What would you guys think the compression ratio would be for a Phase II block w/ a phase I DOHC head that's been milled to min spec? Stock Phase I setup it 9.7:1, think I'm pushing 10:1?

 

If you still have the heads off you can measure and calc it

http://tinyurl.com/3c5d8z

 

I wish they were. It may end up being a mystery unless there's another way to calculate it.

 

Changing your heads from the SOHC to DOHC isn't necessarily going to change your compression ratio, you look at the pistons and the head gasket for that. Now if you had put these heads on say a 2.2L or a 2.0L motor then that's a whole different game altogether. This is going from one 2.5L to another and there should be much difference in CR if any at all. I'd say if there was a change, it'd be very slight like maybe at 9.8-9.9:1 But then it would be as simple matter as changing the head gasket to increase the CR.

If anybody knows that I'm wrong, please correct me, to me this theory sounds right.

 

The compression ratio is worked out using the combustion volume in the head as well. Paulpas is saying that he shaved his heads so the combustion volume in them is reduced thus increasing the compression ratio.

A = volume above piston when it's at bottom dead centre
B = volume above piston when it's at top dead centre


If you know how much you shaved the heads by and you can get the original info on both phase I and II's bore, stroke and compression ratio then you should be able to work out the new compression ratio. Assuming the blocks are of equal height and the head gaskets of same thickness. You'd also have to assume that the combustion volume you shaved off the heads is a simple cylinder.

If someone can verify that phase I and II heads hold exactly the same volume then you just need your engine's bore, stroke and old compression ratio to work out the new compression ratio.

Anyway hope that helps