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Bhp Formula


DUNCAN
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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.

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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.
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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.

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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.

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