dyno hp loss rates

dyno hp loss rates

Postby grumpyvette » November 25th, 2008, 11:43 pm

read thru this
you do realize theres a HUGE difference between FLYWHEEL and REAR WHEEL HORSEPOWER???
keep in mind that any experienced dyno operator can set the parameters the dyno reads to inflate the resulting power curve displayed and, many of them will do that for one reason or another, your best and most useful results will be found in your trap speed and weight being the useful factors, that and your perception of how fast the car is.
does it really make a great deal of difference if you've got a dyno sheet saying you've got 500hp if your in the low ten second 1/4 mile times or one that says your running 600hp?, for bragging rights it may, but the cars running the same ET/MPH either way.
you going to beat or loose to your competition based on et/mph not a dyno sheet


as a general rule rear wheel hp is about 12%-18% lower than FLYWHEEL HORSEPOWER, depending on the drive train and dyno used to test


http://www.edmunds.com/ownership/techce ... ticle.html



Flywheel vs wheel power, what to believe?
I have seen few arguements over flywheel vs wheel power recently. I found this post on another forum and its worth a read IMO.

Many thanks to whoever wrote this


There is in fact no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula we saw in the previous articles - BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the cornerstone of all engine design and development work. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. We'll look at both of these separately.

If we want to know the power of the engine alone then an engine dynamometer (or dyno) is used. This is how nearly all manufacturers rate the output of car engines. The engine is bolted into a cradle and connected to the dyno with a prop shaft which bolts onto the back of the crankshaft (or the flywheel). The power figures measured in this way are therefore usually called "flywheel power". The dyno is essentially a "brake" which can apply a known torque (or "load") to the engine. When the engine is holding a steady speed under a given dyno load then the torque being applied by the dyno must be exactly equal to the torque being produced by the engine. If this were not so then the engine would either accelerate or decelerate. Let's say we want to know the engine torque at full throttle at 3,000 rpm. The throttle is gradually opened and at the same time the load applied by the dyno is increased - eventually by juggling the amount of load applied we get to the situation where the throttle is fully open and the rpm is steady at 3,000. The torque being applied is written down and then the operation would be repeated at say 4,000 rpm. Soon we get a complete chart of torque at all engine speeds. Of course we could also measure part throttle power if desired.

Modern dynos are computer controlled and can generate power and torque curves very rapidly without the operator having to manually adjust throttle and load controls. They can be programmed to measure every so many rpm, say in 250 or 500 rpm steps - or they can measure a continuous torque curve while the engine accelerates at a preset rate. This can be used to simulate how the engine would actually operate in a particular gear when installed in the car.

There are various ways in which the dyno load can be applied. Older dynos use a hydraulic system with a rotor inside a water filled cavity - rather similar to the torque convertor in an automatic gearbox. Modern dynos generate the load with large electric motors. Even a simple friction disk or drum brake will work fine and this is where the name "brake" in Brake Horsepower came from. The important thing is that the load is able to be measured accurately and that there are no frictional losses in the system that escape measurement.

In order for dyno results to be comparable and universally understood there are a number of things that need to be closely controlled during the measurement process:

Operating Conditions
Air temperature, pressure and humidity affect the amount of power an engine produces. Cold dense air means a greater mass of oxygen per power cycle and thus more power is generated (provided of course that air/fuel mixture is properly calibrated for the conditions prevailing). There are formulae that can be used to calculate how much the measured power would change if the test conditions were different. This enables dyno results to be "corrected" back to standard conditions to enable comparison with anyone else's test results. Sadly however there is no one universally accepted set of "standard" conditions because different automotive bodies in different countries use different standards to calibrate to. "SAE" power standards are used in the USA and sometimes in England. "DIN" standards are used on the continent and there are a few other oddball systems just to confuse the issue. So just because your car is rated at 100 bhp and a friends at 110 bhp doesn't necessarily mean that his engine is more powerful - it depends whether both measurements were corrected to the same standard conditions.

One of the tricks I've seen used to get bigger "corrected" bhp numbers is to use a very high ambient temperature reading for the dyno test. If the operator measures the temperature close to the engine rather than well away from it then obviously he will get a reading that is much higher than ambient. When the bhp numbers are corrected back to a lower standard ambient temperature they will increase. I saw an engine dyno sheet the other day where the ambient air temperature in February, in England was supposedly 37 degrees C. Now either that test was done with the temperature probe sat right on top of the engine or it's a part of country I don't yet know about where I would very much like to live !!

Engine Ancillaries
When installed in the car, the engine has to drive a number of items like the alternator and power steering pump which sap power. Also the exhaust and air filter systems will reduce power to some extent. If the engine is tested without any of these ancillaries fitted then it will show much higher power figures. The Americans used to rate their engines like this back in the fifties and sixties and often the installed power of the engine would only be 2/3 of the claimed figure in the sales blurb. This used to be called "gross" flywheel power and if the ancillaries were fitted the power was called "net" flywheel power. Nowadays the gross system, which was very misleading, is not used and all modern published data should be "net flywheel" power. Major manufacturers abide by rigorous standards which set out how the engine should be installed on the dyno to simulate closely the "in car" conditions.

Also called chassis dynamometers, these are used to measure power at the driving wheels. This avoids the inconvenience of having to remove the engine to test it if a tuning modification has been made. However, it means that the power figures obtained will be lower than the flywheel power because of the frictional losses in the drivetrain and tyres. This leads to one of the biggest sources of confusion, error and plain misinformation in the tuning industry. You see, as discussed above, all major manufacturers quote flywheel power so it is understandable that people want to know if the hard earned cash they spent on tuning mods increased the power of their engine and by how much. To know this for certain means knowing how much the transmission losses are. There is enormous pressure on rolling road operators to be able to quote flywheel bhp rather than wheel bhp and most operators now run proprietary software systems which "supposedly" print out flywheel power.


Yes - I know - the whole chassis dyno tuning industry quotes flywheel figures and here's me saying none of it works. So I'd better explain some more and then you can make your own mind up.

First, let's look at how a chassis dyno works. The car is driven onto a rig so that the driving tyres are resting between two steel rollers. The torque is measured at different speeds in exactly the same way as an engine dyno works except that it is torque at the rollers rather than torque at the flywheel. The braking load is applied to one of the rollers by either a hydraulic (water brake) or electrical system again in just the same way as the engine dyno would apply a torque to the crankshaft of the engine. The same universal equation at the top of the page can then be used to calculate bhp at the rollers by knowing the torque and the rpm of the rollers (NOT the rpm of the engine at this stage) - but if the engine rpm is measured simultaneously then we can know roller bhp at a particular engine rpm. The BIG problem with all this is if any tyre slip is taking place. Remember these are smooth steel rollers which over time get quite polished. How much grip do you think you would get if roads were made of polished steel rather than tarmac? The effects of tyre slip are complex (i.e. I don't pretend to fully understand them myself!) but what I do know is that you can get some really strange bhp figures from highly tuned engines on narrow tyres and the readings are invariably too high not too low.

Came across this article about DynoJet dynos (inertia) vs. loading dynos (such as Mustang and Dyno Dynamics) and what it has to say is very interesting. It's a good read.

GermanMotorCars.com: http://www.germanmotorcars.com/Dyno_...0inertia_1.htm

Copied here for those that don't want to click.

This discussion revolves around chassis dynamometer's and is intended to be informative and thought provoking. There are two types of chassis dynamometers on the market, inertia and loading. An inertia dynamometer (such as DynoJet) does not measure torque, but measures acceleration. A loading dynamometer applies resistance that is measured (using some type of strain gauge.)

The most often heard discussion is that what factor can be applied to rear wheel horsepower to reflect crankshaft horsepower. This is where we need to understand how the rear wheel horsepower number was derived. Since the DynoJet seems to be widely used and numbers quoted are those from a DynoJet, we are going to use them as our inertia dynamometer example.

First it is important to have an understanding of how DynoJet gets their horsepower numbers. Power in mechanical terms is the ability to accomplish a specified amount of work in a given amount of time. By definition, one horsepower is equal to applying a 550 pound force through a distance of 1 foot in one second. In real terms, it would take 1 HP to raise a 550 pound weight up 1 foot in 1 second. So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). Dynojet's inertial dynamometer measures power according to the terms just described. It measures velocity directly by measuring the time it takes to rotate two heavy steel drums one turn. It measures force at the surface of the drum by indirectly measuring it's acceleration. Acceleration is simply the difference in velocity at the surface of the drums from one revolution to the next. The force applied to the drums is calculated from acceleration using Newton's 2nd law, Force = Mass * Acceleration. Since the mass of the drums is know and acceleration has been measured, Power (horsepower) can now be calculated. Torque is then calculated using the horsepower number: Torque = Horsepower * 5252 / RPM.

Once they have these numbers a series of correction factors are applied, some made public, some hidden as proprietary secrets. The public correction factor is the SAE correction factor. This formula assumes a mechanical efficiency of 85%. The formula used is: Where: CF= 1.18 * (29.22/Bdo) * ((Square Root(To+460)/537)) – 0.18. To = Intake air temperature in degrees F, Bdo = Dry ambient absolute barometric pressure. This correction factor is meant to predict output in varying atmospheric conditions and is a +/- 7%. The proprietary correction factor is supposed to reflect the loss of power from the crankshaft to the rear wheels.

A Loading Dynamometer applies resistance to the dyne's roller(s) , typically using either a water brake or a current eddy brake. In either case, the amount of force is measure using a strain gauge. The measured force is torque which is a real, indisputable measurement of the actual output at the wheel. Horsepower than can be calculated: Hp = Trq * 5252 / RPM.

A Dynamometer can only measure actual power at the output location. Actual power produced AND delivered by an engine will be highest if measured at the crankshaft, lower at the transmission output shaft and even lower, but more meaningful, still, at the rear wheels. The power that you use is the power at the rear wheels. Some Dynamometer companies add to measured rear wheel power readings a factor that is based on ESTIMATED rear wheel power losses (under what power conditions? 3.0 ltr.? 5.0 ltr.? Under coasting conditions? with a 185/70/15 radial tire? a 335/35/18 radial tire? New heavy radial tire vs. worn old, light, racing tire? Who knows?) In short, there is NO meaningful "average" tire to get a correct rear tire power transmission loss measurement for all cars - so obviously, unless they actually measure the power lost in the rear tires, under driven load conditions, NO dyno company should BE ADDING incorrect power figures into the measured power. It's simply wrong. The fact that they add varying amounts of power to the actual, "true" amount of power delivered and measured to the surface of the drive roller creates a situation that makes it an onerous task to compare power figures from different brands of dynamometer systems. On simple inertial dynamometers, some (most) companies use an average for the inertial mass value of the engine, transmission, driveshaft, axles and rear wheels. This is saying that a 4 cylinder, 2.0 ltr. Porsche 914 has the same rotating mass and same rear wheels as a 8 cylinder, 5.0 ltr. Porsche 928 S+4. This simply is not so and wrong.

It's expensive to measure frictional losses in the engine and drivetrain, requiring the dyno to be able to drive the vehicle with engine off. Add the cost of a 50+hp electric motor, controlled power supply, etc. It's just not likely that $20,000 dyno will be equipped with that equipment. It is also common for dynamometer companies to add to the power readings by adding transmission and driveshaft losses back into the measured power readings. Some companies make a concerted effort try to measure frictional losses and, optionally, add the power to the measured readings. Other companies - some that would surprise you - say that it's not important and give a blanket, single factor for frictional losses in every engine. Some simply say that there is a meaningful "average" for every car,( 4 stroke/ 4 cylinder/ 4 speed transmission, 4 stroke/ 8 cylinder/ automatic transmission) and apply it to every car and that it is not a significant difference. Blanket estimates of "average" losses and corrections are, quite simply, incorrect. At the upper levels of the industry, (we are talking about $150,000 - $500,000 AC or DC 4 quadrant dynamometers) it is not tolerated - shouldn't be - and needn't be. There is a dyno company that actually has different versions of software that displays their own identical data files as different amounts of power depending on whether you use the DOS version or the Windows version of their software!!

True, rear wheel horsepower is the standard of measuring the power that is actually delivered to the rear wheels. It is honest, true, fair and duplicable. It is the ONLY standard that can be duplicated by the entire industry - regardless of the dyno manufacturer. From my experience and that of many others, when comparing True, rear wheel horsepower to DJHP you must apply a factor. It appears that this is a sliding scale based on horsepower but the best estimate is 1.05 to 1.21 (maybe higher). What this means is that for those of you trying to calculate what your crankshaft horsepower is based on DJHP, and are adding 15%, the most common number I hear, you are actually doubling (at least) the factor. Why? Because DJHP already has a puff number added into their DJHP. Lets say DJHP shows 200 hp and you add 15%, you get 230 hp crankshaft horsepower. In reality DJ has already added in 15 or 20% to their 200 DJHP number. How does this help us.? It does not, and is fact harmful to the many dynamometer test facilities that report only what the dyno actually measured. I can not tell you of the many discussions that we have had as to why the horsepower numbers we recorded lower than that of DJ. For those manufacturers that use DJHP as proof of their claims, can you imagine the shock your customers get when the horsepower number of a vehicle tested on a load bearing dyno do not come close to their claim.

Proper tuning, especially on highly modified engines greatly affect the power difference. Due to the fact that the DJ dyno's sweep so quickly on sweep hp tests, there is no way to properly tune a fuel map. What you get is the acceleration and full throttle maps both triggered during the test, ending up over-rich, affecting the horsepower. The other factor that needs to be taken into account is that DJ dynos assume that every vehicle has the same rotating mass - they don't - and that disregard is another reason why the hp conversion figures are different. The most accurate measurement of rear wheel horsepower is in Steady State Mode (inertia is not a factor in power equation.) The inertial mass changes on each car affects the DJ power, but not the true, rear wheel horsepower. There's another message in the above example, besides the average true, rear wheel horsepower to DJHP conversion factor - It's up to the more experienced reader to figure it out.

Chassis dyne HP, What is it? What to call it? DynoJet = "DJHP". It's not really proper to call "DJHP" "rwhp", as neither the Mustang, DynoJet, Fuchs, Superflow or Land and Sea will necessarily produce the same numbers as a DJ dyno, except by luck - and the whole idea of true, rear wheel horsepower is that EVERY dyno manufacturer HAS the capability to provide those numbers! The Superflow chassis dynes, the Mustang, Land and Sea are all capable of measuring power in steady state mode and producing the same numbers - they all measure torque. Torque x rpm / 5252 = horsepower. We've not diddled with physics! The only factor that is added to the measured reading, in true, rear wheel horsepower, is the additional energy (dyne parasitics) required to spin the dyno(s) roller to whatever speed the roller is turning at - logical, proper and required for any measuring instrument, torque x rpm / 5252 = horsepower + parasitic power = true, rear wheel horsepower.

Chassis dyne HP, What can inflate HP readings on a dyno, but not really make more engine power in the real world? A few things can affect HP when using inertia dynos (not a dyne in Steady State Mode) to measure power (what else would you do??:-): Changing to light, worn race rear tires will improve power output on an inertia dyno, but, not improve real world top speed. A heavier (brand new street) tire that replaced the above, light, worn tire, will decrease measured power on an inertia dyno, but not decrease real world top speed. Lighter wheels are a good thing! Better acceleration in lower gears, especially 1st and 2nd (accelerating less inertial mass!). Better handling is possible, too! Driving hard on worn, light tires is foolish and is not being recommended.

Problems with Inertia dyno test procedure and fuel injected vehicles: A Sweep Test (hold throttle wide open and sweep from low rpm to high rpm) will often trigger the Acceleration Fuel Map, along with the Main Fuel Map, causing the fuel mixture readings to indicate dyno operator that the motor is overly rich. This would cause the tuner to lean out the main fuel map. Of course, in the real world, upper gears, the acceleration rate of the engine is much slower than what they tested, doesn't trigger the Acceleration Fuel Map, and the engine ends up a lot leaner in reality in top gear. It's not that common of a problem, since most people never drive that fast for that long to cause engine damage. Work around: Tune full throttle fueling in real world usage at dragstrip (to best trap speed) or in Steady State Mode on different dyno.

You can optimize tuning for a DJ dyno and make big numbers - and you can tune the engine to make the best power under load on a load bearing dyno and blow off the big DJ dyno numbers. Can a tuner cheat and make a load bearing dyno read higher? The only way that could happen is in a Sweep Test - Sweep Tests are the least reliable of all tests, period. There is NO question about that. Since the Rotating Mass is a variable in a Sweep Test (NOT a Steady State Test!), the actual inertia factor entered affects the final HP figure - Tell the software that the vehicle has a lot of rotating mass to accelerate, and the HP number increases. (torque, rpm, acceleration rate and mass are the factors) - just like DJ dyno ignoring the difference in mass of all cars - So - true HP, again - Steady State Test - No acceleration, mass makes no difference, anymore. Torque, RPM and dyne parasitics. Period. True. Can you make a Steady State Test read higher? Really hard to do - The software will NOT take data unless speed and load are completely stable - eliminating cheating. As far as atmospheric conditions making a +/- 10% difference? Unless you REALLY mess with the barometric pressure (and you can look at every atmospheric factor on the test report sheet - it's hard coded to display - and not an option), it is simply, absolutely impossible to do without obvious evidence. Are final tuning optimal dyno settings different on an Inertia dyno vs. a load bearing dyno? For many reasons, final tune settings are different - and, since most load bearing dyno's will do both , there is a choice of tests - from a DJ style Sweep Test to Steady State. Having a choice of those types of tests to do and seeing what the results on the track are, most tuners will choose the Steady State Test over a Sweep Test. Without a doubt - the Steady State test Mode is the most consistently superior method of tuning - anybody who has the capability to do it will echo that sentiment - it's only an arguable point with those who can't do it properly. One of the reasons why the load bearing dyno will provide settings that work better in the real world is that combustion chamber temperatures are more in line with the actual operating temperatures that the engine.

Does altitude make any difference at all in horsepower? The engine couldn't give 2 hoots at what altitude it is tested at - it only cares what the air pressure, temperature and humidity is. Sea level at 28.02 inches baro is exactly the same as 4000 ft at 28.02 inches, as far as the engine is concerned. When tested at 5000 ft, we get virtually exactly the same power (corrected to atmospheric conditions, of course) as we do at sea level - It's just about 24%-25% less on the track! I am confused why some dyno operators insist on putting altitude on their charts and swear that it's a factor.

Crankshaft horsepower vs. true rear wheel horsepower. That's a tough one. As each vehicle is different, the best way is to dyno the engine and then dyno the vehicle to see exactly what the loss is. The best estimate I can give you based on experience and research is take crankshaft horsepower, subtract 14.5% ( search SAE ), take that, and subtract around 10% to 15% and you'll get about true horsepower at the rear wheels. The actual formula contains a curve for power loss through gears and there's another curve for power lost in a tire. Remember, too - that unless you dyno your engine you are only likely to get a crankshaft number from the manufacturer and that's probably a "good" one that the marketing department is providing.

it states plainly that hp loss is a near constant PERCENTAGE of the power run thru a drive train as the FRICTION AND HEAT increase at a near linear rate with the power going thru that drive train.....yes your correct that different designs of auto or manual transmissions will use different percentages of the power, but the difference between designs is just not a huge factor, he suggests you use 18% for a manual trans and 25% for an auto trans as a ballpark guess based on tested results of several cars.
he also states that a larger and heavier trans tends to absorb or use more hp in the power transmission than a smaller trans, which only makes sense, but in most cases a smaller trans won,t hold the extreme power levels a well built engine produces.
swapping from a 12" stock converter to a 9" race converter probably has a larger effect than the model of auto, transmission used if both of the transmissions are similar in size/weight
a 700R4 is at or beyond its strength limits at about 450 ft lbs of torque, a 4L80E will easily handle much more power but it easily weights 100 lbs more, due to MUCH more MASSIVE internal parts, those MUCH stronger parts take more power to turn, but when your in those hp levels theres not really a choice, put a serious engine on a 700R4 and youll replace it FREQUENTLY, theres just nothing you can do to offset the fact that larger heavier parts can be made stronger

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Re: dyno hp loss rates

Postby philly » June 11th, 2014, 7:48 pm

i just happened across this as i was heeading down the rabbit hole and had to bump it up so some new guys can read it. ive told people for years that chassis dynos are stupid toys and not to be taken seriously. hopefully more people will learn this one day

There's never enough money to build it right, but there's always enough to build it twice!

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Re: dyno hp loss rates

Postby 87vette81big » June 11th, 2014, 7:58 pm

Still Reading Phil.
I like Engine Dynos better Too.
Eddy Current Bake dynos are best.
After engine shutdown the Generator load is Fed DC Power Voltage & Current.
Amount of Amps required to spin over at a Given RPM Tells how much parastic drag is present in the test engine.
Can't do that with a Chassis dyno or Water brake dyno.

There are 4- wheeled Eddy current chassis dynos made.
For Indycar & Nascar testing.
Wheels removed & seperate eddy brake dyno brakes bolted to each hub.
Mega expensive testing equipment in the Millions of $&$$$&&$$$$.

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Re: dyno hp loss rates

Postby philly » June 11th, 2014, 10:43 pm

people greatly under estimate the butt dyno... i know im doing something right by the seat of my pants, trap speeds confirm what i already know. the dyno is for bench racers and bragging rights.

There's never enough money to build it right, but there's always enough to build it twice!

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Re: dyno hp loss rates

Postby 87vette81big » June 11th, 2014, 11:08 pm

The chassis dyno was originally meant for a quick test & tune & get a rough baseline.
Its been missused abused since 1979 Phil.
That Long now.

I have been told in the past that the Big 3 , GM, FORD & CHRYSLER HAD THE MOST ACCURATE ENGINE DYNOS In Muscle Motor Era.
But todays Race engines far exceed thier capabilities.
General Motors Chevrolet, Pontiac& Olds were most inovative of the GM Bunch.

Diesel engines undergo extensive Dyno Room testing.
Read a few interesting articles a few years back on the topic.

Butt Dyno & Track times still work great Yes Phil.

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