Long rod(6.125") 355 VS 383/396 strokers



Long rod(6.125") 355 VS 383/396 strokers

Postby B'klyn9C1 » November 7th, 2009, 1:02 pm

A gentleman who is the president of Chicago area F-Body Club wrote on 09/26/09 on the thread "355 or 383 for the money" on the LS1LT1.com forum wrote:

"Don't underestimate a long rod 355 in the torque department. 6.125 rod and Mahle forged pistons on a stock crank and worked stock heads might suprise a lot of people."

I said to myself WOW!! Then I read in his other post featuring his dyno sheet that he made over 492 RWHP and over 512 RWTQ out of a modified stock casting headed 355. I was always told that I needed at least a 383 stroker to get those kind of numbers.

Can you please shed some light on the components that I would need to build such a 355 with the stock nodular iron crank??????

As you know I want to do a quaility budget rebuild of my '96 LT1 longblock using the stock crank which came out of my old 9C1 and now resides in my 1995 9C1 w/the OBD I electronics. The block has over 182k miles on it. My goal is to make over 420 RWHP and over 400 RWTQ.

Since I ran across his post I have been looking into this "BIG BORE, LONG ROD, SHORT STROKE' theory. It seems counterintuitive to what I have beeen taught about high HP engines. I also ran across this article on the Air Flow Research website. "The 350 Engine that Chevrolet Should have Built", http://www.airflowresearch.com/article003. If this link doesn't work googling the title "The 350 Engine that Chevrolet Should Have Built". should get get you a better copy. It was Hot Rod Magazine article from 1997.

Also check out Jim's (aka JPack) thread "So I am almost done with the build....." on the F-body forum on LS1LT1.com. On there he posted his dyno sheet.

I have read quite a few thread RE:Rod to Stroke Ratios (R/S) since. The concensus is they don't matter or at least not as musch as they used to. Due to improvements in head design, piston technology etc. I think is was Joe Sherman's opinion that connecting rod (length) is just where the piston connects to the crank. But how come all the top engine builders with that opinion always use rods of 6" or greater?? They say they do it to use those modern light tight ring pack pistons but end end up getting a R/S ratios that is close to the stock SBC 350's R/S ratio or even a bit greater. If it didn't matter how about running the R/S ratio of the old 400 ci SBC of 1.4 to 1.5

The LS1's R/S ratio is 1.68 (almost 1.7) with a 6.1" rod. And I read that EFI engine benefits more from long rods since we don't have to depend on the strong vacuum signal that the increased piston speed of a short rodded stroker engine.

Bottom line Grumpy and company is the a Long Rodded(6.125 inch ) 355 SBC a good alternative to a building a 383 or 396 stroker for power and fuel economy???????

My 1995 Caprice 9C1 has 3.73 gears, a Tru Trac LSD, a cold air intake and plan to put 4 into1 long tube headers on a 3" dia exhaust (with 2.5" tailpipes) and an "x" pipe. I am also thinking of going to EFI Connection's LS1 PCM and coil-on-plug conversion.


Det. Colin Wilson, NYPD
B'klyn9C1
Brooklyn, NY
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Re: Long rod(6.125") 355 VS 383/396 strokers

Postby grumpyvette » November 7th, 2009, 1:17 pm

in theory longer rods have some advantages,,but its the heads,their flow,the cam timing, engine displacement and compression ratio, and the effective exhaust scavenging that will have far more effect than rod length changes, ID bet a great deal of money that a properly designed 383-396 will beat a similar 355sbc almost every time if your building a combo that can reasonably be used on the street and driven in traffic, and expected to live under street diven conditions for at least 60-80 k between rebuilds

read thru these
http://www.chevyhiperformance.com/techa ... index.html

http://www.purplesagetradingpost.com/su ... ngine.html

viewtopic.php?f=69&t=2645&p=6834&hilit=hemi#p6834

http://www.airflowresearch.com/articles ... 5/A-P1.htm

http://www.airflowresearch.com/articles ... 1/A-P1.htm

http://www.airflowresearch.com/articles ... 6/A-P1.htm
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: Long rod(6.125") 355 VS 383/396 strokers

Postby B'klyn9C1 » November 7th, 2009, 1:57 pm

Thanx again Grumpy,

I already printed out the articles including the one sent to you RE:"the 350 that Chevy should have built". hat 408 ci stroker has a terrible Rod to Sroke ratio of 1.55:1 !

Grumpy have you or your firends have built 350- 357 ci SBC's that have connecting rods of 6" or greatwr and how did they perform on the street and on the track???

I read that these long long stock cube 350's not only do they love to rev much more then a stroker but still make HP past the HP peak. Not a big drop off like most motors.
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Re: Long rod(6.125") 355 VS 383/396 strokers

Postby grumpyvette » November 7th, 2009, 2:18 pm

if you really want the best of both worlds build a 377 sbc with 6" rods , using a 350 crank in a 400 block with special forged pistons and bearing spacers like we did in the late 70s for stock cars , but cheat a bit and build it with a 11:1 cpr and a decent flat tappet cam like this

http://iskycams.com/timingchart.php?pro ... ber=201027

intake
http://www.summitracing.com/parts/HLY-300-110/

heads
http://www.jegs.com/i/Brodix/158/1021001/10002/-1

add a 7-8 quart baffled oil pan,
http://www.cantonracingproducts.com/cgi ... ey=15-240M

a 850cfm carb
http://www.summitracing.com/parts/HLY-0-4781C/



and 3.73 gears and a manual transmission or 3200 rpm stall converter and have fun.
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: Long rod(6.125") 355 VS 383/396 strokers

Postby B'klyn9C1 » November 7th, 2009, 3:38 pm

Nah, my 9C1 is a daily driver and I like my OEM EFI LT1 and starting a single plane carbed car in the wintertime is no fun. Iam just trying tosee if thisl ong rod 350 is a worth alternative to a 383 stroker.

I figure while you are not increasing displacement your are still increasing mechanical advantage in the rotating assembly.
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Re: Long rod(6.125") 355 VS 383/396 strokers

Postby grumpyvette » November 7th, 2009, 5:06 pm

while the difference in rod length looks like a big deal at first its really NOT. heres why,the difference in effective angle between a 5.7" rod and a 6.125" rods is about a 1.25 degree difference, and although the longer rod shorter piston combo tends to be lighter in weight and have slightly less friction in the bore the effective difference is minimal

Table 1
ROD LENGTH RELATIONSHIPS*
(with Crank @ 90 deg ATDC)

Piston Position Crankpin/Rod Angle
Stroke Rod Length Rod Angle from TDC ATDC
3.5 5.70 17.88 2.025 72.12
3.5 5.85 17.40 2.018 72.59
3.5 6.00 16.96 2.011 73.04
3.5 6.20 16.39 2.002 73.60

as a general rule, shorter rods tend to get better results with a tighter cam LSA and larger ports, a longer rod tends to favor slightly smaller ports and a slightly wider cam LSA but again the differences are minimal

POSTED ON ISKY CAMS
Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.

info thats well worth the time to read thru


http://victorylibrary.com/mopar/rod-tech-c.htm

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

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

viewtopic.php?f=53&t=110

viewtopic.php?f=53&t=1017

viewtopic.php?f=53&t=1168

viewtopic.php?f=53&t=510

I get asked all the time if 6" or 5.7" rods are the best selection for the 383 sbc builds, the truth is both have some advantages and disadvantages, as an example I selected two very similar forged KB pistons to compare with similar compression ratios that might be used in a kick butt racing combo, with 12.5:1 cpr for a weekend warrior style 383 sbc
so lets look at the differences as they will have similar traits to other pistons


heres a link to KB pistons, Ive used dozens of these sets with good results
http://www.kb-silvolite.com/

MORE SOURCES

viewtopic.php?f=53&t=2208

advantages & disadvantages of the 5.7" rods and pistons, in a 383 sbc


Image
less clearance for the counter weights , to pass under the skirt, higher thrust loads on cylinder walls
higher reciprocating weight
481 grams piston / 132 grams pin
rings fully supported in oil ring groove, as the piston pin is lower
slightly longer and potentially more stable bore skirt contact area



advantages & disadvantages of the 6" rods and pistons, in a 383 sbc
lower reciprocating weight
http://www.kb-silvolite.com/test/icon/i ... s&P_id=391
more clearance for the counter weights , to pass under the skirt, lower thrust loads on cylinder walls

Image

418 grams piston / 132 grams pin
THATS 63 grams lighter than the 5.7" piston
rings not fully supported in oil ring groove, as the pin by-sects the lower oil ring groove
slightly shorter and potentially less stable bore skirt contact area

related info

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

http://www.rustpuppy.org/rodstudy.htm

http://victorylibrary.com/mopar/rod-tech-c.htm
Image
Image

ISKY CAMS POSTED THIS INFO
Tech Tip - 2005

Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.

http://www.iskycams.com/ART/techinfo/ncrank1.pdf
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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