M30 questions...

[CookeD]

Hey Guys, some M30 questions. I know we all love 'em...!

I'm wondering if anyone give me a ball-park range for what sort of numbers I should be getting from a compression test and/or what is normal for a non-rebuilt M30? Some background: '73 motor, stock head, 74k miles, and the car runs well. The compression test results:

#1 - 156 psi
#2 - 155 psi
#3 - 158 psi
#4 - 160 psi
#5 - 137 psi
#6 - 144 psi

#5 (low) is just over 14% away from #4. So the questions: Are these numbers normal for a non-rebuilt M30, and is a 14% diff. across cylinders any kind of a problem? I've heard that there aren't actual problems until you're on the other side of a 15% variance, but clarification from someone that knows M30's well is much appreciated.

Thanks in advance!

John

 

[ScottAndrews]

Hard to say. Usually a 10% variation is cause for concern. All of the various M30 variantsa run different compression ratios, so the absolute numbers are hard to assess. My M90 635 engine runs 185 +/- about 5 PSI.

A much bette assessment of an engine is a leak test. See may article at Bigcoupe.com on this.

http://bigcoupe.com/phpBB2/viewtopic.php?t=658

S

 

[x_atlas0]

Indeed, your CR will determine the "correct" psi rating. Low CR = lower psi values, higher CR = higher psi values. 160 sounds about right if you have an 8:1 CR motor, as my 10:1 M60B40 sits at about 190.

 

[Bill Cutler]

compressions

The compression test should always be done with the engine at normal operating temperature........... But I agree that a leakdown test is way more telling of engine condition.