Chassis dyno losses - please critique my thinking.

[Chargermal]

I just want to make sure my thinking is correct on this.

I’m aware that various makes and types of chassis dynos produce different results, and that different external conditions as well as differing drivetrains, tire pressures and size, gearing etc can also have an effect.


BUT.....Lets take to exactly similar cars ,drivetrain, dyno, tires, operator, weather etc.

The one difference is one has a 350 crank HP small block, the other has a 500hp Big block.

BOTH make peak power at 5500RPM

Question - Am I correct in thinking the parasitic losses through drivetrain will be the same for both cars at 5500 RPM - as a numerical figure - as the RPM at peak power is the same for both ?

(I’m aware that the % losses will be greater for the smaller motor as its making less total HP)

Or have I missed something?

[travis]

At steady speed the parasitic losses should be almost identical. Parasitic losses should also be the same if both are accelerating at the same controlled rate. In a vehicle running down the road at WOT however, the higher Hp engine will have more parasitic losses because it will accelerate all the drivetrain parts harder…which takes more power to accomplish.

[Chargermal]

Thank you...that makes perfect sense.

[rebelrouser]

What little I messed with a chassis dyno, the rollers are the main issue, how you load the vehicle and the rate at which you load it really affects slippage on the dyno. At the college where I taught we had a good Mustang chassis dyno, so many times I had different students running the same pulls on the same car. On lower HP vehicles, results were much more repeatable. On higher HP vehicles, the results many times were all over the place. So the operator and how the pull is done affect the results a lot, just from my limited experience. And the main thing I noticed was the amount of rubber dust on the dyno after a pull. If I was to buy and trust a chassis dyno, I would have to buy a hub dyno, where you bolt the axles to the dyno and not have any slippage on the rollers.
And for some real fun, for open house the college several times did a dyno day, for the public, we had 200hp street cars, to 1,000hp drag cars, it was a lot of fun, and the students got a lot of real world experience with the public.

[Circlotron]

The one difference is one has a 350 crank HP small block, the other has a 500hp Big block.

BOTH make peak power at 5500RPM

Question - Am I correct in thinking the parasitic losses through drivetrain will be the same for both cars at 5500 RPM - as a numerical figure - as the RPM at peak power is the same for both ?

Say your hypoid diff at the above conditions has an efficiency of 90%. It's going to lose as heat 35hp and 50hp worth respectively. So the parasitic losses will be different. And if the torque converter is identical in both cases then the 43% greater torque of the larger motor it is probably going to make it have quite a bit more slip. If it is operating entirely as a fluid coupling at that rpm then maybe something approaching 43% more.

So as a percentage, the losses might be kinda constant, but as an absolute lost hp figure, no.

[vannik]

To be clear, what is meant by chassis dyno:

• Hub dyno, or
  Rolling road dyno?

They have different losses, and on the rolling road dyno the way power is calculated also differs from make to make.

[Rick!]

Drivetrain parasitics are as Circlotron stated. You can figure it out using the loss at each element or just use a rule of thumb.
15% of crank power is the typical drivetrain loss if using hard tires on Clayton rollers.
Using the big inertial roller dynos is a bit different and I don't have much experience with them except for doing some testing on an older diesel pickup several years ago.
Engine parasitics, like cooling fans and alternators, would be constant between the two engines, assuming they run the same RPM.
YMMV.

[David Redszus]

The one difference is one has a 350 crank HP small block, the other has a 500hp Big block.

BOTH make peak power at 5500RPM

Do they both have the same bore and stroke?

[F-BIRD'88]

Don't try to convert power on a chassis dyno to that on a engine dyno or another type of chassis dyno.
There are just too many variables.
Test are only comparable same engine on same dyno
to see +/- power changes due to tuning and parts swapping. It is what it is and thats what the testing is good for.

If you want to know engine dyno tested power, put the engine on a engine dyno.. Often a heart breaker

[houser45]

I have a pretty good amount of testing on a chassis dyno with sprint cars. My question has always been if you took two engines that you made as equal as possible, cylinder heads, compression ratio‘s, cubic inches, one of them had a 50 pound crankshaft with an 1800 g Bobweight. The other a 39 pound crankshaft with a 1730 g Bobweight. Will the engine with the lighter rotating assembly show up as more horsepower on the Dyno? I know acceleration times are also tested besides just horsepower and torque. I’m sure the lighter crankshaft engine should be able to accelerate the drum faster

[BCjohnny]

By 'chassis dyno' I guess you're referencing 'rolling road', aka tyre hysteresis, slippage, drum size, co-efficient of friction of same, tyre pressure, tie down ..... how many people are sitting in the boot, blah blah

As a scientific tool, apart from the vehicle you have on it at a particular time doing back to back tests, comparative results are almost meaningless

I sold my last 'rolling road' two years ago ...... I'm building a hub dyno

[77cruiser]

If you really want a heart breaker run it at the track & use one of the calculators to see how much power you have at the time.

http://users.erols.com/srweiss/calcmhp.htm

[bob460]

If you really want a heart breaker run it at the track & use one of the calculators to see how much power you have at the time.

http://users.erols.com/srweiss/calcmhp.htm

That's a good guide/calculator right there.

[Chargermal]

This answers my question in part -

“ Engine parasitics, like cooling fans and alternators, would be constant between the two engines, assuming they run the same RPM.”

But what about trans and diff losses?

I can see the same diff losing the same power at the same RPM....but do converters....or is engine torque a determining factor?

I wanted to ascertain if drivetrain losses are constant even if one motor makes more torque than another at the same RPM?

[digger]

your hypothesis is incorrect, the higher power at same rpm the higher losses in absolute terms but its not the same % either

as someone said above you will never get anywhere thinking about it so don't bother

focus on getting repeatability and proportionality in results through setup procedure and forget about actual losses %/absolute