Assuming a fixed percentage parasitic losses to derive the engine output has led to numerous people deducing the wrong engine output. The parasitic loss is a fixed torque loss through the tyranny ie you have to calculate this loss and add it to the wheel torque to ascertain what your engine output is. To elucidate what I am saying, if you measure torque at the wheel to be 100 ft.lbs: using a fixed percentage loss of 20% will give you engine output of 125 ft.lbs at that rpm applying a fixed torque loss of 20ft.lbs will give you an engine output of 120 ft.lbs at that rpm. Let us now tune that engine such that we now have 150ft.lbs of torque at the wheel, then applying a fixed percentage of 20% parasitic losses will falsely give us an engine output of 188 ft.lbs at that rpm. applying a fixed torque loss of 20 ft.lbs will give us an engine output of 170ft.lbs of torque at that rpm. Torque loss is a function of mass,rpm,diameter,friction. The question then is how do we know what the torque loss is since dynometers measure the torque at the wheel? Well, you take what the torque from the manufacturer's torque-curve and subtract your dyno torque from it reading off the rpm abscissa. The difference is your total torque loss through the system.