Cold compression test

NZ911 · 07-27-2014, 03:14 PM

Hi all

Im looking at purchasing a 3.0SC engine on webers and the seller has provided me a compression test done when the engine was cold as it is not in a car any longer. Can anyone give me an insight on what these numbers look like?

From the seller -

We hooked up the gearbox and the starter (included with the engine) up to battery, a jump starter and then connected to a battery charger. Used thick cables and we were able to get good cranking speed to do the test.

I do realise that a cold compression test does have limitations of what can be discovered, but the results were pleasing.

1. 145 PSI
2. 158 PSI
3. 157 PSI
4. 140 PSI
5. 145 PSI
6. 145 PSI

That shows from the lowest to the highest that minimum percentage value is 88.6% which according to Wayne Dempsey's - How to Rebuild and Modify Porsche 911 Engines 1965-1989 is above 85% minimum for a healthy engine.

Any thoughts?

afterburn 549 · 07-28-2014, 01:47 AM

I would count on them lower up to temp. that is the safer road to take. Its a pretty big span.

JJ 911SC · JJ 911SC's Garage

It is within the 10% spread and it would yield higher number when the engine is hot.

afterburn 549 · 07-28-2014, 05:42 AM

15% is a big spread.
Maybe allowable ....

JJ 911SC · JJ 911SC's Garage

Quote:

Originally Posted by afterburn 549

15% is a big spread.
Maybe allowable ....

Oops, I meant 10% (15 psi in his case)

afterburn 549 · 02:13 PM

In a P Car engine most of us are anal about stuff.
Everyone is allowed to do what they want, but to buy a untested engine with those numbers is maybe looking for answers in the wrong places.
Perhaps - Overhaul your own engine and not be out the fishing expedition money ?
It would have to be CHEAP for me to be interested.

NZ911 · 07-28-2014, 02:09 PM

Thanks for the replys... food for thought

cgarr · cgarr's Garage

You could change the cam timing and get the numbers up

porschetub · porschetub's Garage

Air cooled engines take up when they are warm ,a compression test is a variable not a dead set test,as a marine engineer we test with the engine running (hot) and generally do it several times and reach average over several tests,it does vary somewhat on each test.
A cold test is helped by turning the engine for a while to blow out any carbon deposits with the plugs out,more important if the motor has been sitting for a while.
Old school mechanics used to squirt oil done the plug holes and then retest ,never done it myself but I assume it would lift the pressures somewhat or provide a better indicator of rings sealing (or not),IMO the OP's test isn't too bad.