The most often heard discussion is that what factor can be applied to rear wheel horsepower to reflect crankshaft horsepower. This is where we need to understand how the rear wheel horsepower number was derived.
AZSPEED’s Mustang (Loading Dynamometer) applies resistance to the dyne's roller(s), using an eddy current brake. The amount of force is measure using a strain gauge. The measured force is torque which is a real, indisputable measurement of the actual output at the wheel. Horsepower than can be calculated: Hp = Trq * 5252 / RPM.
A Dynamometer can only measure actual power at the output location. Actual power produced and delivered by an engine will be highest if measured at the crankshaft, lower at the transmission output shaft and even lower, but more meaningful, still, at the rear wheels. The power that you use is the power at the rear wheels.
Proper tuning, especially on highly modified engines greatly affect the power difference. The most accurate measurement of rear wheel horsepower is in Steady State Mode (inertia is not a factor in power equation.)
AZSPEED’s Mustang dyno is capable of measuring power in steady state mode and producing the same numbers - they all measure torque. Torque x rpm / 5252 = horsepower. The only factor that is added to the measured reading, in true, rear wheel horsepower, is the additional energy (dyne parasitic) required to spin the dyno(s) rollers to whatever speed the rollers is turning at - logical, proper and required for any measuring instrument, torque x rpm / 5252 = horsepower + parasitic power = true, rear wheel horsepower.
As far as atmospheric conditions we measure dry air temperature, barometric pressure, and density altitude to enter into the calculations on baseline and final test after we have verified the correct air fuel ratio before, and after tuning.
Steady State test Mode is the most consistently superior method of tuning.
Crankshaft horsepower vs. true Rear Wheel horsepower. As each vehicle is different, the best way is to dyno the engine and then dyno the vehicle to see exactly what the loss is. The best estimate we can give you based on experience, take the rear wheel horsepower and divide by 0.7 to arrive at crankshaft horsepower. However, some items that can have an effect on crankshaft vs. rear wheel horsepower are, tire slippage, clutch slippage, drag from transmission, axles, U-joints, and CV joints.
AZSPEED has both engine and chassis dyno so we are equipped to determine the parasitic losses if needed.
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