Trying to understand... the rolling road dynamometer appears to measure instantaneous drum rotational velocity and torque, then based on V and T, the drum inertia, applied load, drum acceleration, and probably some other factors calculates corrected HP. It seems to me that this method should produce a result that is independent of gear since regardless of gear the torgue-velocity product should be constant. However, I've read that certain gears are preferred for this test.

Shouldn't the same results be obtained in any gear minus driveline losses?

If corrected HP is independent of gear, then in order to reduce the effect of driveline losses, would it not be better to test in top gear?

 

Gear choice is pretty much transparent, as far as horsepower measurement goes ... BUT ... the lower gears will show less power since acceleration is faster and inertia factors are amplified ... the higher gears will show less power due to heatsoak since the complete run takes a longer amount of time. For consistency's sake, most MINI shops dyno in 4th gear.

 

If the weight of the car were known then the HP could be calculated from an open road (or drag strip) acceleration-vs-time curve. Is there a calibrated fifth wheel device available with which to make acceleration (or speed-time) measurements?

 

Like this?

http://forums.audiworld.com/vag/msgs/457.phtml

 

Fascinating. But it's not really a fifth wheel is it. I assume (please correct) that it is monitoring RPM hence wheel revolutions vs time but not distance vs time. So some calibration is required? How is this done - traverse a known distance and record number of engine revolutions?

Regards,

--
Curious

 

The logging that I did in that example used VAG-COM (which gets the diagnostic measuring block info from the VW ECU). In the MINI, road speed is calculated by the ABS system based on the average speed of the rear wheels. If you change tires to a different rolling diameter, you need to compensate for that, but my logs at the dragstrip showed an excellent correlation between elapsed time (+/- 0.1s over the 1/4 mile) and trap speed (+/- 0.1 MPH over the 1/4 mile). You could always use a fifth wheel to get the readings (either the actual wheel kind or the optical kind) or use an accelerometer (a la G-Tech). The only problem I have found with G-Tech, AP-22 and the like is their formulas for calculating trap speed are way off. I'm not sure why they do it, but I've seen traps 5 mph higher than reality, even if ET is close.