Wheel HP vs Crank HP

Chris Lesperance

Chris Lesperance

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I'm not sure if I am missing something. Is there a way to see the wheel horsepower of a car in the physics spreadsheet or in Mod Dev? I believe there are frictions that are set within the "Gear" and "Differential" tabs on the physics spreadsheet but I'm not sure what amount that is. My assumption is that the engine.ini is horsepower at the crank.

So a bit of background, I'm working on a car in which one owner quoted the engine at 325hp. Another owner quoted 305hp. The person quoting the 325hp, I know has access to an engine dyno. Other person, I'm pretty sure did a wheel dyno with his car. In game, I have the engine built to 325hp. I just recently discovered the GeneralTorqueMult and GeneralPowerMult in the HDV. Note states: "You may want to incorporate some of the driveline friction into the engine directly. This may be better because friction often increases with increasing input forces." I can math the percentage to make that 325 hp to 305hp, but I just want to make sure that the friction from the diff and gearbox do not drop that HP any lower. Seeing the final wheel HP would be helpful.

One other side question, would I be accurate in assuming that the proper way of adding an air restrictor to an engine, that would be controlled by the FuelAirMixtureTable within the engine.ini? The 325hp engine runs with a 32mm restrictor in certain events. That configuration is about 300hp at the crank. At the moment, I just run a different torque curve. But if it is the FuelAirMextureTable is the proper way, I may look into doing that.
 
I think you can check curves from the wheels through CarStat app. If it opens a car. I don't know how legit is that. But whe nyou think about it, should be nothing but just reduced amount by percentage, although more realisticalyl it would probably have slight more losses on the higher end of engine load ?

To me these nuances also gives plenty of guessing. Good thing is that there are somewhat known top speeds usually, laptimes. The drag is usually either known or is not impossible to interpolate reasonably well by comparing to similar cars, taking frontal area into account, car size.

For modern historics it is tricky, because power levels went up. Probably not unusual is that engine that used to have particular power at the crank, now has it at the wheels.

And I have just read (again) an interesting article about drivetrain losses. There is no generalisation for power losses. Due to different circumstances it could range from 2% to 20%, and maybe even more. 4wd having most losses. FWD with front engine, or RWD with rear engine having less losses.

https://www.motortrend.com/how-to/modp-1005-drivetrain-power-loss/#google_vignette

I don't know about air restriction simulation.
 
Manta, just read that article, in the end i think the only thing i can take from it is that there is no simple calculation that is applicable to different cars. It would have been so much easier if there was a nice formula for me to try and work out.
I suppose in the end i will just have to live with what we have and as the lack of a formula applies to all cars, perhaps the status quo will remain.
 
Without tested data, we're always guessing, but they're educated guesses.

@Chris Lesperance has some power numbers and presumably weight of the cars plus maybe some acceleration figures, but that still leaves tire drag, tire grip during acceleration, aerodynamic drag, and drivetrain losses as variables. A couple of coastdown tests on the street will narrow down aerodynamic drag & tire drag. Driver skill and course conditions when checking lap times (2-3% just from good to great drivers, but maybe 5% for the aliens).
 
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Absolutely, but this what makes it all more interesting to me. If you can have lots of data, that will narrow guesswork, you still will have some spaces left for guesses. And with more variables know, plus some logic and comparisons and interpolations we can narrow down guessing windows to rather good educated guess.

These cars, even modern ones won't be all exactly the same anyway. Even same car probably won't always be the same as it was. Some differences of conditions, different tires, differences in lubrication, maybe something will be starting to wear down and percentage or two might get lost.

At the end the car just have to be doing things it is doing IRL.
 
Thanks for the info. Good to know we're all on the same page and I'm not missing anything.

@Emery , as you have mentioned about the other variables, the aero drag on the physics spreadsheet isn't accurate to what is within the game. I discovered it with Raintyre and he posted about it. I have some variables to play around with but I know we're getting close.
 
Just wanted to follow this up. After being confident that I had my aero and drag numbers correct. I tested my wheel HP engine vs my crank HP engine. I confirmed that the crank HP engine matched the performance of the real world data.

Just putting that out there if anybody was questioning.
 
Emery said:
Without tested data, we're always guessing, but they're educated guesses.

@Chris Lesperance has some power numbers and presumably weight of the cars plus maybe some acceleration figures, but that still leaves tire drag, tire grip during acceleration, aerodynamic drag, and drivetrain losses as variables. A couple of coastdown tests on the street will narrow down aerodynamic drag & tire drag. Driver skill and course conditions when checking lap times (2-3% just from good to great drivers, but maybe 5% for the aliens).
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My only exposure to Dyno charts was a leaked Porsche Dyno Pull back when Sportscars in America split. The Nascar backed Grand Am limited HP more than the rival ALMS(Don Panoz's series) The Porsche was limited to a certain number, but the chart, which was done at a dyno facility with only the engine tested, not the entire drive train, revealed the Porsche exceeded the limits through-out the powerband. My only assumption was the organizers only tested at the rear wheel with a rolling chassis dyno. If my memory serves(and it usually does not these days) the increase was around 8 to 12% depending upon the rpm.
 
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