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Message: <blockquote data-quote="Silver95bird" data-source="post: 1511626" data-attributes="member: 21743">Neat work, glad to see you're still kicking blak. The pictures really don't do the npi heads justice. The PI heads do a much better job of tapering as they go further from the valve. The NPIs have this weird pinch like 1" or so in, that the cutaway pic is better at showing. People see the port at the gasket and assume that's the big difference. Not so. The NPI also has this weird casting that makes the intake port kind of ovaled, like a tongue shape. I can also tell you they aren't all the same size. They're roughly 1.10 x 1.50 iirc, I'm not sure how that changes the airflow. Perhaps trying for more swirl? When I started doing the quick job on my 96, I used a 1/2" drive impact socket until I found the one that would just barely pass through the tight spot, and then I ported all 8 ports so that the next size bigger would both pass through, and slide across side to side at that point. Could I have taken more? Maybe, but I didn't want to break through, so I took what I could get. Blak would be better able to say how far is safe without cutting the wrong spot too thin. My humble opinion is that on an npi casting, you would have to be very thin in a few places to make oversized valves worth it. The ID of the seat is enough to support a fairly ported port as is, given that the seat really needs a 15/30/45 angle port. Bigger seat IDs require bigger ports to support it. Starting at the seat, there should be a 2% taper each inch back. The mapping of the ports can show you the area at each point back, so you can calculate the area of each level and see exactly where and how much needs to be cut. You'll find that at the gasket face you really don't even need to cut for flow, you're cutting for mating the intake to it.</blockquote>