Relationship: Compression vs Leakdown?

[jyl]

I am confused about the relationship between compression and leakdown. I am looking for a late 3.2, and had a PPI done on a '89 coupe with these results:

#1 180 0%
#2 170 10%
#3 180 10%
#4 180 20%
#5 180 10%
#6 180 0%

From reading earlier posts my impression is that the compression readings would be considered fine and the leakdown results on #2-#5 would be considered problematic.

Question: How does a cylinder have good compression but not-so-good leakdown? Mechanically, what is going on in there?

More information - the car is driven daily, had a DIY valve adjustment within last 10K miles, the mileage is just under 140,000, and it drives very nicely. Don't know the detailed history, as the seller has only owned it a year. The mechanic heard leakage in the crankcase and in the intake. He also heard some leakage where his tester seated in the #4 plug, and thought there could possibly have been a helicoil there.

[dickster]

correct me if i'm wrong but they are related.

comp. gives a quick test of the engines "overall" condition, but results can vary greatly depending on how its done.

i would suggest the leakdown is more subtle and provides more info. ie if you got low comp. then the leakdown can indicate where its coming from. crankcase hiss = rings, muffler hiss = exhaust valves, etc.

i would have expected higher leakage on #2???

i also like checking things like color of tail pipe (check that its not "wet/sooty", condition of spark plugs (nice light brownish), and color of oil (nice and fresh looking). these all give there own story.

why's he selling??

[Sputterbug]

I had a similar result in one cylinder when I had my '85 tested last week. Cylinders 1 and 3-6 were all between 180-185 and 2-6% leakdown. Cylinder 2 was 155 and 70%!

I thought that result was highly questionable, as does my mechanic. He heard air leaking from the crankcase, which (as confirmed by him and others here) implies rings. He suggested that I use a bottle of Techron and drive the car for a couple k-miles, then we'll retest. My car also had oil leaks in the 4 return tubes, as well as the left side chain tensioner hose and line. Those were all replaced (so we'll see how my oil consumption is now).

He also commented that my car runs "really strong," and that he was surprised. So I'm pretty much not worried about it. I would worry more about the results of your '89's leakdown, since 4 out of 6 cylinders are quite high in that department.

[304065]

When the new owner of my '88 Carrera had a PPI performed, the motor showed 6% in #1 and 2% in the rest of the cylinders, at 96,000 miles. I would say that anything 10% or higher is indicative of a problem- particularly if the mechanic is hearing air leaking out of the breather.

Was the motor up to operating temperature when the test was performed?

To add to Nelson: the compression test is total cylinder pressure with the intake and exhaust valves closed. If a valve is not seating properly, that reduces the pressure: if the rings are worn and compression is not being generated, that reduces the pressure. The beauty of a leakdown test is that by pressurizing the cylinder with compressed air, you can find out WHERE the problem is- you listen for air escaping from the induction system (intake valve leaking), out the tailpipe (exhaust valve leaking) or from the crankcase breather (ring blow-by).

Who's your mechanic in Jersey? I'm looking for a car now and would appreciate any references.

[Formerly Steve Wilkinson]

The "compression" number is an instantaneous value achieved and maintained for a split-second, when the piston goes up and the valves close during the compression stroke. Unless the rings and valves are terminally shot, an engine can achieve perfectly acceptable compression numbers while cranking, while there's no time for the compression to decline due to leakdown.

The leakdown number, therefore, is a measure of how well the engine _holds_ that compression. If the rings are not up to snuff or the valves or guides need some work, the compression value (in psi) will slowly--or rapidly--"leak down" as the compressed air in the cylinder slowly squeezes past the valves or rings.

You can also sometimes hear where the engine is leaking. If it's the rings that are bad, you'll hear it hiss into the crankcase, if it's the exhaust valve you'll hear it hiss into the exhaust system, intake valve into the intake manifolding. Dunno how this works on a Porsche with a muffled exhuast and no direct-to-crankcase oil-pour access; I've only done the listening on aircraft engines, where it's easy to hear the hissing through a variety of orifices.

Stephan

[Sputterbug]

John,

My wrench in NJ is Performance Plus in Oakland. 201.337.5678 and http://www.performanceplusauto.com

They participate in numerous PCA activities with the NNJR and I have heard only good things. Tony, the owner, was polite and good to deal with. The few times I have stopped by for fun there have always been a bunch of road and race Porsches in various states of disassembly.

[jyl]

Thanks, all, for the answers.

richardnelson asked why the owner is selling. He says it is for personal including financial reasons and I believe him - the economy is not too good here in Northern Calif, lots of job losses, and plenty of Porsches, BMWs and similar "nice-to-haves" are for sale.

Anyway, he is having the valves adjusted and the leakdown retested, perhaps that will make a difference.

[Wayne 962]

I just wrote about 3000 words on this topic for my new Engine Rebuilding Book.

Everything mentioned here in this post is true. However, there is one important thing to note. Both compression tests and leakdown tests are good for determining the condition of the engine. However, they are not the end-all of tests. They have their vulnerabilities, and as such can only be used as one data point in diagnosing the engine.

Unfortunately, you don't have access to the engine all of the time, so they are your only means for determining the condition.

Leakdown tests and compression tests cannot adequate assess the condition of the engine when it is running (dynamic conditions). If the car is cold when the leakdown test is performed, or there is a piece of carbon stuck in the valve, it can give off-readings.

Compression numbers by themselves are almost useless. You want to compare cylinder to cylinder, and that will give you a comparison for how the cylinders are compared to each other.

In your case, the numbers don't support each other, so I'd suspect that there are some unforeseen factors involved.

What is my recommendation? Based on the compression numbers, it sounds ok. The leakdown numbers are suspect because they don't back up the compression numbers. I would use this data to negotiate a better price. The leakdown numbers don't necessarily paint a bad picture, but they certainly *increase your risk.* As any good bond rater will tell you, increased risk means a discount in price.

140,000 is not a low amount of miles. The 3.2L engines are long-lasting, but they do suffer from some problems (rod bolts & valve guides). I'd guess that this engine is beginning to get tired. It may last another 100,000 miles, it may last another 10,000. If you're looking for less risk, then spend the $$$ and get a car with fewer miles.

Also pick up one of Leffingwell's new Buyer's Guides:

http://www.pelicanparts.com/catalog/shopcart/911M/por_911M_documt_main.htm

It's pretty useful for the first time buyer...

Hope this helps,

-Wayne