Adjusting for crank horsepower

I am confused on how to calculate for modified crank hp using the stock hp ratings. Using the WRX as an example, they come stock at the flywheel at 227 bhp, but they are dynoed to be right at 171 whp. Now, let's say that this WRX made 100 more hp to the wheels. How do you calculate the hp to the flywheel now? Is it measured by a percentage of the drivetrain loss?

(227/171=~75%) --> (1.25 X 271=338hp)

Or is it measured by adding the 56hp that was lost in the drivetrain at the beginning?

(171+100=271) --> (271+56=327hp)

You get two different, but logical, answers. But which one is the correct number?

 

Neither. Most likely the drivetrain loss is ore accurately represented by a curve. It's going to be near impossible to derive crank numbers from wheel numbers and vice-versa on an AWD car. I don't even think SOA has a way to figure that out.

 

RN is right, this is pretty complicated, and nobody deals with it the same way.

People who want to brag simply assume a 25% driveline loss, so a 300WHP must make 375BHP. Which it doesn't, but that doesn't stop people from claiming it.

If you want to be safe, you simply add back the same drop you see on a stock car, so a 300WHP car would be assumed to make 356BHP. Again, it's probably not right, but then at least it's not dishonestly large.

Something closer to reality would be to assume that a certain portion of the loss is a fixed value, and the rest is a percentage. So if we assume that the first 20% of the driveline loss is fixed (about 45HP), and that the other 5% is proportional to wheel output, then we would predict a 300WHP car to have 300 + 45 + (300*.05) or 360BHP.

In this case, the 360BHP figure is probably the closest to true, but in fact, we have no way of knowing for sure (short of pulling the engine and putting in on an engine dyno).

Now, that 5% is a number I just pulled out of my ass. Probably not unrealistic, but not exact either.

 

And in the end, who gives a ****? It's what you put to the road that matters anyhow.

(that's one restrictive language filter there. I mean if I'd said **** or **** or *** I could see it, but ****?)

 

Yeah, if someone is giving you crap because your dyno numbers say 170whp in an AWD car whereas his 2WD car makes 200whp then he's a dolt and doesn't understand how different drivetrains work. Heck I usually tell people I only make 165bhp because that's what the factory specs are. It's just a number after all

 

In the end, there's no final answer. Well, I'd like to go around bragging (online is fine ), but I'd rather let them racers find out the hard way. } Thanks for your input guys!!

 

Well, you COULD get an actual answer if you removed the powerplant from the car and hooked it up to an engine dynamometer. But that would cost an awful lot of money to get a number that in the end, doesn't mean all that much.