Dave Searle, editor of Motorcycle Consumer News used a great amount of background information from Factory Pro Tuning to produce this article, laying out, what I think is the most accurate description of how dynojet horsepower numbers got inflated from the true horsepower numbers.
Dave was responsible for the additional pointed out fact that the VMax that Cycle tested for their Superbike Shootout wasn't a stock 1200cc. Indeed, according to Searle's other sources, it was a 1400cc "sleeper" bike. The idea that it "might have been" a "sleeper" was the result of Marc Salvisberg, during the many hours of telephone interviewing with Dave, happening to walk into his attic and there, on top of a pile of old mags, was the Cycle mag 1985 Superbike Shootout that indicated that the VMax was significantly stronger than the then current batch of big bikes (which a stock VMax wasn't).
Dave makes a comment that djhp was only 5% to 10% higher than
the true horsepower.
That's not correct. That "10%" might have come from one of the other dyno companies and that's what they might tell their dyno owners. There are one or two magazines who don't use dynojet dynos - and that might be the factor that they use to bump up the true number to something that is "+/- 10%" of a dynojet hp number (according to the former owner that company at PPIR).
The real inflation factor ranges from 5% to 30%. depending on which dynojet one is comparing the true power to and that fact that the inflation factor increases % as the true value increases (example: 12% at 80 hp true and about 35% at 275 hp true)
An idea of a "measurement scale" is so that data can be compared, regardless of brand of test equipment.
The result of the fact that magazines voluntarily use a secret inflated dynojet hp scale makes it so that only a dynojet dyno can provide "djhp". (and ONLY if it's calibrated correctly)
Any other dyno company that attempts to make a dynojet horsepower reading is only guessing with some kind of guessing accuracy and that's a disservice to the dynamometer industry, tuners and the riding public.
Motorcycle Consumer News \ Motorcycle Consumer News, Dave Searle, Measures, Dynojet Oct,2009.pdf