Real HP loss numbers

[Gary Derian]

Inertial effects in a drive train are considerable and vary greatly with
gear.  The effective polar moments of inertia increase with the square of
the gear.  Axles have little effect.  Driveshafts have more.  Engines and
flywheels have the most.  They have both high inertia and spin faster than
anything else (in the lower gears).  I ran some calculations once on
flywheel inertia.  A 30 typical lb. flywheel effectively adds about 1000 lbs
of mass to a car in first gear.  Of course if you are traction limited in
first gear, it doesn't matter.

An inertia dyno can take all of these inertia factors into account and give
the same numbers as a brake dyno.  If you are dyno testing to generate
numbers, then go through all the inertia calcs.  If you are dyno testing to
optimize fuel and timing maps, it really doesn't matter what the number are,
as long as they are repeatable.

Gary Derian <gderian at oh.verio.com>

>>I have wondered about this myself.  I have heard anywhere from 15%
>>to 25% in drivetrain losses.  What I can't figure out is where the
>>energy is going.  If you have a 300HP engine with 25% drivetrain
>>loss, then you are losing 75HP somewhere.  Since it doesn't just
>>disappear, something has to be soaking up 75HP of energy.  My
>>guess is that the loss would be in the form of heat which would
>>mean of lot of drivetrain parts would have to be getting really hot
>>(assuming the 300HP load on the engine).  Since I have only seen
>>my transmission get warm, it doesn't really make much sense unless
>>I just haven't had the load on for a long enough period.