[s-cars] Accurate Drive-Train Loss on UrS?

QSHIPQ at aol.com QSHIPQ at aol.com
Wed Apr 30 08:56:48 EDT 2003

[ Picked text/plain from multipart/alternative ]
Some of us older gear heads don't really care what's at the crank, it's what
gets to the ground that counts.  The best method to confirm "driveline" loss
is to get the RWHP, estimate, then get a quarter mile time.  With any HP
figgr you come to, it either results in a faster car or a slower car.  A guy
can get a faster car matching gears to his HP/Torque curves, than the one
bolting on parts to the motor, BTST many times.

Specific to the below, the real interest is peak HP and torque values.  As
such you really only need to figure DLL for those peaks.  In terms of S cars
on the awd dyno, the accepted figgr is between 25-30%DLL  crank to wheel.
The torsen cars actually give the least accurate HP figgrs anyhow, since the
torque shifts fore aft on the rollers .

Remember guys, ANYONE who quotes flywheel HP, unless put on an engine dyno,
is giving an extrapolated HP figgr.  Looking around to other published
numbers, rwd/fwd hit around 13-15%, awd hits around double that.  Without
doing all the math, that sounds pretty reasonable to me.

Right now the best comparo IMO for quattro, is the RWHP figgr, since that's
the most common to see.  For the turbo I5, you hit 300hp on the chassis dyno,
you are at least 400 at the crank.  401 or 425?  Who cares?  But the guy
hitting 383 at the wheels is closer to 500hp at the crank than you are with
300 at the wheels.

These discussions can be found dating back decades.  Awd or S car, sure
doesn't change the amusement gained from them.

In a message dated 4/29/2003 11:56:27 PM Central Daylight Time,
calvinlc at earthlink.net writes:

>Conversely, why should the driveline require more power to be spun when
>connected to a tuned motor, as opposed to a more pedestrian stock motor? It
>would seem to me that to spin the halfshafts at 60 or even 200 RPM would
>take the same effort, regardless of what HP the engine was capable of
>putting out. Rotational inertia does not factor in here, as HP is a steady
>state measurement, is it not?

Two points here:
1) Inertia does factor in.  Torque = I * Angular Acceleration.  Since HP =
Torque*RPM/5252 the higher the I, the more torque it robs.
2) However, I am along with you for the fact that the percentage is a rotten
way to say it.  It is a parasitic loss that is going to be dependent upon
Drivetrain Inertias, as well as RPM for the frictional losses.

I believe the reason you see things expressed in percentages is because in
general the larger the HP &Torque output the beefier the drivetrain needs
to be; example would be the clutch/flywheel on my '72 Pontiac vs. a Honda
Civic.  HUGE difference.  Now, if you keep the drivetrain components the
same, and then do stuff like better exhaust, more boost, etc. then the
losses should go down as a percentage.  Also, this will depend alot upon how
brute force oriented the manufacturer is to solving problems.  Example would
be using a larger gear in the tranny with thicker teeth as opposed to using
a higher strength material in the gear and keeping it small.


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