One of THOSE discussions.... Driveline loss-Page 2: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Oct. 13, 2011 1:00 p.m. ransom HalfDork

Keith wrote:

Ah, here's that dyno run. Same car, same dyno - the only difference is that one run went through the sweep in 15s while the other took 25s.

http://www.flyinmiata.com/tech/dyno_runs/NC_sweep_times.pdf

One nice thing about our dyno is that we can control the rate of acceleration, which does keep inertial losses as a constant.

It's true, and it's funny. It seems to me that to find out how much power you're really making, you'd want the sweep to be really slow, or even "stepped" with plateaus at whatever resolution you were concerned about.

Inertial effects are a bit like having the weight of your car affect your horsepower values. It has a bigger impact than chassis weight, but it's the same idea; it tells how how much power is leftover to accelerate the car after the driveline's been accelerated, but having a horsepower number that can fluctuate with a set of wheels just seems screwy (though that's the way it's done pretty much, right?)
Oct. 13, 2011 1:02 p.m. Brett_Murphy HalfDork

In reply to ransom:

That is a fantastic picture.
Oct. 13, 2011 1:04 p.m. ransom HalfDork

In reply to Brett_Murphy:

Credit where it's due (belatedly). It came from http://www.camnetics.com/
Oct. 13, 2011 1:11 p.m. corytate HalfDork

we did coastdowns to measure rough drivetrain loss on the dyno
Oct. 13, 2011 1:14 p.m. rotard Reader

Why don't you go drag race it? Real world numbers>all
Oct. 13, 2011 1:18 p.m. 92CelicaHalfTrac SuperDork

rotard wrote:

Why don't you go drag race it? Real world numbers>all

I agree with you... it's not really my car that originally sparked this.
Oct. 13, 2011 1:28 p.m. alfadriver SuperDork

Does it help that if a dyno is absorbing 800hp vs 200hp under the exact same conditions that the forces going through the gears are rougly 4x greater, in ideal conditions?

Under the same 200hp conditions, then the friction will be the same, given the same parts. But with the mythical turbo, then the driveline passes 4x more force at 800hp.

Does that help?

Ransom's description is really, really good, but it's almost as if you may be missing the small part that 800 = 4x200. And since power = (force)x(speed) = (torque)x(rotation speed), given the same speed- torque/force has to be 4x higher, and since friction = (miew correction)*normal force, and normal force =4x higher, then friction is then 4x higher.

Otherwise, the reality is that it's not all that impiortant for anyone other than advertisers- drivers feel wheel power, and vehicle economy has all frictions incouded at the wheels. So you only care about what's happening at the wheels.