Perth-WRX.com - View Single Post

XT_for_me · #24 05-05-2012, 04:45 PM

I got a laugh from it as well. The facts are that measuring engine torque on a dyno is never going to be realistic. If you want to know why there is an easy explanation. The torque figures you are used to seeing are measured on an engine dyno where direct torque measurements are easy to achieve and calibrate. Measuring torque at the wheel is likewise easy to do, and accurate. Where things fall apart is in the translation. The figure you see is a guesstimate of the engine torque based on the wheel torque readings, however the unknown factor in the equation is the gearing in the car.

Some theory..... In a static condition or almost so the torque fed into the gearbox is multiplied by the gear ratio. Same happens at the diff. So if for example the Foz generates 320nM (as quoted by Subaru in stock tune) then at the wheel we will measure engine x gear ratio x diff ratio which is 320x0.972x4.444=1382nM

Where the dyno read out falls over is that it does not know the gear ratios to do the reverse calculation. You may also notice that I have not mentioned drive train loss. In a static situation as described by the simple calculation above there is no loss, however at anything above 0 rpm of course the losses due to friction and inertia increase with speed.

Bottom line is that torque displays that try to emulate the flywheel output are not going to be very accurate.

Just to touch on the subject of power readings, I am sure you all know that some dynos read much higher or lower than others. It really doesn't matter when the dyno is used as intended, and that is as a back to back measure when adjusting a tune.

I am surprised that people want to mock something they clearly do not understand.