Doc, I think you nailed it. There can't be any argument that whatever the driveline loss is, at a fixed speed that loss is fixed; whether you have a 500 HP motor or a hampster. We try to talk HP on chassis dynos, but as I understand it, they measure torque (on the drum or whatever the drag device may be) and CALCULATE HP, based on time/i.e accel. of the drag device. Obviously, the greater the acceleration, the greater the power required (power loss) to accelerate the device and rotational items like wheels (hence the 'bigger tire/wheel combo = lower dyno HP reading' theory), but I maintain you're not measuring HP, you're measuring the acceleration rate. The reduction from your engine dyno HP is a combination of actual drive train loss (fixed) PLUS the HP required to accelerate that drive train (which will vary with the 'rate' of acceleration). You wouldn't get a real HP reading until the RPM stabilized/0 acceleration, which 'never hoppens'. You quit the pull.
Al Wohlstrom's dissertation above seems to say the same thing.
I would suggest that the "HP" reading off a chassis dyno isn't really a measure of HP but of the 'accelerative' force available in one's engine/drivetrain combo. It provides a good index of comparison in how fast your car will accelerate. It is directly related to HP, but it is NOT steady state HP. This would support the logic that the bigger the motor, the bigger the 'apparent loss' due to it's greater acceleration.
To get an actual HP reading, I believe you would have to run it up to near the Rev. limit, increase the drag until the RPM starts to drop, then read the highest steady state torque/power figure you get.
RE. increased heat losses due to increased friction from higher powered engines, I believe those would be relatively insignificant.
In a nutshell, for bragging rights, I'd lean towards engine dyno figures; for racing discussions, chassis dyno numbers would be more relevant.
You can get an accurate estimate of quarter mile performance from RWHP (it's all acceleration), but I bet you couldn't calculate the top speed of a car (steady state)from it's chassis dyno HP figure. Any Bonneville types out there have any chassis dyno numbers vs. salt flat speed?
That's my 2 cents worth.
Merry Christmas to all.
Mike
Al Wohlstrom's dissertation above seems to say the same thing.
I would suggest that the "HP" reading off a chassis dyno isn't really a measure of HP but of the 'accelerative' force available in one's engine/drivetrain combo. It provides a good index of comparison in how fast your car will accelerate. It is directly related to HP, but it is NOT steady state HP. This would support the logic that the bigger the motor, the bigger the 'apparent loss' due to it's greater acceleration.
To get an actual HP reading, I believe you would have to run it up to near the Rev. limit, increase the drag until the RPM starts to drop, then read the highest steady state torque/power figure you get.
RE. increased heat losses due to increased friction from higher powered engines, I believe those would be relatively insignificant.
In a nutshell, for bragging rights, I'd lean towards engine dyno figures; for racing discussions, chassis dyno numbers would be more relevant.
You can get an accurate estimate of quarter mile performance from RWHP (it's all acceleration), but I bet you couldn't calculate the top speed of a car (steady state)from it's chassis dyno HP figure. Any Bonneville types out there have any chassis dyno numbers vs. salt flat speed?
That's my 2 cents worth.
Merry Christmas to all.
Mike