Compression Test Results

Post here for mechanical and engine topics such as fuel issues, transmission problems, rough idle, exhaust, electrical issues, etc

Moderators: GRNSHRK, ron, bfons

Post Reply
MyEuroM6
Posts: 2
Joined: Thu Aug 31, 2017 8:31 pm
Location: Boston

Compression Test Results

Post by MyEuroM6 »

New member here, considering purchasing an M635. Compression test results came back as follows:

140/135/130/130/125/125

Engine is a M88/3 which has had some piston work done but I don't have the details. It is running a turbo system - perhaps they did some work to lower the default 10.5-1 compression of the M88.

The car was inspected by a very reputable shop which gave the car a solid rating in the inspection. It has a "low, lumpy idle" but "pulls great" under full throttle and has "good boost".

Should I be overly concerned with the compression results being on the lower end?
User avatar
hornhospital
Platinum Member
Platinum Member
Posts: 2931
Joined: Sun Jul 11, 2010 6:46 pm
Location: Silverhill, AL
Contact:

Re: Compression Test Results

Post by hornhospital »

If it's boosted they NEED to be on the lower side. Those are low, but not so much. I'm more concerned with the low-to-high variation. 15psi is a lot of variation between cylinders.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'85 325e "Hazel Ann"; '95 M3 "Ashlyn"
MyEuroM6
Posts: 2
Joined: Thu Aug 31, 2017 8:31 pm
Location: Boston

Re: Compression Test Results

Post by MyEuroM6 »

Thanks. Would you put more weight into the compression test results vs. the solid overall inspection report? The car pulls strong as you would expect with a 350-400+ hp turbo kit.

Valuation is tricky - seller wants mid 20's for the car. It has been repainted and has under 100k miles but no service records. Interior is original and in pretty good overall shape. Car is very dry, engine bay looks clean. I believe the market value is around the $23-25k mark based on BaT sales.
User avatar
Da_Hose
Platinum Member
Platinum Member
Posts: 3236
Joined: Fri Mar 07, 2008 8:15 am
Location: Santa Ana, CA.

Re: Compression Test Results

Post by Da_Hose »

If it was turbo'd, I would expect lower compression. Test compression again, and make sure it's warmed up well. All cylinders within 10 is good. Hit the low ones with a spray of oil. If it goes up, you have a ring issue.

Comp. test is really important on a custom engine with weak maint. history. I have a spreadsheet with every upgrade and service on my M6. If the seller doesn't have really thorough records on such a unique car, I would be nervous.

Jose
1987 M6 - My dream car
User avatar
Brucey
6 Series Guru
6 Series Guru
Posts: 10077
Joined: Fri Apr 09, 2004 7:17 am
Location: Cambridge, UK

Re: Compression Test Results

Post by Brucey »

worth doing a leakdown test on an engine like that.

135psi indicates a static compression ratio of about 9:1, which is feasible for a turbo motor.

125psi on both #5 and #6 suggests that it is possible that the head gasket has a slight blow between the cylinders (which is not unheard of). The leakdown test should find it, if it is there.

Use an improvised stethoscope (length of tubing) into the adjacent cylinder via the spark plug hole to confirm leakage into that cylinder during the leakdown test.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Post Reply