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    Max. to take off cyl. head for compression bump?..

    Hi All

    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. Still I think its best to ask some experts who have experience with this engine whether I'm on the right track.

    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.

    Thanks, Tom

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    Big Dog
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    Re: Max. to take off cyl. head for compression bump?..

    My late model track car (for sale BTW) uses the same motor and we shaved the head by .5 millimeter with no problem at all. Still runs pump gas (93 if I can find it, 91 otherwise) and never gives any hint of pinging. There are two caveats you need to know.

    1. You will need to take the front timing chain cover to the machine shop with the head, it has to be shaved also to make sure the deck height is the same for both pieces.

    2. The cam timing will be affected by about 2 degrees (retarded) which will give you a very slight bump in top end power with no noticable loss in the bottom end. For a track car this is ideal, the revs stay up most of the time.

    Dog

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    Re: Max. to take off cyl. head for compression bump?..

    Awesome response Big Dog. I think I will push it just a little further since this is a spare head and I have the thicker head gasket to fall back on. Thanks very much for the reminder about the timing cover. I would have forgotten....

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    Re: Max. to take off cyl. head for compression bump?..

    P.S.S. The valve timing would still be altered, as you said. I wonder if BMW makes offset keys to adjust for this (like Mercedes). I'm guessing that upper timing gear flange is located on the camshaft with a key, it's been a while since I've had it off...

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    Big Dog
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    Re: Max. to take off cyl. head for compression bump?..

    There are adjustable cam gears but they are pricey. I don't think it's going to be that significant, the extra top end torque is nice but probably not much of an issue for a street car.

    Dog

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    Re: Max. to take off cyl. head for compression bump?..

    P.S. Dog

    Honestly, I don't think the timing is affected. I once lost the pin that locates the rotor holder inside the Motronic cap and I found that I could play with the "timing" or location of that rotor within a wide range without changing the performance one bit. The Motronic is what tells the system when to fire, not the rotor position. As long as the rotor is close enough to each pin inside the cap to transmit it to the correct spark plug.....

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