Chassis Dyno Testing

[Geary L. Sarno]

My question is : What is an average power loss through the drive train. I've seen rear wheel power loss figures in articles ranging from 20 to 40%. Our first dyno figures were 369 RWHP. The torque was about the same . Considering that we found a damaged spark plug after the testing and the secondaries were running rich, I was'nt disappointed with the results. I'm running a 403 FE with Toploader and Jaguar Rear. -Aggressor

[MJN]

My dyno guy reckons 15% loss on a T5 and 8.8 rear end with synthetic lube. If yours is a top loader or older design trans and with the Jag rear I would bet that your drive train loss is a fair bit higher. If I had to guess I would say around 25%. If it's as high as 40% then you have a real problem. 40% sounds like an auto trans number. It will be much higher with an auto trans compared to a stick.

[kountzecobra]

I have an old friend that worked for Roush Racing in their Craftsman truck division that told me something I had never thought of. He said, on an average vehicle, the drivetrain loss is about 15 to 24% until you get to around 250 to 300 RWHP. After that, it seems to lessen percentage-wise on up the horsepower scale. If you took a car running at 250 hp and replaced the engine only with one that produces 500 hp, the loss would not be 15%, but maybe 9 or 10%. However this was a theory at that time and hadn't been proved well enough to get real numbers. To simplify, he used 15% for manual trans and 20 to 24% for auto trans. These are with parts in top condition. Something for you engineer types to think about. Me, I don't know.

[GeorgiaSnake]

David

I have often wondered the same thing. If it takes X amount of hp to overcome drive train friction then the formula should be total flywheel hp minus x. The percentage would vary considerably. Just a thought.

Randy

[jwoodard]

had my car dynoed at Bob Norwood's facility here in Dallas. Bob uses 16% as the factor for drivetrain loss. So if you had 369 RWHP you have in the neighborhood of 440 at the crank.

Jim

[Excaliber]

http://cobra.terraworld.net/forums/s...309#post195309

We were also discussing this very issue in the above thread! My thinking is power loss is fairly constant regardless of h.p. So a "percent" number could be VERY misleading from "stock" to "built" motor.

Ernie

[Hal Copple]

While it is fun to put your car on a Chassi dyno, and then try to see about what your engine's power is, (i did this too), i think dyno's are not very comperable. For instance, one of the Hot Rod magazines i get, just tested about three dozen "built" 350 CI engines, at three reputable dyno's in the us, and found that the California engines were putting about 30+ hp more on the average, than comperable engines at the two other east coast facilities. No real explanation, just stating that obviously different dyno's, even engine ones, can give pretty different results on similar engines.

The magazine is running the "championship" on the California dyno, no doubt it will give the best hp numbers.

So calculating flywheel hp from any dyno, even a flywheel one, can give a variety of gestumations of real engine power.

But it is fun, and when i got my numbers, and did the backward calcuations, i found my engine has more power than the Titanic had!!

[kountzecobra]

It seems to me that the dyno operator can have an effect on the results too. I would think best use is for tuning and one should use the same dyno with an operator with lots of experience for the most repeatable results possible. Then you can lie like a SOB about HP numbers when bench racing with friends!

[Cobra #42]

Guys, If your are really interested, and your dyno guy is sharp and will take the time, he can do a reverse dyno run and by measuring the coast down time and give you your exact loss numbers. But for most dyno operators that have done 1000's of pulls , they use the 16% number unless you have micro polished your ring gear and pinion, also same process to the trans. Use your dyno time with an O2 sensor and get it dialed in. 369 RWHP will kick alot of 500+ HP ass, that has never been on a dyno !!!! everytime

What I like to see is all the 500+ HP Bull Sh...ters walk the walk and get on a dyno or just talk the talk.

[GeorgiaSnake]

42

My question is still unanswered. Take your car - install a 300 hp dyno tested motor in it - put it on the dynamic dyno and see what it delivers. My guess is a good bit less than the static dyno and for good reason - drive train friction. For arguments sake lets say it pulled 240 hp at the wheels which is a 20% loss due to the drive train. Now yank the motor and put in a 500 hp dyno tested motor - go back to the same dynamic dyno duplicate the pulls and compare the numbers. Unless I'm missing something here it should produce 440 hp (500 minus the 60 hp established in the first test) at the wheels not 400 hp as the 20% rule would dictate. Adding hp cannot increase the drive train friction beyond what is was before.
I really don't care about the numbers I just want my car to haul a$$, but this 20% rule seems to throw science out the window.

Randy

[Jeff Frigo]

15-20%. The higher the hp, the less the percenage of loss.

[Cobra #42]

Georgia, There are as many questions as answers in these discussions. For example, when you start taking a engine that has been "dyno tested" and measured at the crank, what type of headers were used, are they the same ones you are running in the car, are the collectors the same ?? Mufflers, how long are the exhaust pipe, have both dyno's been calibrated and are using the using same temp inlet air. Carbs, type of oil, rear end lube,the list goes on and on. That why if you get into a dispute over who did what, drag both cars to the same shop same day and see what happens. The other issue may be when was the dyno last tested itself ?? That's why I don't concern myself about the HP numbers, what I like to see is getting my fuel air ratio to be even though out the entire RPM range and let the driver do the rest.

[A Snake]

The only chassis dyno in my area at the time I had the engine dyno'd would only handle 250 rwhp so I couldn't do it.

Unless chassis dyno setups are different than they used to be, it is tough to get a light weight high hp car to stay on the rollers even with chains or straps and even tougher to get an accurate, consistent reading.

BTW friction does increase with load. Sorry I can't offer a pat formula, there are too many variables.

Al

[petek]

Friction does increase with load. Think sandpaper. If I remember correctly there are exceptions, some plastics demonstrate a reduction in friction with an increase in load (to a point).

The dyno would have to present a greater load to verify the higher horsepower, the greater load would cause greater friction in the gears in the diff and tranny. Things like synthetic lubes help by decreasing the friction ... so the rear wheel hp can be greater with synthetic lubes.

I've got a feeling that the relationship between hp and drivetrain losses is not linear (i.e. you can't just use a static percentage of loss). If anything, the greater the horsepower, the greater loss in the drivetrain.

Then again, perhaps the accuracy of the dynos also changes with load. Perhaps the greater the load, the greater of the internal losses within the dyno ... and the greater the inaccuracy of the final reading.

Yup, I've got a 1000 hp 351W ... it's just that there is a 60% sum of drivetrain loss and internal dyno friction.

[Geary L. Sarno]

Hey guys - Thanks for the feedback.
Another related question. My Edelbrock manual states that 13:1 fuel/air ratio is the best for wide open throttle operation. On the dyno my ratio was 15.6:1 for a wide range and that's where the FE made the best power. When my secondaries started to open the ratio dropped to 13.6 to one and the power/torque curves fell in a similar manner. I'm running dual Edelbrock 600's (1405's) on a 1967 Shelby GT500 intake. I think the problem relates to air speed/volume through the secondaries in the dual carb setup.
My reasoning is that the primaries will supply all required air up to the point where the vacuum demand starts to open the secondaries. The carbs are designed to provide fuel/air through the secondaries based on feeding 100% of the vacuum over what the primaries can support. In this case the carbs would see 100% of the vacuum, but each set of secondaries would provide 50%. Would this reduced air volume contribute to the richness.
If air pressure rises in a geometric curve would 50% volume yield a less than 50% vacuum operated secondary air-plate opening because of reduced speed. - Aggressor

[Excaliber]

Petek,,,,
I tend to agree that the h.p. loss due to friction vs high or low h.p. in not "linear", that it is not a "constant". But I also think the "percent" change is not as high as you suggest. 5000 rpm represents a certain "parasitic loss", wether thats from a 600 hp beast or a 225 hp stock motor. The "stress" on the gear train would remain relatively constant for a given rpm regardless of h.p. applied at that rpm.

However, I STILL tend to agree with you that there is SOME greater parasitic loss with the 600 hp vs the 225 hp running the same set up. Why? Well,,,,like you said, it's a "gut" thing. I got a "feeling" it might be true, but I can't explain it.

Ernie

[MJN]

Don't forget the drivetrain inertia effect on a dynamic dyno such as the Dynojet. This is never really accounted for in any of the previously quoted losses it's just 'part of the percentage'. You can very nicely lower your 1/4 mile ET by reducing your drivetrain mass/inertia by using a lightwieght reciprocating assembly, aluminum flywheel and driveshaft and lightweight wheels and tires. Making these changes would show up as a gain in HP on a Dynojet even though no HP was actually added to the engine. For a car with a heavy drivetrain, with say a jag rear end that has the extra U/J's and driveshafts, the mass to be accellerated is much higher than for a live axle and should show lower HP numbers on a Dynojet and slower acceleration times, all else being equal.

By the way 15:1 A/F ratio is waaaayy too lean under load. For a lean burn cruise it may be OK. You will burn up something in very short order running with that A/F ratio. 13.6:1 is probably ideal for a N/A engine. The power changes you are seeing are certainly not due entirely to the A/F ratio. Be carefull.

[Jeff Frigo]

Found on the net, looks pretty good.

REAR WHEEL HP:
Now let's discuss "Wheel" HP. Additional losses occur between the flywheel and the tire, primarily from losses in the transmission and differential. The difference between Net and Wheel power should only be around 3% (stick shift) to 8% (automatic). But because most dynos utilize rapid acceleration to determine power, the inertial requirements push this up to universally accepted numbers of 15% to 20%. Here are the details:

TRANSMISSION AND DRIVESHAFT:
We typically find a 1.5% power loss in the 'gear section' of the trans and driveshaft (i.e., a stick shift or the non-converter section of an automatic). A torque converter's losses depend on the slip ratio, but up near the power peak well over stall speed for this argument, the slip ratios usually result in a power loss of around 4% to 5% for a stock converter. If you have a lock-up converter that is locked during the test, that's a 0% loss.

DIFFERENTIAL:
Also around 1.5% power loss.

INERTIAL REQUIREMENTS:
OK, so far we've accounted for a 3% (stick) to 8% (auto) power loss from "Net" to rear wheel. The other loss (that is incorrectly labeled 'inefficiency') is the inertial power requirements of accelerating all the rotating hardware in the system during the test; crank, flywheel, converter, even the two drive wheels.. etc. If the test could be done at a zero rpm per second accel (like on a hill, or like the factories test for Net!) then there would be 0% loss here. But the whole concept of an inertia dyno is to accelerate a known inertia (the drum) and calculate what power is required to do so. The guys at DynoJet simply do not add on the extra inertia of your wheels, axles, driveshaft, trans parts or crank-related hardware because they vary on every vehicle, and they don't want to mess with it... so they get a lower number. I've done lots of work in this area and turn all of these extra inertias into "pounds" and just add it to the vehicle weight with very good results. When testing on a DynoJet, of course, the gear ratios and all come into play, but as long as the test isn't in low gear, this 'extra mass' is commonly in the 7% to 15% range. This gives us the "15 to 20%" rule mentioned by others.

BOTTOM LINE:
Unless you want to do the specific inertial math, I tend to use 17.5% for automatic cars with stock converters and 13.5% for stick shift cars or automatics that were tested with the converter locked up.

Bill Watson

[Cobra #42]

I totally agree with MJN on the fuel /air ratio, DON"T RUN that thing at 15/1 you will hole a piston or worse. We always shoot for 13/1 to make good power for the long run. Some one also mentioned that they dyno could only go to 250 RWHP ( find another shop). We dyno Honda's with over 1000+ FWHP , boy do they make some noise. But nothng sounds like a V8 above 7000 rpm.

[GeorgiaSnake]

Jeff,
I agree completly with the Bill Watson's quote for a given car on a given day, however it doesn't hold true if you take the same car and increase the hp. The power to overcome inertia and friction remains the same.

Example - A Mustang 351W with a supercharger. Drive it onto the dyno take the belt off the supercharger and run a test. You will get the rear wheel hp (lets say 200), which would indicate about 240 hp at the flywheel using the 20% rule. All of the variables are included. The power loss is 40 hp or 20%.
Now put the belt on and run another test. You will get the rear wheel hp just as before, the power to overcome inertia and friction will not change. Everything is the same except for additional hp. Assuming a 50% increase in hp with the supercharger now working you should have about 360 hp at the flywheel and 320 at the rear wheels not 288.

Just my simple answer to a complex question.

Randy