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07-28-2010 06:51 PM #1
Max. to take off cyl. head for compression bump?..
I have built several nice street performance engines and am currently doing some more work to my already very strong Euro spec. 1984 535i (M30B34) engine. I am installing a later model, 1989 535i (M30B35) head which has the slightly larger (47mm) intake valves and manifold. I plan on running this with my original Euro spec camshaft which has slightly more lift than US spec. (I think the cam journals should be identical?) and while the head is out I figured I would bump the compression up a bit. My old engine has never given me a hint of pinging, even under hard acceleration on the occasional regular gas, so I'm sure there is still quite a bit of leeway to mill down the new head. I was thinking of going as high as 0.75mm off the standard thickness (129.3mm?). Then, if that's a bit too much for street use, I can always use an optional thicker head gasket to ease back up.
Am I in the right ball park? What do you people who have done this think? The head cost me very little so I am willing to experiment, but of course I want to make reasonable changes that have a good chance of success.
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