911porschefan

What would be acceptable leakdown and compression numbers? If there is a huge variation between cylinders is that a warning of something to come? How long should the car be running to get accurate numbers?

Indycam

The tests are done to see how the cylinders are .
If one or more cylinders are not up to snuff , you will maybe need to find whats going on with that cylinder/s .
If you have the test/s done and then post the numbers on rennlist ,
a few people will look at the numbers and tell you their thoughts .

As for what is and is not acceptable ,
the acceptable numbers for a race car , single engine airplane going out over the ocean etc ,
are not the same as the acceptable numbers for an old beater dump run truck .
I once drove a Mercedes-Benz W111 220s with zero compression in one cylinder .

raspberryroadster

i am advised (& based my buy decsision on these numbers).....leakdown between 1 & 3, outside 4% is ok.

compression tests are done at low rpm & therefore less reflective of engine at stress - but are a reflection of overall health of the engine (if compression numbers are strong & consistent and if compression numbers are strong & very consistent....a leakdown test may not be warranted).

the car i purchased ppi'd (compression) at

#'s 2,3 & 6 175p.s.i
#'s 1,4 & 5 180 p.s.i
....which i am informed are v.g. to excel.

be better if more informed members chimed in on this one.

cheers

augsburg911

Fwiw, when I had a leak down test performed on my car, I believe the numbers were roughly around 11%. The shop that conducted the test said that these numbers were good for a 16 year old car, with 78k miles on it.

ilko

Under 5% - awesome
Under 10% - good
Under 15% - acceptable
Over 20% - bad news for your bank account

meek

^^ What Ilko said.

steve g

I understand the % values but what is the baseline number ?

Steve Weiner-Rennsport Systems

Thats excellent,...

For reference, a brand new engine thats been run sufficiently to seat the rings usually shows 2% (sometimes less, depending on what rings are used).

Bearclaw

If the car hasn't been driven much recently or has been driven in a granny-like fashion, I suggest getting it up to full operating temp and giving it a good blast across the countryside before testing it. I rejected an '86 a few years ago when I was looking, in part because because one cylinder showed 8%, as opposed to the rest showing 2-4%. It was owned by a 70-yr-old guy who I know really babied it. From what I've read since, I wonder now if that wasn't because of a piece of carbon on a valve seat or some such thing. It wasn't investigated further because the car had some other things I didn't care for. But the price was really right and if those other things could have been worked out, I might have passed on a car that was really ok.

Bull

Once you establish the leakdown numbers (Ilko's ranges are a good guide), you then need to determine where the leakage is happening....which can be done by listening. Is the sound caused by leakage at an intake valve, exhaust valve or rings? then you have more info to determine likely extent of the issue, if any.

the car should be up to normal operating temp when the leakdown test is taken. If you are under 12% leakage, yet close spread between all cylinders, I would be less concerned than if you had 4 or five at something like 5% and the others at a much higher number. Of course 5-6% on all is excellent for a used engine.

raspberryroadster

so what about ACTUAL compression test numbers....my porsche mechanic says that initial ppi should always start out with compression and that the cylinders should test out close together.
any views on the numbers that my ppi reflect for compression
ie all cylinders at between 175 & 180 p.s.i.

Steve Weiner-Rennsport Systems

JMHO, but for me, leakdown figures are FAR more useful for determining engine condition, rather than compression test results as the latter is skewed by engine temperature and carbon deposits.

Further, leakdowns allow you to pinpoint WHERE any leakage is coming from; intake valves, exhaust valves or rings. When you find a leaky valve, staking the valve can improve the figure by loosening the carbon.

Compression tests tell you nothing about that whatsoever and in fact, can a be a red herring.

911porschefan

as usual thank you guys for the help. I guess the important thing for me to watch for will be the leakdown numbers. hopefully they will come back with relatively low percentages or at least consistent ones between all cylinders.

raspberryroadster

thks steve, understand porsche maybe different than past cars (most recently cobra, 2 alfas & bmw)....you are a pro - i am not, but have always understood compression test was 1st sign of engine health, ie if an engine is basically a pump it needs strong compression to run efficiently, start easily and perform strongly....particularly any differences in compression between cylinders is a red (?) flag.

understand compression is run without stress....measure of health of bottom end - piston & rings....and guess what you are saying not indicative of health of top end-which is a deeper issue with a porsche

sorry, just trying to learn....1st porsche!!

thanks, gerry

dfinnegan

What is "staking the valve"?