I have had motors on the engine dyno start to nose over around 5000 RPM and then start to gain power again above 5300 RPM, more than once if you test it some more on the engine dyno try taking it 500 RPM above peak HP rpm to see if it is done or not (unless it is starting popping and backfiring through the intake )
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
(unless it is starting popping and backfiring through the intake 
)
