Compression questions

[Goober]

I'm looking at several 96 - 98 GT's. One I have found has a bad motor. I also have a 93 Mark VIII engine. What would the compression ratio do if I put the 2v NPI heads on a 4v rotating assembly? I read somewhere that putting 4v heads on a 2v shortblock would lower compression (I think na svt said that on Corral). So I assume this swap would raise it, but to what?

[94tchikinv8]

I think with the NPI heads it would be around 10.5:1 from what I've read. I think if you put PI heads on the Mark shortblock it would go up to 11.5 or 12:1 with their smaller combustion chambers.

With at least a set of PI cams and a PI intake on the NPI heads and mark shortblock, it should be a pretty good running engine. PI heads are not the end all, be all of the modular world that everyone assumes. Most of the gains between the NPI and PI are from the intake and cams.

-Rob

[Goober]

PI heads, aggressive cams and a TFS or PP intake would be in the future somewhere. I thought PI's had more flow to them. Is that true?

[NXcoupe]

Yes that's true, but his point is that most of the power increase comes from the cams and intake. Those little motors don't utilize all the flow in stock trim of the PI heads.

[na svt]

I'm looking at several 96 - 98 GT's. One I have found has a bad motor. I also have a 93 Mark VIII engine. What would the compression ratio do if I put the 2v NPI heads on a 4v rotating assembly? I read somewhere that putting 4v heads on a 2v shortblock would lower compression (I think na svt said that on Corral). So I assume this swap would raise it, but to what?

PI heads on a mark motor would result in a CR of about 11.8 to 1.

[cstreu1026]

PI heads, aggressive cams and a TFS or PP intake would be in the future somewhere. I thought PI's had more flow to them. Is that true?

I would skip either of those intakes (especially the Pro. Products junk) unless you have forced induction. The TFS will gain a few ponies but you will sacrafice low end power and gain heat soak and more weight on the front end. The plastic PI intake works pretty well on these engines. I still stand by the idea that the PI head is the best option for the average enthusiast. The SVO/FRPP head flow better but cost a ton if you can find them and they have the same limitation on cam lift that the NPI heads do. It appears that you can port NPI heads to flow as well or even better than ported PI heads but that too cost big money. In additon the results can be sketchy. I know one person on here made less power with ported NPI heads and comp cams than I did with stock PI parts.