It completely depends on the dyno, I could go on a dynapak right now and make 350rwhp easy. They read so high. Dynojets read a bit high too, I'm probably at like 330whp on a dynojet. The dyno I used (Mustangdyne) reads more accurately when you consider the whp/weight vs. performance figures (performance seems to match what you would expect 310whp/3300lbs to do). If I were to dyno on a dynodynamics I would probably put down 300 or less because those read low.
So you have to know which dyno you are talking about. This is my rule of thumb:
Dynapak: +15%
Dynojet: +10%
Mustang: 0 (base)
Land&Sea: 0
DynoDynamics: -5%
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