Dusterguy
-You probably understand the 4-stroke cycle as Intake/Compression/Power/Exhaust. What you may not understand is overlap/scavenging. George (post 110) has a good handle on this.But with bigger cams there is a 5th cycle called overlap/scavenging that is supposed to occur during the overlap period when both valves are open. That is the whole idea of having tuned headers and overlap period in the first place. This scavenge-cycle does more than just help exhausting; it actually helps to pull in a larger fuel/air charge than the engine can physically pull in by itself. So if the engine could pull in 9% more charge with scavenging, it will feel about 3% bigger than without (only about 1/3 of the energy gets to the crank).That's an easy 11 cubes and at 1.1 hp per cube that's 12 horsepower.If you prevent scavenging from working, by forcing the exhaust into positive pressure with too small an exhaust system, then scavenging cannot work.
-But it is worse than that.If you force the exhaust into positive pressure, the pressure backs up, and when the exhaust is supposed to be leaving the engine, it is possible, during overlap, for the exhaust to back up and enter the intake manifold. It can get so bad that the exhaust will try to exit the intake through the carb.Guess what that does to power.Your cam has a lot of overlap, making this phenomenon a very real possibility.If the exhaust ends up in the intake, it reduces the pressure differential across the venturies, which means a much smaller A/F charge can enter.
-So now your engine is under a tripple whammy. Loss of scavenging, loss of induction, and intake charge dilution.This can add up to a lot of power.