The really only good way is to have the car dynoed on a chassis dynomometer. I would suggest going to the DynoJet web site for a list of facilities that have those in your area. There is some disagreement here at MW as to whether the Mustang Dyno or DynoJet is best for this.
After you get what you believe is the correct rear wheel horsepower, you must try to convert that to what the engine produces at the flywheel under certain specified conditions.
The DynoJet uses SAE (Society of Automotive Engineers) conditions and does convert to this, I'm not sure about the Mustang Dyno. To convert from rear wheel to flywheel, I use a driveline efficiency of 83% for a standard transmission and 75% for an automatic. So divide the chassis dyno figures by either .83 0r .75 to get your estimated flywheel hosepower and torque figures.
Standard dyno conditions (used by engine dynos like SuperFlow) are about 4.5% higher if the same accessories and exhaust system are used, due to conversion to more favorable atomospheric conditions. These are what most magazine stories are reporting.
Hope this helps a little.
Rev
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'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.
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