Here is some info I found on the interwebz
Mustang dyno is a much better tuning tool. they set the resistance to your cars weight. that allows the engine to perform under the same load that it would see on the road. A dyno jet provides a set amount of resistance that has no relationship to what the actual engine load on the street would be. I believe (and could be wrong) that the resistance on the Dyno Jet is equal to a car weighing 2100#
Use a Dyno Jet for Dyno Queens The mustang is the real tuners tool.
Mustang measures torque and computes horsepower as HP = torque * RPM / 5252 while the Dynojet measures the acceleration (difference in RPM) of the drum and computes horsepower from that with a much more complex formula.
The Dynojet will have an increasing error as the horsepower goes up. For instance, you dyno stock and get 310 HP, then do heads, cam, boltons, etc, and dyno 450 HP. That number should be higher because the greater amout of power accelerates the drum more rapidly, throwing off their power calcualtion. Even as a tuning tool, the numbers should be taken with a grain of salt. The Mustang does not suffer from this phenomenon.
In theory, the Mustang should be closer to "true" horsepower.
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