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Message: <blockquote data-quote="96blak54" data-source="post: 1404587" data-attributes="member: 14727">If you think about it.....Its not overkill when you consider stock npi performance compared to upgraded npi performance. And lets not forget....for both valves in that tiny bore, shrouding is guaranteed.70% of exhaust is evacuated directly as the valve cracks away from the seat.(thats why you hear the bang) The remaining 30% is the bleed down and 1.4" is adequate. Maybe not a 5.4l or for more rpm from a 4.6l, but adequate. The biggest 4.6l head restrictions are pinched ports. Opening the ports and grinding out the pinchs brings the heads to a little better than pi heads. Valve jobs, chamber work further improves, but who in their right mind will spend the coin on npi work? "Hey just pi swap it"....thats not me. Now lets crank it up and do this bigger intake valve and shift the head(or combo of both bigger bore and shift, reducing the needed amount of shift).Its the piston intake stroke choking the engine out. As one piston is in combustion,....7more are in limbo preparing or finishing out a cycle. That one combustion does alot ...and also propels the cars weight. If the engine cant make it self suck the air in easily, performance is compromised. Its true the power potential is within the exhaust bleeddown. Slow bleeddown coupled with reduced intake restriction can carry us to the next level with stock (junk) heads.</blockquote>