That's a spread of 56 wheel horsepower for the same car with the same mods, significant. Just changing the correction on the Dynojet to SAE changes the numbers to 378 horsepower to the wheel. 12 horses less than in STD correction on the same dynojet and 44 horsepower more than on the Mustang dyno. The next time one is comparing dyno results remember that the numbers can very based on the dyno, conditions, and correction and the same car and show very different numbers due to this.
Additionally, the Mustang Dyno does not always read low. This is commonly cited but AMS showed recently how Mustang numbers can be manipulated rather easily. Dyno numbers are just part of a picture but more often than not used for bragging rights. One thing is certain, it's easier to skew dyno numbers than a timeslip.
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