2014 Ultimate 1 Ton Challenge results

4x4ord wrote:
I have thought a bit about why a chassis dyno might read higher horsepower numbers in higher gears and have come up with what I think is a logical explanation: When power is transfered from the vehical tires to the chasis dyno's rollers, the torque exerted on the tiny contact patch distorts the tire. This distortion of the tire sidewall cosumes power. Running the truck in a higher gear would reduce the torque and therefore the distortion and power loss at the tire/roller contact point. The inertia type chasis dyno numbers are probably more valuable to dragsters interested in a 15 second run down the track. The conditions associated with a truck pulling a loaded trailer up a hill might be hard to duplicate on an inertia based chasis dyno.

You are thinking about it in too much depth, it's much simpler. The longer the engine has to load the more power it will make.

There is also a big difference in torque, and sometime hp, based on how the run is started. 1400 rpms with no boost will make a lot less torque and possibly hp than 1400 rpm with 10 psi at the start of the run.

A 5th gear run from 1400-3200 has 46 mph to complete the run in, but 6th has 64. That extra speed of the roller means the engine gets closer to full load.

Correct, conditions on a hill are very difficult to duplicate on a chassis dyno. I make 30-32psi on no grade, and only 28ish on the dyno. That's load.