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Message: <blockquote data-quote="96blak54" data-source="post: 1221334" data-attributes="member: 14727">The npi heads are still contenders in the 2v battles, ....they just need a little more love.1. The intake valve has that darn swirl dam to help promote swirl. Laying the dam back a little will unshroud the valve and help with flow, but dont cut it out totally. We still want lots of swirl. Removing it totally will increase advance timing. Also removing the material of the combustion chamber around the valve, laying back to the gasket/ bore size helps a lot. This reduces the chamber cc but makes a word of difference in flow. From my discoveries with pi vs npi 4.6l rwhp differences with stock untouched heads the pi was only 10rwhp more different over the npi all in stock form with same HEAD CAM INTAKE (PI) and compression. Doing this swirl dam trick places the rwhp equal between the 2 but the npi torque is alittle more.....both tested at the same compression!2. The npi exhaust valve is smaller of the heads but not really a wall for flow. I believe Ford switched the exhaust valve size to a 4v intake valve size to cut down on manufacturing.(a 4v intake valve is much long than a 2v valve and will not directly swap) This flow wall is the material around the valve guide under the npi exhaust valve. You can really go crazy removing this material but leaving a little at the roof of the port to aid in some support for the guide. Doing this takes a port that stalled air flow at just .300" lift to an accelerated flow to max lifts.3. The intake port actually is pinched early into the port. More pinched than the tear drop shape you see at the manifold side. The manifolds ports are actually bigger!  Again opening this up improves flow. 4. The valve guide length is what kills the use of a higher lift cam. Simply grind off around .150" off the top of the guide under the valve springs. This will add clearance between keepers/retainer and valve seal. Only do this if you plan to use a higher lift cam with more than .550" lift. If you do this modification a different valve seal must be used. Instead of a 2v seal get 4v valve seals. The 4v seals are shorter than 2v. The typical remedy is to deepen the spring pocket and cut the top off the guide to fit the 2v seal. 5. Their is to much combustion chamber. You're better to mill the heads .100" and compensate the compression with piston dish. Keeping all the combustion in the cylinder head as much as possible and not in the cylinder is more beneficial to flame travel/combustion characteristics/timing. The combustion chamber is essentially the same as a pi, just more of it. Also the deck thickness on most npi heads are roughly .080" more than any pi head. Earlier npi heads are .150" thicker.6. Port the ports! You can literally take .100" from the walls off the intake port walls. All the way around!  So a port I.d. of 1.400" to 1.600" is possible. That's crazy! Same material for the exhaust but their is more material on the floor than the roof. Intake valve: Rough untouched under the valve seats. Blend the bowl with little material removed keeping it rough. Cut out the rest of the port from there. Port the floor short radius under the seat keeping a good radius but opening it up from there. Open the sides up also. All this while trying to keep a funneling shape in mind.</blockquote>