i posted this befor and i think it will help some people understand that their mods might not give them as much as they think.
"well, 15% is deffo about right for a FWD car and 17% for a RWD cause i have been talking to some people. In real terms, you not even getting the real power of the engine at the wheels. Even on an engine dyno, its at the flywheel, not the crank, if the flywheel was off, the engine would prioduce more power.
Power is the rate at which work is done, and the more power an engine has, the faster it can do work in any given time. By definition, one HP is 33,000 ft pounds per minute (550ft/lbs per sec). No man or machine know what the actual power is since HP is just a marketing term designed by James Watt because he, needed to be able to rate the power output of his steam engines in order to advertise them.
Anyway, when you put your car on a dyno, the dyno measures the torque of the engine with a brake (hence Brake HP) where once the engine can stay at a constant RPM at full throttle, the braking force applied by the dyno is its ft/lbs.
The formula to convert torque into power is:
HP = torque x 2 Pi x rpm/33000 or simplified into:
HP = torque x rpm/ 5252
You divide it by 5252 because the is the point where all engines Bhp and torque curves will cross (assuming they are on the same axis) on the dyno graph. If yours doesnt, then the dyno you went on doesnt work.
And if you know the BHP, then you can rearrange the equation to caculate torque:
torque = hp x 5252/rpm
back to the dyno, when you run your car on a dyno, the computer measures the amount of torque at the rollers (not the wheels) and so can work out the BHP at the rollers, by using the speed of the rollers (rpm). If the engine speed is measured at the same time, then we know roller BHP at any given engine rpm. Another problem is the slipping of the tyres and the rolling resistance of the tyres as they are placed between to huge cylinders to simulate the road. However, this pinches the tyre and squeeses it, sapping power.
So is there any way of really measuring the true transmission loss of a car? Yes - only one - by measuring the flywheel power on an accurate engine dyno, the wheel power on an accurate chassis dyno and taking one away from the other. There is no way on Gods green earth of finding out the true transmission loss just by measuring the power at the wheels.
The measured wheel bhp can change depending on tyre pressure, tyre size, suspension angles and other things which wont affect flywheel power - so the actual transmission loss % will also change. It pays to try and standardize as many of these things as possible if you intend to do a series of power runs over a period of time. Always use the same tyre pressure because this is a factor which can easily change from day to day.
VW technical also quote their cars as losing, on average, about 15% of the flywheel power in the transmission and tyres.
The chassis dyno division of Bosch UK also suggest 15% as being a realistic estimate of transmission losses"
