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-   -   M20b25 compression test result prior to boost install (http://www.maxbimmer.com/forums/showthread.php?t=150190)

The Twin 04-17-2012 02:15 PM

M20b25 compression test result prior to boost install
 
cross-posted from the F/I sections...figured get more exposure here.
if Mods feel it's clutter, please nuke the appropriate one.


My 1990 325ic will be undergoing some updgrades shortly.

in preperation for it to accomodate the boost setup that will be installed, I went ahead and performed another compression test...with results that kind of surprised me.

Please note, I overheated my engine last summer during the heatwave while stuck on Decaré in MTL...I suspect HG damage, which will be replaced prior to boost.

regardless, I wanted to get a feel for the health of the block.

COLD TEST RESULTS:

145, 150, 120, 125, 150, 150

JUNK.


WARM ENGINE RESULTS...WITH oil added to cylinder, AND open throttle.

160, 180, 180, 180, 200, 180

...ya.

I'm being told a few things, such as "complete rebuild" to a new ring job+hone+HG.

Would it be wiser to scrap this engine, and just get a new(used) one or even with the "better" results of a warm compression test, still reveals it has issues?

I'll be running 8psi with a .60AR turbo...

thoughts?

Ceeker 04-17-2012 04:08 PM

you may run into the same problem changing the motor.

The Twin 04-17-2012 04:14 PM

yeah...after talking to a boatload of folks, I think the concensus is to simply run it as is (of course change the HG+get head done)...

if bottom end goes, I'll have a descent head to play aroud with come next engine.

NOTORIOUS VR 04-17-2012 04:15 PM

you should have done the test warm but with no oil... and then one with oil... as that will tell you if the issue is with the top or bottom end of the engine

The Twin 04-17-2012 04:16 PM

can do it again this evening with no oil, and warm engine...

The Twin 04-17-2012 09:04 PM

Quote:

Originally Posted by NOTORIOUS VR (Post 1560861)
you should have done the test warm but with no oil... and then one with oil... as that will tell you if the issue is with the top or bottom end of the engine

Did test again this evening on warm engine. Solid 150 across all 6 cylinders. Maybe a +\- 2,3 psi variance but all where basically the same
Started testing hot, and by end was down to 1/4 temp. No oil in cylinder. Dry test.

SamE30e 04-18-2012 09:34 AM

**** it... boost it.

Fix it when it blows up.

NOTORIOUS VR 04-18-2012 12:35 PM

Quote:

Originally Posted by The Twin (Post 1560928)
Did test again this evening on warm engine. Solid 150 across all 6 cylinders. Maybe a +\- 2,3 psi variance but all where basically the same
Started testing hot, and by end was down to 1/4 temp. No oil in cylinder. Dry test.

sounds about right... with that little variance I wouldn't worry about the bottom end.

MaxBell 04-18-2012 10:51 PM

Yeh, sounds like the bottom end is fine. Get your head checked for warps/cracks. Lap your valves, etc etc.

I'd recommend Dave at that machine shop at Bronson and Chamberlain. (down that ramp, across from the Drummond's)

He did excellent work on my bike and M50 builds.

Recommended.


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