Bhp Formula

DUNCAN · April 13, 2004

Does anyone know the formula to convert brake horse power at the wheels into bhp at the flywheel ?

By the way, while it was on the dyno the exhaust emmisions would have sailed through an MOT with the pre-cats removed.

Maz · April 13, 2004

so how much you got atm Duncan? from the rolling road i mean.... :) :) :)

aido · April 13, 2004

Is this any good Duncan:

http://www.ktrperformance.com/services/dyno_faq.htm

If you need to know the flywheel bhp then you will have to estimate it - there's no other way short of using an engine dyno. The corrections you need to make for cars with manual gearboxes are these:
The average rear wheel drive road car with between 100 and 200 bhp loses about 17% of the engine bhp as transmission losses. We find Porsche transmission losses are 12% to 18%, depending upon specific model and the age of the vehicle. We use 15% as a rule, and find the estimates of flywheel power compared to factory figures very reliable.

No car in the world loses anything like 30% of the engine's power in the transmission and tires as many rolling road operators would try to have you believe. So take the wheel figure and divide by 0.85 for RWD and that will get you as close to the true engine bhp as you are ever going to know. A rule of thumb we use which is quite accurate is to treat the losses as being 12% of the flywheel power plus 10 bhp for RWD cars. This equation "loads" low powered cars more than high powered cars which is more closely like what happens in reality. Remember, these percentages are just good realistic averages. The measured wheel bhp can change depending on tire pressure, tire size, suspension angles and other things which won't affect flywheel power - so the actual transmission loss % will also change.

DUNCAN · April 15, 2004

Is this any good Duncan:
http://www.ktrperformance.com/services/dyno_faq.htm

If you need to know the flywheel bhp then you will have to estimate it - there's no other way short of using an engine dyno. The corrections you need to make for cars with manual gearboxes are these:
The average rear wheel drive road car with between 100 and 200 bhp loses about 17% of the engine bhp as transmission losses. We find Porsche transmission losses are 12% to 18%, depending upon specific model and the age of the vehicle. We use 15% as a rule, and find the estimates of flywheel power compared to factory figures very reliable.

No car in the world loses anything like 30% of the engine's power in the transmission and tires as many rolling road operators would try to have you believe. So take the wheel figure and divide by 0.85 for RWD and that will get you as close to the true engine bhp as you are ever going to know. A rule of thumb we use which is quite accurate is to treat the losses as being 12% of the flywheel power plus 10 bhp for RWD cars. This equation "loads" low powered cars more than high powered cars which is more closely like what happens in reality. Remember, these percentages are just good realistic averages. The measured wheel bhp can change depending on tire pressure, tire size, suspension angles and other things which won't affect flywheel power - so the actual transmission loss % will also change.

Thanks for the info mate.

ColinBarber · April 15, 2004

An average would be 15% loss for a manual transmission and 20% loss for an auto. It's easier to guess with a manual, on a dyno you can drop it out of gear and the dyno will try and calculate the drivetrain losses.