milling heads for comp ratio increase

nm9stheham said

OK, OP, very good summary. Looks like you did some homework on this before. Glad to hear the piston height are indeed level with the deck; that is a lot better than a stock 318! The small displacement of the 273 makes small changes in chambers, etc. a lot more critical to static CR.

I ran your number through with 68 cc chambers, .028" head gasket, and about 4 cc's volume in the piston eyebrows and down to the top ring, and come up with 8.17 static CR.

As for the milling, several folks here have reported that they can mill up to around .030" off the bottom of the heads and not have to mill the intakes and things still line up. So assuming you did that, that would would cut around 5 cc's from the chambers. I'll guess that the flat-faced exhaust valves will save around 1 cc, and if you did that, shaved the heads .030", and found/used a .021" head gasket, then your Static CR would be up a bit over 8.9. That is a pretty good increase for those small changes, and that will definitely show up positively in street performance.

I am now curious about your cam duration specs. Being solid with .470 lift, it might be of pretty long duration and that can become a low rpm torque killer.

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Thank you heaps for working out the static CR. :) I was actually going to get an accurate CC on each one first before I put the numbers in myself. But that has given me a good idea - but pretty low isn't it! :eek: . I have looked at youtube and there are plenty of videos showing how to do the measurements for cc.

But that is good news about the 30thou milling amount - it seems a lot compared to what is recommended! I can easily do a dummy assemble of heads and intake and check how much room I have to play with and how the bolt holes line up. Maybe first with a 40thou Felpro and then with no gasket and look at how it changes etc. If I can take 30 thou off, that is a very good start. If it is tight and almost there, I don't see why I can't run a rat-tail file through the intake holes to facilitate lining up.

You may be right about the cam - here attached are the specs. It is an 'Ol Skool Schneider cam, valve lift 0.48, 290 duration and look at the lobe separation - only 107. So this cam may be a little rough idle and perhaps more suited to a hotter 340, but it's the only one I have and I don't see why it couldn't be made to work pretty well. Schneider didn't give me the RPM range but at a guess I would say 1800-6200, Does that sound close? (just a guess) I am running a 2500 stall so that will help -final drive is 3.23 and as I mentioned, it is a light-ish vehicle around 2750pounds