| it is most possible that your engine doesn,t want to rev any higher as it is peaking to its max rpms, to get more rpms from your engine the duration of the exhaust port must be advanced, say if your engine that has had work done to it, has its exhaust duration set at lets say 170 deg, to get more rpms you must open up this duration to atleast 176 deg, also your pipe must work with this setting, maybe your pipe is coming on to early as well. | 
