Good discussion about the difference between Mustang and DynoJet Dynos here:
http://osr.albeeno.com/forum/viewtopic.php?t=588
Scroll to about halfway down the first page to skip the junk

Quote:

Mustang dyno's will always give you a LOWER number than a DynoJet. Why? Because Dynojet's were the first dyno's aimed at the tuner crowd, and to have your car read 10-15% more HP than on any other dyno is a good marketing plan.

Another reason Dynojet's aren't that great, is that they are simpler and a lot less accurate. Honestly, would you rather tune your $50,000 blown 383 with accuracy, or marketing hype ?

The Mustang Dyno uses an "Eddy Current" drive that let's you tell the dyno operator the weight of your car. It can simulatea true 1/4 mile pass. The DynoJet just spins a "known mass" - some higher-hp cars only get a 2-second pull on a dynojet!!!