Re: Q: Is 150-155psi acceptable compression

Basically I think what your mechanic is saying is that because it is not below like 120 psi you should have no problem. Remember, you are buying a used motor and BMW does not use 3 piece rings so you do have quite a compression loss through the ring. I know that on most US M30s if you have a compression of 145psi you are good. I would say that the M5 should be higher because of the higher compression, but if the manual says 175 as the low number, I would think that 150 would be low. Personally I would do a leak down test on it too. If the leak down comes out fine, then I would say the motor is fine and that it is probably just a little carbon build up on the valve seats. This can be removed by using a MotorVac. Hope this helps
Rob
M535i SIG Chief Technical Advisor
www.m535i.org

I am looking to buy an e28 M5 and have had it checked by a reputable tech. He stated the compression was fine @ 150 - 155. He states that this is the specification in the BMW manual. Everything else on this board states it should be 175-185? Please clarify. Thanks in advance for your advice.

I have a U.S. spec E28 M5 that I purchased new in 1989. It currently has 92,000 miles on. Runs like a scalded dog. I've always had it serviced (frequently and regularly) at a factory BMW dealer or a highly reputable independent BMW shop. I just had the compression checked in December (2000) and it was: 205, 205, 200, 205, 205, 210. I'm no wrench, but I'd have to say 150-155psi compression on an E28 M5 is low. Further investigation and tests would be in order before I'd proceed farther with a purchase on that one. Good luck with your quest.

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