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we are having a bit of debate on a different forum about rolling road dyno's and the general feeling is you don't bother working out the actual engine rpm and then the engine torque as its a waste of time far better to just deal in wheel torque.
What is the experienced tuners feeling on this? When is one or the other important?
have you watched Andre's webinar on tuning automatic transmissions and also the one on dyno setup? He gives a lot of very valid arguments as to why you need the engine RPM
Basically your ECU uses engine speed to judge how much fuel and ignition is required, if your using wheel speed what happens when you get tyre deflection or wheel spin?
That makes sense,
so if on a rolling road dyno we have the bottom scale in engine rpm and not KPH (wheel speed) is the torque that is being display as torque based on rpm not kph (If the later you just change gear and get a different torque at a given KPH)
If the dyno can give a definite RPM from either an inductive pick up or data stream such as OBDII then it will compensate for different gearing but if not then you can change your figures by changing gear. I can do this on my dynapack as it doesn't compensate, once the raito's are programmed in it uses hub speed to work out RPM so if I change gear and get to the same hub speed the torque figures will differ
If you don't use a tacho pickup or through OBD then the torque reading will be not that accurate. It is more prone to error when done with roller rpm. I know many dyno operators use roller rpm cause their hardware is not working or just plain sucks in this department and then you get excuses like it does not matter. Engine rpm is important for accurate readings.