Malibu Vette

iTrader: (1)

I have been thinking about the losses on rear wheel dyno's. I dynoed 335 with my stock 01 Z06. That is 13% losses and 50 hp. My friens made 355 rwhp on his stock 02 Z06. That is just under 13% losses and still 50 hp. Now with mods I am making 420 rwhp. If I use the 13% rule My losses have increased to 62 hp. (420/.87=483.75) and my friend has increased to 490 rwhp using the same math his losses are 73hp. My question is why would it take more horse power to run the same drive train jujst because the engine makes more power. My therory is that the rwhp losses are fixed to the drive train and the & is only applicable to the stock engine and as the engine horsepower increases the losses stay constant so the percentage of losses gets lower. I am open to other ideas, not that this is that important, just something I was thinking about.

P Mack

iTrader: (6)

If you are on a dynojet, the more power you have, the faster you accelerate the drum. The faster the drum accelerates, the faster your drivetrain has to accelerate too. It takes more power to accelerate your drivetrain faster, and that is less power that actually makes it to the rollers.

That is just the inertia part. There is also a friction part, so it's not a set percentage or a set rwhp, but somewhere in between.

dynocar

It is not a fixed loss, besides enertia losses, anytime we increase the pressure and/or velocity the friction and generated heat increases which increase or losses. If it were not for this we would simply do a drivetrain coastdown before a dyno run, compute the loss and add this amount to the measured power. But when we accelerate and load (bind up) the drive train, those numbers increase. When we compare RWHP to the manufacturer's advertised HP, there is another huge factor that we seldom discuss which is called proceedural losses. On chassis dynos we normally measure HP using the RPM sweep method but the manufactures use the RPM step method. Anyone who has ever operated an engine dyno and switched back and forth between the two proceedures see big differences, often up to 10% or more depending on RPM sweep rates. But for rules of thumb the 15% for Dynojets and 25% for other brands seem to work well enough for manual transmissioned cars.

EJ