Compression and leakdown, what's acceptable?

[rrental]

I have put money down on this really nice 1986 930. The PPI came out great, with some issues that are repaired and replaced by the Porsche dealer.

Tomorrow the compression and leak down test result will be available and it will be the deciding factor as everything else is good and agreed upon.

What would be considered acceptable values for these tests?

Thanks!

[JBurer]

All depends on whether the test is performed on a cold or operating temperature motor. Cold motor, anything over 20% indicates rebuild time.

You're also looking for consistency between the cylinders. One of the cylinders (Number 3, if I recall correctly) typically has a slightly lower value as it is not cooled as effectively by the fan. As long as it's not more than a few % difference, it's fine.

[Jhalsey]

Had my 89 930 w 31,000 mi tested by TLG in N Hollywood:

Compression 125-130 psi
Leakdown was 1-2 percent on all cylinders.

[rrental]

Good information, thank you!

[WPOZZZ]

Was the leakdown done on a cold or warm motor?

[JBL930]

I believe you can expect around and under 3% on a new or rebuilt motor, but i'm told anything up to 10% is acceptable..... If it's in the teens then you need to plan a rebuild soon

[JBurer]

JBL - is that in reference to an operating temp motor?

Franz Blam told me they put a cutoff at 20% on a cold motor.

[flat_6]

I was told that the rebuild point is 10% on a race engine and 20% for street.

[rrental]

Thanks a bunch guys! Greatly appreciated! The results came in today. Did not see the report yet but got them over the phone. Leak down comes to 1-2% and compression between 120 and 125.

Its my understanding its done with a warm engine.

Will post exact numbers when I have the report.

[WPOZZZ]

Originally Posted by rrental

Thanks a bunch guys! Greatly appreciated! The results came in today. Did not see the report yet but got them over the phone. Leak down comes to 1-2% and compression between 120 and 125.

Its my understanding its done with a warm engine.

Will post exact numbers when I have the report.

They should do the leakdown on a cold motor.

[Flat6pac]

But the motor in operation is not cold. Performance cold has little to do with a running engine.
Bruce

[JBL930]

Originally Posted by JBurer

JBL - is that in reference to an operating temp motor?

Franz Blam told me they put a cutoff at 20% on a cold motor.

I think that was on a warm engine, I can't see that you would be able to do a leak down on a 'up to temp' hot engine anyway, unless you were wearing oven gloves.....
I've never witnessed a leak down test, so honestly can't tell you..... Let's say it was on a hot engine, would you test one cylinder, then start the car again and get it up to temp and do the next, then do the same for each??? Seems like a pretty laborious task.
I got the figures I quoted off a tread on the 993 section on Rennlist, those are what RS Tuning say are acceptable figures

[911nut]

I think one could be at risk to gall a spark plug hole thread if doing a leakdown on a cold engine. Besides, the engine compartment is so tight you'd burn the crap out of yourself attempting it.
I previously worked in engine manufacturing and 20% was the cold leakdown limit on field-returned engines so that's also a good value for us, IMO.

[jwasbury]

I did my own leakdown test on my engine prior to rebuild. I ran the engine up to proper operating temp (by driving it), then pulled into my garage and did the test with the engine "warm." Did not run it between testing cylinders and yes the spark plugs were hot, but its manageble.

I also performed/witnessed leakdown testing on a cold motor, fresh zero hour rebuild with replated cylinders, new pistons using total seal rings. The leakdown was <5% on that one cold.