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[Rick MacLaren]

Does 'loose' mean that the engine cylinders have more play within the chambers than, say, a 911?

What're the tolerances for a 1 overbore piston/cylinder combination?

How about compression tests on the turbo?

How important is it that the test be done on a hot car?

What's an acceptable compression figure for a test done on a cold car? What percentage of loss is acceptable?

I need to establish some kind of baselines, post rebuild...so any data you can input is greatly appreciated :smile:

[ This Message was edited by: Rick MacLaren on 2002-09-20 05:35 ]

[numbers]

If an engine is put together loose, it just means that clearances for bearings and pistons are slightly greater than normal (specs). It used to be a common practice for racing engines, as it will reduce friction, and therefore give you a little more usable horsepower.
A loose engine is generally not the way to go for street use, because a loose engine needs to be fully warmed up before you get into the throttle or you will likely through a rod. Also, loose engines use more oil than an engine that has close tolerances. They rice burners use very close tolerances, and you know how high they rev, and how long they last. As far as compression checks, if the engine is not fully warmed up, the compression may read low on one or more cylinders, and the varition between cylinders may be larger than a fully warm engine. The difference in warm and cold readings will not be tremendous. I.E., no compression cold, will still be no compression warm.

[Rick MacLaren]

Well mine came within the acceptable specs range on compression with a cold engine, and within 2 percent on each cylinder, which I guess is good. But I'm still low, and was wondering how that worked.

EBS says I can expect 5-7 percent increase on compression with a hot engine.

I took it to a German 911 tuner to do a compression test, crack the head, replace a valve spring. He's is all bent out of shape over the clearances on the pistons, yet the cross hatchings are still evident and clean - no scrapes or scoring. Like new. Evidently, according to Mahle, the pistons are 86.5 with 87.0 rings, which is in accord with the Mahle catalog.

I'm glad you answered my post, I was getting worried. So how much difference can I expect between cold and warm results on the compression test? Have you done one on your 931? Has it been bored out?

I found this old post in the archives:

From: Colin I
Email: colin@cingram.demo.co.uk
Date: 01 Jul 2001
Time: 04:51:55
"the official "acceptable" oil consumption is 1.5 litres / 1000 km ( ie quite a lot!) official workshop manual says acceptable compression range is 8 to 11 bar ( 118psi to 162 psi ) with a max difference between cylinders of 3 bar ( 44 psi ) - yours sounds ok. remember, turbos have a low compression ratio which, with boost, takes it up to an effectively high compression when operating. you only see low pressures when doing a cranking compression test, so don't compare the number you get with friend's naturally aspirated motors. you should really do the test with a hot engine ( difficult working on it, though!)"

[ This Message was edited by: Rick MacLaren on 2002-09-22 07:46 ]

[numbers]

Nope, I have never ran a compression check on the 931. Maybe next time that I have the plugs out.