepower and engine speed go up, and as the components get bigger, then aluminum rods become a more practical choice.
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Re: connecting rod info

Postby grumpyvette » February 23rd, 2013, 10:40 am

540 rat posted this info
I don,t agree with everything but its an interesting veiwpoint


"Yes, even though this topic has been beaten a number of times, it’s connecting rod time again, and here’s why. It was brought to my attention that one of our magazines had a tech article awhile back that had a section on connecting rods. It was written by someone I’ll only identify as Mr. K, and he knows who he is. In that section about rods, he used some of my words that came out of the “Rod Strength Analysis” that I did a few years ago, some of which I’ve posted in the past. He also of course used some of his own words. But, while he was straddling the fence, trying to be neutral and not offend advertisers, he contradicted some of what he’d put in that piece, which ended up confusing people. I have been asked about all that, and asked to post something to clear up the confusion he created.

So, that is why I’m posting the following info, which is an UPDATED version of some of what I’ve posted in the past about connecting rods. If you are interested in the details, read on. Otherwise, close out now while you still can.

I’ll say right up front that I do NOT sell connecting rods, so I have no vested interest in what rods people buy and run. People can of course do whatever they want. But, there is so much misinformation, misunderstanding and confusion about connecting rod design, that I’ve put together a brief overview for those who are interested in knowing the Engineering FACTS, rather than relying on the incorrect info that is so common on the Internet and elsewhere.

It is best to avoid H-Beam rods in general, no matter who makes them, and no matter who else uses them. Because as you will see below, an H-Beam rod is never the best choice. They were originally made by someone who “thought” they might be better and/or cheaper to make, without benefit of any Engineering analysis. So, the maker didn’t even know what the H-Beam shortcomings were. Then other makers copied them, and eventually people started to think they must be good because they kept showing up. And because they looked different than stock rods, some figured they must be trick parts that are better.

But, you will only find the H-Beam style being used in the aftermarket Automotive Industry where it is common for companies to create parts without using any Degreed Engineers. A lot of the aftermarket companies “just make stuff” without even knowing what they are doing. No competent Degreed Mechanical Engineer would ever design an H-Beam rod, because an H-Beam rod is a textbook case of how NOT to design a connecting rod. So, buyer beware.

A rod’s max compression loads are determined by the amount of HP being made. It’s a simple matter of the higher the HP, the higher compression loading on the rod. And an Engineering “FACT” (NOT opinion or theory) is that the I-Beam rod design has about twice the strength in compression, compared to a comparable H-Beam rod. So, that makes an I-Beam rod a far better choice for any application, and particularly for those at higher performance levels, such as those making over 1000 HP.

But, a rod’s max tension loads are determined by the mass of the parts involved, the rod length, the stroke length, and the max rpm. That’s it. The max tension loads will never change, no matter if you throw Nitrous, a Turbo, or Blower at it, as long as the short block and redline don’t change. That max tension loading occurs at TDC on the exhaust stroke. And that has absolutely nothing what so ever to do with the amount of HP being made. In order to change the max tension loading, you’d have to change the short block configuration and/or the redline. Both types of rods have similar tension capability, since that is only a product of the beams cross-sectional area.

In High Performance engines, connecting rod “compression loading” is ALWAYS considerably higher than the “tension loading”. Here’s an example using an 800HP, 540ci BBC with a 7,000 rpm redline:

Max compression loading on the rod is about 21,000 lbs or 10.5 tons.

Max tension loading is only around 11,000 lbs or 5.5 tons.

So, as you can see in this particular example, the compression loading is about twice as high as the tension loading. But, if the HP increases, the compression loading will also increase. And “THAT IS WHY” a rod’s compression loading capability is important to consider when you are in the market for a new set of rods for a High Performance engine.

An I-Beam rod made from high quality material such as 4340 forged steel will provide plenty of “Margin of Safety” with regard to compression strength. But, a comparable H-Beam rod’s margin of safety can be iffy, and it only gets worse as the HP levels go up. For an H-Beam to catch up to the compression strength of an otherwise comparable I-Beam, the H-Beam would need to be FAR heavier than the lighter, stronger and more efficient I-Beam design. So, by using I-Beam rods, you will have the capability to increase the HP later on, without worrying about the rods being strong enough to handle the extra HP.

The superiority of the I-Beam, is why it is the structural beam design of choice for countless Professional Engineering applications. So, the next time you need a set of rods, you might want to do yourself a favor, and only consider I-Beam rods which are a significant UPGRADE over H-Beams. And this is why you see I-Beam rods in countless OEM engines, including the Supercharged Corvette, which were designed by actual Degreed Engineers who knew what they are doing.

BOTTOM LINE: No matter what anyone tells you, there is simply NO good reason to ever use an H-Beam rod. So, it makes no sense to buy H-Beams when the clearly superior I-Beams are readily available.

If you are still having a hard time accepting all this, consider the following:

Lunati’s recommendation for their rods:

• H-Beam Rods - ideal for High Performance street & mild race engines.

• Pro Series I-Beam Rods – perfect for Street Rods, Street-Strip Engines and all-out Race Engines

• Pro Mod I-Beam Rods - perfect for any racer needing an ultra-strong I-beam design

They also say that every Lunati connecting rod is forged from premium quality 4340 alloy steel for strength.

So, as you can see, Lunati knows what they are doing, mirrored what I said above, and got it right about H-Beams, I-Beams and forgings.

And speaking of that topic, no one “needs” a billet rod either. Forged rods have desirable grain structure and desirable residual compressive stresses, but billet rods DO NOT. Forged parts are always better than billet parts. For example, all fracture critical jet aircraft parts are forged, NOT billet. Billet parts are simply cheaper to manufacture in small quantities, even though machining time will be higher. Because billet parts do not require the horribly expensive forging presses and dies. But, when parts are produced in high enough mass quantities to spread out the cost of the forging presses and dies, then forged parts can end up being both superior and more affordable, because forgings don’t need as much final machining time
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Re: connecting rod info

Postby grumpyvette » February 23rd, 2013, 10:42 am

y2k496 posted this


Coming from an engineer, I have to agree that there is a substantial amount of misinformation out there. I also did an analysis of connecting rods for a term paper for strengths of materials and material science. Connecting rods operate primarily as a 2 force member (compression and tension). The cross sectional geometry of the rod greatly affects the member's resistance to bending (not a typical load of a connecting rod in general). In terms of compression and tension, it comes down to cross sectional area, material selection, surface finish, grain structure, and type of loading. Aluminum tends to "remember" fatigue as it is stretched and compressed- and is a ticking time bomb in an engine. I have found that the typical I-beam rod has a greater cross sectional area than the typical H-beam rod at similar points along the member. This finding could be what is used to come to the "greater strength" arguement. If the I-beam has a larger cross section area, and they are both made of 4340 steel- it will have a higher capacity until yeild and failure under tension and compression. The rod's material decides primarily the amount of force it can take, the surface finish will inhibit crack formation and propogation thus limiting crack formation and propogation in the rod. Shot peening the surface, and removing surface imperfections goes a long ways in terms of strength. Once a crack has started- it travels easily through a reciprocating, shock loaded member. Introducing a bending moment, when you consider friction on the bearing and piston pin (or tight clearances/distorted bore) start to bend the rod elastically- where it will return to the original position. At the speed that this fluctuation occurs, a small crack combined with tension leads to a broken rod and a very effective oil pan/block sawzall. Connecting rod fasteners are of high concern. Thier design impacts the overall strength of the big end of the rod considerably. I have to cut this short, but basically.... Your 190,000psi bolt is rated for 190,000 psi or 190ksi in tension before yeilding. Bolts stretch and return to thier normal position. When you torque a fastener, you stretch it to yeild. This is the theoretical highest clamping force that fastener will hold before stretching into the plastic range...where the stretch becomes permanent..and the bolt is now longer than when youstarted. A 190,000psi bolt... 3/8" diameter has a cross sectional area of just over a 1/10th of a square inch, and a maximum clamping force of just under 21000 pounds. Once you exceed this force, whether it be by torquing or by inertial loading as the piston reverses direction on the exhaust stroke at TDC, you will stretch the bolt...deform your big end bore, and be on the verge of engine damage. The big end of the rod can deform without stretching the bolts...all based on design.

Good design, good material, surface finishing, loading and harmonics all come into play. I have H-beams in my 496 and I feel as though they have a considerable margin of safety at my power levels. One thing you learn quick in materials is....aerospace doesn't mean anything...that means the lowest bidder made the bolt just strong enough for the application with a consideration so the design margin of safety. a 300lb rated rope will hold 600lbs if loaded slowly...design factor of 2. Tie a knot in it and it is now 300lbs....So dont go "kinking" up the details of your engine build...get the right parts...correct assembly tolerances, and operate within the constraints of which you built it for. Just bored and figured id waste some time before breakfast.... Very interesting stuff though.
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Re: connecting rod info

Postby grumpyvette » March 3rd, 2014, 11:48 am

http://horsepowercalculators.net/tuner_ ... eat-debate

By: Haitham Alhumsi

When it comes to wringing every last ounce of performance out of your engine combination (within the limitations of a specific displacement or a set of racing class rules), some engine designers head to the little known secrets of messing with the engine’s rod length to achieve an ‘ideal’ rod length to engine stroke ratio (which I’ll call RSR from here on out).

Most designers like to simplify engine design by sizing engine parts (such as camshafts, headers, intake systems…etc) based on horsepower figures or based on ‘average airflow’ through the part. However, more advanced simulations break engine flow into 4 distinct regions corresponding to the four strokes of the engine.

As the each piston moves up and down in the bore they create a variable amount of vacuum and compression depending on where the piston is exactly in the stroke. Similarly, depending on when the intake and exhaust valves are operated, the intake system and exhaust system become exposed to this positive (compression) and negative (vaccum) cylinder pressure at different levels. The combination of piston position with camshaft timing produces a direct effect on intake and exhaust system airflow velocities.

Let’s look at piston position in the four engine strokes:

Intake Stroke:

As the piston moves away from top dead center, the volume in the cylinder is expanding creating a ‘vacuum’ which outside air rushes in to fill (if the intake valves are open already). At the moment the intake valves open: The lower the piston is from top dead center and the faster the piston is moving away from top dead center, the more engine vacuum is presented to the intake system and the higher the peak velocity of the intake air will be.

Compression Stroke:

As the piston moves up from bottom dead center, the volume in the cylinder is contracting creating compression (if the intake valves are closed). At the moment of closing the intake (and exhaust valves and sealing the combustion chamber for compression): The higher the piston is in the chamber, and the slower it is moving upwards, the less peak compression pressure we will have during the time of spark plug ignition, and the less peak combustion pressure we’ll have during the power stroke.

Power Stroke:

As the piston moves away from top dead center in the power stroke, as the result of the expanding gases from the combustion of the air and fuel mixture, the volume in the cylinder is expanding thus lowering the combustion pressure, at the same time the force on top of the piston applies in a rotational torque applied through the connecting rod to the crankshaft. The lower the piston position in the bore, and the faster it moves away from top dead center, the less combustion pressure applied to the piston and the less power is delivered from the combustion process to the engine*

*With some reservations as I’ll explain later when we talk about effective stroke.

Exhaust Stroke:

As the piston moves away from bottom dead center, up the bore, the exact time that the exhaust valves open, the piston position, and the piston velocity up the bore will affect peak exhaust gas velocity and peak exhaust gas pressure.

Now let’s stop with the qualitative and lengthy jargon and dive into the technical.

The piston, connected to the crankshaft, through the connecting rod, creates a unique mechanical transfer. The perfectly circular rotation of the crankshaft is translated to a lateral piston movement through the angularity of the crank and rod combo. This means that the perfectly uniform angular velocity of a crank journal operating at a constant rpm, gets translated into a non uniform lateral velocity and lateral acceleration depending on the x and y components of the angular movement.

A quick way to ‘visualize’ this is as follows:

At a steady rpm, the crank moves steadily in a uniform rotation. It does not need to accelerate to hold that rpm, nor does it ever change direction. It just spins, uniformly, clockwise (or counterclockwise depending on the engine manufacturer) at a fixed number of rotations per minute (RPM).

The piston on the other hand, at a steady rpm, moves both up and down the bore. This means that for every engine revolution, the piston has to change direction four times. This means that the piston has to accelerate when moving away from top dead and bottom dead center and decelerate (in preparation to changing direction) when moving towards top dead and bottom dead center.

Because the piston has to speed up, then slow down, then change direction, then speed up again, then slow down again, then change direction, then repeat that process once again, all in a single revolution of the engine, it becomes visually very clear that piston velocity and acceleration are always changing, even when we are holding the engine at a constant steady rpm.

If you add engine acceleration and breaking into the mix (such as building rpms or letting off the gas) then those accelerations get further added into the piston motions.
So by now, if you’ve been following thus far we’ve established that piston position affects engine dynamics in the 4 different strokes, and that piston velocity and acceleration are variable within each specific stroke.

So what determines the acceleration and velocity profiles of a piston at any point in the engine’s rotation?

Enter the rod length to stroke ratio:

The piston position at any point in the engine’s revolution is described by the following triganometric equation (this equation is derived from the triangle shape created between the piston’s axis of motion up and down the bore, the center line running through the connecting rod between the piston pin and the rod journal, and center line running through the crankshaft between the rod journal and the center of the crankshaft)

Image
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Re: connecting rod info

Postby grumpyvette » March 3rd, 2014, 11:49 am

You can see the rest of the piston equations (velocity and acceleration) by clicking on the link above.

So now we can calculate piston position vs crank angle for any engine, no matter what stroke it has and what rod length is there. If we combine this piston position with the exact valve timing events (intake opening and closing and exhaust opening and closing) then we can pin point exactly where the piston will be at those exact events and exactly how much pressure or vacuum has been built up when the intake and exhaust systems are connected to the cylinder (when the valves open).

This has direct and indirect effects on power delivery… first let me list all the effects, then we’ll go into some details:

Direct Effects:

1- Dynamic Compression Ratio
3- Peak compression pressure and peak combustion pressure
5- Peak engine Vacuum and peak intake velocity
6- Engine Effective Stroke

Derivative Effects:

1- Combustion Duration
2- Piston dwell time & optimal ignition timing
3- Cylinder wall friction
4- Average and peak piston velocity and safe redline

The effects listed above in bold font are effects that are modeled in our virtual dyno. The effects listed in regular type are not modeled in our virtual dyno at present but I will still discuss them here because it’s good to understand exactly what’s going on.

Direct Effects:

1- Dynamic Compression Ratio

The dynamic compression ratio is simply calculated as the ratio between the volume of air in the cylinder head plus the volume of air in the cylinder bore at the moment the intake valve closes, divided by the volume of air in the cylinder head when the piston reaches top dead center.

Holding intake valve closing (IVC) constant at 40 degrees after bottom dead center for example, an engine fitted with a longer rod (or a higher rod length to stroke ratio) will have a piston that is farther up the bore than an engine with a piston with a shorter RSR. This results in the longer rod engine having a marginally lower dynamic compression ratio (lower by 0.1 to 0.3 compression points at the extremes).

This means that running an 11.3:1 piston in a long rod engine will end up having the same dynamic compression ratio as an 11.0:1 piston in a short rod engine.

Affects similar to compression ratio (such as higher boost pressure and higher nitrous injection levels) will also follow the same trend with the longer rod engine able to take a slightly higher dose of ‘radical’ while still having the same net dynamic compression as the shorter rod engine running a ‘tamer’ superficial figure of compression, boost, or nitrous.

3- Peak compression pressure and peak combustion pressure

When the air and fuel mixture is ignited via the spark plug, the combustion of the air / fuel mixture spikes cylinder pressure to about 4.5 times the original cylinder pressure at the moment of ignition.

This is to say that igniting a mixture that is already at 10bar compression pressure results in about a 45bar final combustion pressure, but igniting a mixture that starts out at a lower 9bar compression pressure reaches a lower peak of 40.5 bar combustion pressure.

An engine with a longer rod will have a lower dynamic compression ratio compared to an identical short rod engine (as explained above) and thus the final compression pressure (which is highly related to things like detonation and head gasket holding pressure) will have a lower peak combustion pressure allowing it to tolerate higher levels of ‘radical’ in compression, leanness, boost pressure or nitrous before reaching the fuel’s octane limit or the internals’ pressure leakage limits. At the same time this means that potentially the longer rod engine can make less power (because it peaks at a lower pressure level) if not for dwell time which we’ll talk about later.

5- Peak engine Vacuum and peak intake velocity

Longer rod engines spend more duration near top dead center and bottom dead center, and less duration transitioning between the two ends of the stroke. This as discussed earlier results in a lower dynamic compression ratio (because the piston moves through the mid part of the stroke so much faster than a shorter rod engine) but results in more ‘dwell time’ near top dead center both on the power stroke and in the exhaust stroke.

In the power stroke, the piston spends a significantly longer duration near top dead center in a longer rod engine vs a shorter rod engine.

In the exhaust stroke, the piston spends a significantly longer duration near top dead center in a longer rod engine vs a shorter rod engine.

As a result, the longer rod engine holds its peak vacuum (in the intake stroke) and its peak pressure (in the exhaust stroke) for a longer duration of time (even with the same intake and exhaust cam duration as the duration I’m talking about here is controlled by the piston position inside of the overall intake and exhaust stroke durations).

This means that the longer rod engine can ‘make due’ with an undersized intake and exhaust system better than a shorter rod engine because the engine breathing happens over a longer duration in the intake and exhaust strokes and thus the peak intake and exhaust velocities are closer, in a longer rod engine, to average intake and exhaust velocities as the induction is smoothed out over a longer period requiring a slightly smaller intake and exhaust pipe.

6- Engine Effective Stroke

This is hard to visualize but easy to explain mathematically. The angle between the connecting rod and crankshaft position controls the amount of torque that is delivered from the piston (and the combustion) to the crankshaft.

Longer rods create shallower angles with respect to the crankshaft at every point in the rotation compared to shorter rod engines. This means that long rod engines deliver less torque, to the crankshaft at every point in the rotation compared to a shorter rod engine. This makes longer rod engines typically produce less torque (from the same amount of air and fuel mixture) compared to a shorter rod engine giving it a ‘shorter effective stroke’

To calculate effective stroke, re-arrange the piston position equation such that stroke can be calculated as a function of piston position, crank angle and rod length.

Taking stroke as a variable (a result) in this equation rather than a fixed dimension (an input) allows us to calculate the effective stroke of different rod length combinations and estimate the difference in torque delivery to be gained or lostby altering the RSR.

http://www.stahlheaders.com/Lit_Rod%20Length.htm

Summary of direct effects:

Before we even get into the derivative effects of long rod vs short rod engines, it’s already becoming clear that there is an inherent trade off between long and short rod engines…

Long rod engines can be built to more radical specs in terms of compression, air to fuel ratios, boost pressures …etc which lends them to producing higher power figures. Shorter rod engines produce more combustion and compression pressure, as well as a higher effective stroke from less radical specs effectively lending them to producing higher torque figures and wider powerbands.

Long rod engines transition faster between top and bottom dead center making more sensitive to valve timing events which is typical of ‘tuned’ motors. Shorter rod engines build vacuum and pressure over a wider range crank angles making them less sensitive to exact cam timing, but more sensitive to peak flow velocities, and undersized intake and exhaust parts which is typcial of most street engines (that respond rapidly to ported throttle bodies or oversized intakes but less favourably to wide changes in cam timing settings).

Derivative Effects:

1- Combustion Duration

The combustion duration of a mixture in an engine is a very complex issue to study. I’ve read many PHD dissertations on this topic, which goes to show you the high level of research, theory, and experimentation it takes to come up with any meaningful data on this issue.

Usually these experiments involve taking a dual plug engine (like the Golf TDI or porsche engines, or even a dual plug cylinder head conversion on a harley v-twin). Then, swapping out one of the spark plugs with a pressure transducer / infra-red camera integrated unit and measuring both the infrared image as well as the pressure readings of the combustion process inside the cylinder. Once all this data is synchronized with the crank angle, we can then read meaningful readings of combustion durations and start to vary different engine parameters to see their effect on combustion duration.

One clever article I read had fitted a 4 cylinder engine with different pistons, each cylinder had a different static compression ratio, and so the researcher was able to use the same engine (without having to tear it down and rebuild it) to research the effect of compression ratio on combustion duration.

In my research for writing the virtual dyno and the power calculator, I can summarize certain ‘trends’ that affect combustion duration as follows:

a- Combustion pressure
b- Turbulence & fractal flame-fronts
c- Distance of travel

a- Combustion pressure

The higher the combustion pressure, the more kinetic energy is stored in the compressed mixture. The higher your engine’s density ratio (boost pressure x air density) and the higher the engine’s dynamic compression ratio (static compression ratio altered by cam timing and cylinder filling) the more kinetic energy gets stored in the air fuel mixture before it is ignited. Once it is ignited this energy is released into the mixture as an accelerent of combustion.

This is why more aggressive engines (higher boost pressure, higher compression ratios, tighter cam timing with less overlap and higher cranking pressure) require less timing advance for optimum torque delivery… because all these factors shorten the combustion duration and so to optimize power delivery to always ocurr around 17* after top dead center, a faster burning mixture with shorter combustion duration will require less advance to hit that timing mark.

As I just mentioned above, combustion duration is affected by the engine’s dynamic compression ratio, which is affected by the engine’s RSR as discribed earlier and so timing advance is affected by the engine’s RSR where engines with shorter rods will have a higher dynamic compression ratio and a lower combustion duration.

The virtual dyno uses dynamic compression rather than static compression in it’s combustion duration calculations and so you will see optimal timing variations in the virtual dyno due to different rod to stroke ratios. The optimal advance though is not a straight forward endeavour as there is a balance between optimizing timing advance for peak combustion duration, and between the force delivered to the piston due to the longer rod’s longer dwell time.

So longer rod, doesn’t always mean more timing advance… it depends on the balance in the trade off between combustion duration getting longer (due to a lower compression pressure from the longer rod) with the increase in dwell time due to the longer rod angularity.

b- Turbulence & fractal flame-fronts

Many studies have show that flame propagation in the combustion chamber under combustion pressures and due to air turbulence (swirl and tumble) is actually faster and non linear compared to fuel being ignited in free air conditions.

Image



Part of the reason for this is that the combustion process is very similar to a nuclear process where the energy released from an atomic split is enough to split the next 2 atoms, creating an nuclear chain reaction… the air fuel mixture in the combustion chamber possesses a similar amount of stored kinetic energy that gets released during combustion. The spark plug ignites the air and fuel pocket right around it. That pocket combusts and releases the fuel’s stored energy thus raising the pocket’s localized temperature in the center of the combustion chamber (for hemi heads with centrally mounted spark plugs). The edges of that air/fuel pocket now reach combustion conditions (optimal pressure and temperature) and they ignite too creating multiple pockets of elevated pressure and temperature around the original air/fuel pocket and this process continues as the combustion effect spirals outwards from the source of ignition to the rest of the combustion chamber.
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Re: connecting rod info

Postby grumpyvette » March 3rd, 2014, 11:56 am

Engines that have high swirl and high tumble effect design built into their intake port and the top surface of the piston increase the turbulence inside the chamber which enhances the creation of these pockets and speeds up the chain reaction. Also, we know that timing advance builds with rpm up to a certain point. As the engine accelerates (trying to outrun the combustion duration) we advance timing to synchronize power delivery at 17* ATDC. However, past a certain crossover rpm (around ~3500 to ~4500 rpm in most engines) the turbulance created inside hte chamber due to the speed of the piston motion enhances the fractal effect which accelerates the combustion duration and so we no longer need to advance timing and timing levels off. This is why most engines have peak timing occurring at around ~3500 to ~4500 and then held steadily afterwards.

c- Distance of travel

As more and more fuel is burnt, the cylinder pressure keeps rising in the combustion chamber. To reach peak cylinder pressure, almost all of the air/fuel mixture has to be ignited, and to do so the fractal flame front has to travel all the way from the spark plug to the farthest point in the combustion chamber.

Engines with a larger diameter bore, and engines with a side mounted spark plug will have longer combustion durations than engines with smaller bores and engines with centrally mounted spark plugs.

In engines with a centrally mounted plug, the distance the flame front has to travel is half of the bore.
In engines with an offset mounted plug, the distance the flame front has to travel is around 2/3rd of the bore.
Engines with the plug mounted next to the exhaust ports (side mounted plugs) require more advance because the flame front has to travel the entire length of the bore.
Engines with twin-plugs have a total distance of travel of at most 1/3rd the distance of the bore so they require less advance, have a better high rpm power deliver (by about 4%) and better emissions because of a more uniform combustion across the entire air and fuel mixture in the entire surface area inside the bore.

2- Piston dwell time & optimal ignition timing

As mentioned earlier, optimizing timing advance targets delivering peak cylinder pressure at 15 degrees to 17 degrees after top dead center. However, as explained earlier, due to the angularity of the piston motion, and due to the effective stroke (which is a result of the angle and leverage the connecting rod has over the crankshaft) and due to the amount of time that the piston spends near top dead center (dwell time) being acted upon by peak cylinder pressure, we find that the total amount of force delivered from the combustion process to the rotating assembly has more to do with the sum of force applied to the piston over the duration of the expansion / power stroke .

That is to say, though we are trying to optimize timing such that peak cylinder pressure occurs ‘around’ the 17* atdc mark, the exact timing that will deliver peak torque will vary with effective stroke, and dwell time because power is delivered to the piston over a period of time, and a range of crank angles (for example 0* atdc to 71* atdc for a given rod length) and what becomes important is maximizing the total energy delivered to the crank over that duration.

The total energy delivered is the summation of the force applied over that window and the force applied is a result of the combustion chamber pressure force multiplied by the mechanical advantage that the connecting rod has on the crank lobe due to the triganometry of the assembly.

In general, the longer rod will have a higher dwell time near top dead center which will improve the energy transfer between the combustion chamber and the rotating assembly at higher rpms when the amount of time (in milliseconds) that the piston spends near top dead center gets very tight.

On the contrary, the shorter rod will be less efficient at energy transfer at higher rpms, but because it has a higher mechanical advantage (due to its longer effective stroke) will produce more lower rpm power and better torque figures.

The virtual dyno calculates power differences for different ignition timing advances and for different RSR’s by calculating this exact energy integral over the window of duration that corresponds to the ingition and camshaft timing events and uses that to judge power variations due to different RSRs and different timing settings.

3- Cylinder wall friction

The shorter the RSR the more lateral force applied on the piston as the side skirt is literally dragged into and rubbed against the inner cylinder walls. If you think about this mathematically, you can break the piston acceleration into it’s x and y (cosine and sin) components in relation to the angle between crank center-line and the connecting rod centerline.

The larger this deflection angle, the larger it’s x-component (the cosine of the relevant angle) and the larger the amount of force applied on the piston against the cylinder wall. The inverse is correct for longer rods and shallower angles.

This mechanical friction can make longer rod engines produce marginally higher power figures due to lower internal friction losses. There are also ways to reduce this friction loss on shorter rod engines such as using lighter pistons with shorter side skirts and using friction reducing coatings on the side skirts.

In general, this effect is hard to measure and not modeled in the virtual dyno. The most important thing to consider here is application design:

Engines designed for endurance racing, continuous high rpm operation, and sustained high rpm loads will always perform better with a longer RSR and a longer rod combination because of the reduced internal stress, friction, thermal buildup, and power loss on the piston skirts and the cylinder side wall.

This is why engines like Formula1 engines and 24 hour endurance racing engines will always choose a very high 2.1:1 RSR to make sure that these engines can last , easily, and happily, the entire duration of the test they will be put under.

4- Average and peak piston velocity and safe red-line

Finally, (and related to the point above) the piston velocity using the equations I’ve listed in the Wikipedia article is not constant throughout the engine revolution. Even though most designers use quick rules of thumbs for piston stroke, and average piston speed to ‘judge’ a safe redline for the engine , it is not really that accurate.

The accurate way of judging safe redline for the rotating assembly is to use the exact angularity of the internals to calculate both peak and average piston velocity. Furthermore, by taking a 3rd order derivative of the piston position equation we can produce an equation the describes the stress applied by this motion on the connecting rod (for a specific RSR). By knowing peak and average velocities and comparing them with peak and average stress applied on the connecting rod, and by comparing those figures with the metallurgic properties of the chosen connecting rod material or alloy, only then can a ‘safe red-line’ truly be set.

Yes a shorter RSR will apply more lateral load (sideways bending force) on the connecting rod and a higher average piston velocity, but these stresses and loads do not necessarily mean that the engine has to have a lower redline…. the use of a lighter and stronger connecting rod alloy (such as the Titanium connecting rods that Porsche swaps into their GT3 engines in place of the OEM rods) can allow for a ‘sub optimal’ RSR to operate at a higher redline rpm safely without worry of shattering the bottom end due to lateral stress.

The takeaway:

The RSR debate will forever live on the Internet forums as people argue right from wrong ways to setup an engine. Since I mostly focus on streetable engines and that is what most of our readers and customers are building my advice is to use the shortest possible rod that is going to be safe at your target redline rpm. Yes this will cost you some power in the higher rpm range (compared to a longer rod combo) but you can always add more power up top with a few more PSI of boost pressure or a slightly more oxygenated race fuel. However adding more torque to long rod engine, and widening your powerband (by using a smaller , faster spooling turbo or a higher compression ratio) is not always that easy to do. Smaller turbos / superchargers come with top end trade offs and changes in static compression require rebuilds and so I will always err on the shorter end of the RSR to give a more streetable car with a wider power band, so long as that RSR is still safe at my intended redline rpm.

If you’re building a track only race car…. and you want all your power near redline… and you want a more zingy rpm near the top end (which might be the case for a larger displacement motorbike engine or a track only race car, then sure maximize the rod/stroke ratio) … but for most people the wider power-band benefits of the shorter rods will be very evident in daily usage of the vehicle.

This is partly why stroker kits are very impressive on the street. They not only increase displacement by moving the crank journal upwards… they also force the use of a shorter rod and a shorter RSR which moves peak the peak torque rpm further down the rpm range, as well as boost the static and dynamic compression ratios as a result of the increased engine displacement AND the shorter RSR. For a single modification, they are pretty effective at transforming a street car into something of a torque monster and if you’re planning on staying naturally aspirated with your engine (or spraying a little bit up top) this is a definite way to widen your powerband and make a car that feels much faster and versatile compared to stock.
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Re: connecting rod info

Postby 87vette81big » March 3rd, 2014, 12:37 pm

I read your latest posts on Short Rod Vs Long Rods Grumpy.
I am still using DD2000.
Do You have Virtual Dyno ?
Some of those High End Engine simulatiin programs cost over $500.
I don't think it was Kurtis Levington Speaking.
Some ideas are similar.
But Kurtis has his own too.
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Re: connecting rod info

Postby grumpyvette » March 3rd, 2014, 12:43 pm

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
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Re: connecting rod info

Postby 87vette81big » March 3rd, 2014, 12:56 pm

:mrgreen:
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Re: connecting rod info

Postby grumpyvette » March 6th, 2014, 1:36 pm

http://www.strokerengine.com/RodStroke.html

http://www.strokerengine.com/BBCengines.html

http://www.e30m3project.com/e30m3perfor ... /index.htm

http://www.strokerengine.com/RodStroke2.html

http://www.stahlheaders.com/Lit_Rod%20Length.htm

http://www.wallaceracing.com/dynamic-cr.php

http://www.users.interport.net/s/r/srweiss/tablersn.htm

http://victorylibrary.com/mopar/rod-tech-c.htm
CLICK LINKS
http://www.wiseco.com/Calculators.aspx
http://www.grumpysperformance.com/strk1.png
http://www.grumpysperformance.com/strk2.png
http://www.grumpysperformance.com/strk3.png
http://www.grumpysperformance.com/strk4.png

Ford 302 Block Height 8.20
306 Stroke 3.00 Rod 5.090 Ratio 1.69:1 CH 1.608
306 Stroke 3.00 Rod 5.562 Ratio 1.85:1 CH 1.130
331 Stroke 3.25 Rod 5.400 Ratio 1.66:1 CH 1.170
347 Stroke 3.40 Rod 5.400 Ratio 1.59:1 CH 1.090
347 Stroke 3.40 Rod 5.315 Ratio 1.56:1 CH 1.175
Many people feel the 347 piston is too short for the street,
so Probe offers a 5.315 so you can use the 331 piston,
I recommend their use.

Dart Iron Eagle 4.155 Block Height 8.70
369 3.40 5.700 1.68:1 1.300
380 3.50 5.700 1.62:1 1.250
380 3.50 5.850 1.67:1 1.100

Windsor Block Height 9.50
351 3.50 5.956 1.70:1 1.774
393 3.85 5.956 1.55:1 1.608
393 3.85 6.250 1.62:1 1.325
408 4.00 6.000 1.50:1 1.490
408 4.00 6.125 1.53:1 1.350
408 4.00 6.250 1.56:1 1.250
418 4.10 6.000 1.46:1 1.450
418 4.10 6.125 1.49:1 1.325
418 4.10 6.200 1.51:1 1.250
418 4.10 6.250 1.52:1 1.200
427 4.17 6.200 1.48:1 1.215
434 4.25 6.200 1.45:1 1.175
The 408 and 418 are best for strip & street
The 427 makes torque, great for trucks.

Cleveland Block Height 9.20
356 3.50 5.778 1.65:1 1.672
356 3.50 6.000 1.71:1 1.450
393 3.85 5.956 1.55:1 1.319
393 3.85 6.000 1.56:1 1.275
408 4.00 5.956 1.49:1 1.244
408 4.00 6.000 1.50:1 1.200

400M Block Height 10.30
408 4.00 6.580 1.71:1 1.647 (0.070 below deck) Factory number
408 4.00 6.635 1.66:1 1.670 .005 above deck BBC H-Beam Rod Cleveland KB piston 76 FT 10.6:1
418 4.10 6.700 1.63:1 1.550
427 4.17 6.700 1.60:1 1.515
956 1.55:1 1.300
Ford Big 460 Block Height 10.30
466 3.85 6.605 1.72:1 1.770
520 4.30 6.605 1.54:1 1.545
520 4.30 6.800 1.58:1 1.350
545 4.50 6.700 1.49:1 1.350
545 4.50 6.800 1.51:1 1.250

IDT Block 4.700 Bore
624 4.50 6.800 1.51:1 1.250

IDT 11.3 Block (coming soon)
729 5.25 7.550 1.44:1 1.125

IDT 12.0 Block (coming soon)
763 5.500 8.000 1.45:1 1.250

SBCScat9000.jpgBBC_crank_1_sm.jpgSCAT-SBF351-4340_Crank.jpg

Chevy Small Block Height 9.00
355 3.48 5.700 1.64:1 1.550
355 3.48 6.000 1.72:1 1.250
383 3.75 5.565 1.48:1 1.561
383 3.75 5.700 1.52:1 1.433
383 3.75 6.000 1.60:1 1.125
395 3.87 5.700 1.47:1 1.363
395 3.87 5.850 1.50:1 1.213
395 3.87 6.000 1.55:1 1.063
408 4.00 5.700 1.42:1 1.300
408 4.00 5.850 1.46:1 1.150
408 4.00 6.000 1.50:1 1.000

400 GM Block 4-1/8 Bore
352 3.25 6.000 1.85:1 1.375
377 3.48 6.000 1.72:1 1.250
406 3.75 5.700 1.52:1 1.433
406 3.75 6.000 1.60:1 1.125
420 3.87 5.700 1.47:1 1.363
420 3.87 5.850 1.50:1 1.213
420 3.87 6.000 1.55:1 1.063
434 4.00 5.700 1.42:1 1.300
434 4.00 5.850 1.46:1 1.150
434 4.00 6.000 1.50:1 1.000
You can see that put a 4.00 crank in the 350 & 400 block
is the limit of what is possible, not what is ideal.

Dart 4.20 Iron Eagle 9.320 Height
457 4.125 6.125 1.48:1 1.133
457 4.125 6.000 1.45:1 1.258
471 4.250 6.100 1.43:1 1.095
471 4.250 6.000 1.41:1 1.195
471 4.250 5.850 1.37:1 1.345

Big Block Chevy Deck Height 9.78
460 4.00 6.135 1.53:1 1.645
460 4.00 6.385 1.59:1 1.396
489 4.25 6.135 1.44:1 1.520
489 4.25 6.385 1.50:1 1.270
503 4.37 6.385 1.46:1 1.208

Truck Block Deck Height 10.20
489 4.25 6.535 1.54:1 1.540
489 4.25 6.800 1.60:1 1.275
503 4.37 6.535 1.50:1 1.478
503 4.37 6.700 1.53:1 1.313
503 4.37 6.800 1.55:1 1.212
518 4.50 6.585 1.45:1 1.415
518 4.50 6.700 1.49:1 1.250
518 4.50 6.800 1.51:1 1.150

4.5 & 4.60 Big Bore BBC 10.20
540 4.25 6.535 1.54:1 1.540
540 4.25 6.800 1.60:1 1.275
557 4.37 6.535 1.50:1 1.478
557 4.37 6.800 1.55:1 1.212
572 4.50 6.585 1.45:1 1.415
572 4.50 6.800 1.51:1 1.150
598 4.50 6.535 1.45:1 1.395
632 4.75 6.700 1.41:1 1.125
The 632 doesn't need to rev high, but many are in race cars.

World Merlin III 4.5- 4.680 Bore 11.625 (Oliver Rods)
632 4.75 7.100 1.49:1 2.150
632 4.75 7.750 1.63:1 1.500
632 4.75 8.000 1.68:1 1.250
705 5.30 7.750 1.46:1 1.225
750 5.60 7.650 1.37:1 1.175
805 5.85 7.500 1.28:1 1.200
These monster engine push the limits.
The only way to get a good rod length compromise is with limited lifetime
custom made aluminum connecting rods and pistons.
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
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Re: connecting rod & rod length too stroke info

Postby grumpyvette » March 8th, 2014, 1:08 pm

65_Impala wrote:Since people can understand pictures so much better here is a graph of piston position vs crank angle for a full revolution of the crankshaft to compare the 5.7" rod and 6" rod.

Image

Big difference, right? :rotfl: Just for your reference the biggest difference is 0.0179".


Here's the rod angle.

Image

At least this has a difference. A whole 1.026 degrees. An argument could be made that this would cause less wear on the bore. However, it occurs at the mid-way point of the stroke and I find that argument rather baseless unless someone can post up proof of an 5.7" rod 383 engine with the worst bore worn in the middle. The most wear occurs at the top of the bore where the rod angle is very little.

I really don't see where these ~huge~ changes between the 2 rods are going to make a hill of beans difference for a street going engine.

Now, if you wanted to wring every single possible HP out of the engine then maybe.

The ones making comments about how this makes such a huge difference in a street going application really need to post proof of the difference. Links to nice engine build aren't proving anything.
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
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Re: connecting rod & rod length too stroke info

Postby 87vette81big » March 8th, 2014, 3:20 pm

Smokey Yunick has done more back to back Dyno testing with SBC Engines than anyone.
Chevrolet footed all the bills.
You have to go back to his Vintage 1970's & 80's articles.
He liked long rods.
Eddy current brake dyno used.
Real accurate. Few have today.

One thing I have noticed .
Pontiac V8 Engines built with Chevy BBC Rods don't always run as hard as expected.
Times slower at times 1 second or more than called on.
Professional engine builds. Not mine.
Real Pontiac Spec Forged Rods for race non existent today.
Sell BBC Instead.
Crower made what I wanted.
Costed $$$$.
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Re: connecting rod & rod length too stroke info

Postby grumpyvette » September 2nd, 2014, 9:35 am

one factor I seldom see pointed out is the fact that increases in the piston or bore diameter , has the effect of rapidly increasing the piston surface area the cylinder pressure has to push against. if we compare a 327 with a 4" diameter bore and a 400 with a 4.125" bore the increase of an 1/8" in bore diameter may seem to provide a minimal benefit, after all its an increase of from (32) times 1/8th" in diameter to, one more or (33) times 1/8th" in diameter or about a 3% increase in bore diameter, yet the surface are increase from 12.5 sq inches to the 400sbcs of 13.39square inches is a 7% increase in surface area
http://vimeo.com/66357583
Image
now 7% may not seem important but with near 600 psi peak pressure in a cylinder on an average engine thats easily an extra 800 pounds of effective cylinder pressure over the piston at near tdc.and keep in mind PEAK pressure is rapidly lost as the piston on the power stroke moves away from TDC, in fact almost all the effective power or torque is generated or imparted to the piston and rod assembly and thus to the crank shaft as it descends into the bore in the first 45 degrees of crank rotation past TDC, and by the time the cranks rotated to 45 degrees past TDC the pistons moved down the bore less than 1" from its previous location at TDC.
now you generally can assume youll get 1-to-1.2 hp and ft lbs of torque per cubic inch of displacement , adding about 7% to the piston surface area makes the engine noticeably more efficient if the cam timing and exhaust scavenging allow you to extract that power advantage.
Image
Image
the short version here is that a engine thats properly configured with a larger bore has, or at least in theory has an advantage, over its whole rpm range.

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

viewtopic.php?f=69&t=5123&p=14765#p14765
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Re: connecting rod & rod length too stroke info

Postby 87vette81big » September 2nd, 2014, 5:44 pm

Sorta explains why all the above readings that Some Pro built Pontiac V8 modern stroker engines don't perform worth a Crap on the street with E Edel heads I have seen Grumpy.
R/S ratio in 1.4 range & E heads that flow 300-425 cfm range intake side.
One I recall guarenteed 900 HP, costed $40 K.
All iron 421 Poncho bored .090" over to 440 ci Ran Mid 9's N/A.
New expensive Race Pontiac Pro Big inch 500 + ci engine in same car would never hardly get out of 11's.
High 10's twice N/A.
Avoided E Heads since.
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Re: connecting rod & rod length too stroke info

Postby grumpyvette » September 2nd, 2014, 7:43 pm

By David Reher, Reher-Morrison Racing Engines wrote:
“An engine produces peak torque at the rpm where it is most efficient.”

Recently I’ve had several conversations with racers who wanted to build engines with long crankshaft strokes and small cylinder bores. When I questioned them about their preference for long-stroke/small-bore engines, the common answer was that this combination makes more torque. Unfortunately that assertion doesn’t match up with my experience in building drag racing engines.

My subject is racing engines, not street motors, so I’m not concerned with torque at 2,000 rpm. In my view, if you are building an engine for maximum output at a specific displacement, such as a Comp eliminator motor, then the bores should be as big as possible and the stroke as short as possible. If you’re building an engine that’s not restricted in size, such as a heads-up Super eliminator or Quick 16 motor, then big bores are an absolute performance bargain.


I know that there are drag racers who are successful with small-bore/long-stroke engines. And I know that countless magazine articles have been written about “torque monster” motors. But before readers fire off angry e-mails to National DRAGSTER about Reher’s rantings on the back page, allow me to explain my observations on the bore vs. stroke debate.

In mechanical terms, the definition of torque is the force acting on an object that causes that object to rotate. In an internal combustion engine, the pressure produced by expanding gases acts through the pistons and connecting rods to push against the crankshaft, producing torque. The mechanical leverage is greatest at the point when the connecting rod is perpendicular to its respective crank throw; depending on the geometry of the crank, piston and rod, this typically occurs when the piston is about 80 degrees after top dead center (ATDC).

So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously there are other factors involved.

The first consideration is that the cylinder pressure produced by the expanding gases reaches its peak shortly after combustion begins, when the volume above the piston is still relatively small and the lever arm created by the piston, rod and crank pin is an acute angle of less than 90 degrees. Peak cylinder pressure occurs at approximately 30 degrees ATDC, and drops dramatically by the time that the rod has its maximum leverage against the crank arm. Consequently the mechanical torque advantage of a long stroke is significantly diminished by the reduced force that’s pushing against the piston when the leverage of a long crankshaft stroke is greatest.

An engine produces peak torque at the rpm where it is most efficient. Efficiency is the result of many factors, including airflow, combustion, and parasitic losses such as friction and windage. Comparing two engines with the same displacement, a long-stroke/small-bore combination is simply less efficient than a short-stroke/big-bore combination on several counts.

Big bores promote better breathing. If you compare cylinder head airflow on a small-bore test fixture and on a large-bore fixture, the bigger bore will almost invariably improve airflow due to less valve shrouding. If the goal is maximum performance, the larger bore diameter allows the installation of larger valves, which further improve power.

A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don’t travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced. In a wet-sump engine, a shorter stroke also cuts down on oil pressure problems caused by windage and oil aeration.

The big-block Chevrolet V-8 is an example of an engine that responds positively to increases in bore diameter. The GM engineers who designed the big-block knew that its splayed valves needed room to breath; that’s why the factory machined notches in the tops of the cylinder bores on high-performance blocks. When Chevy went Can-Am racing back in the ’60s, special blocks were produced with 4.440-inch bores instead of the standard 4.250-inch diameter cylinders. There’s been a steady progression in bore diameters ever since. We’re now using 4.700-inch bores in NHRA Pro Stock, and even bigger bores in unrestricted engines.

Racers are no longer limited to production castings and the relatively small cylinder bore diameters that they dictated. Today’s aftermarket blocks are manufactured with better materials and thicker cylinder walls that make big-bore engines affordable and reliable. A sportsman drag racer can enjoy the benefits of big cylinder bores at no extra cost: a set of pistons for 4.500-inch, 4.600-inch or 4.625-inch cylinders cost virtually the same. For my money, the bigger bore is a bargain. The customer not only gets more cubic inches for the same price, but also gets better performance because the larger bores improve airflow. A big-bore engine delivers more bang for the buck.

Big bores aren’t just for big-blocks. Many aftermarket Chevy small-block V-8s now have siamesed cylinder walls that will easily accommodate 4.185-inch cylinder bores. There’s simply no reason to build a 383-cubic-inch small-block with a 4-inch bore block when you can have a 406 or 412-cubic-inch small-block for about the same money.

There are much more cost-effective ways to tailor an engine’s torque curve than to use a long stroke crank and small bore block. The intake manifold, cylinder head runner volume, and camshaft timing all have a much more significant impact on the torque curve than the stroke – and are much easier and less expensive to change.
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Re: connecting rod & rod length too stroke info

Postby 87vette81big » September 2nd, 2014, 9:42 pm

There is obviously more going on than most realize Grumpy.
Not Arguing with above observations .
I will say what works on Chevy won't work on a Pontiac V8.
And Visa Versa.
A few Pontiac 455's have ran 8's & I recall one that ran 7's Normaly aspirated 20 years ago.
All production iron Blocks, Crankshaft & Heads.
Spun to 6,500-7k.

Most Hog the Ports out too Big.
No Velocity left.
87vette81big

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