Acceptable Compression & Leak Down Ranges?
 

I am having a PPI done on a '80 911sc with 142K miles. Topend redone with new headbolts at 65K.

What are the acceptable ranges of compresion, and acceptable differences between cylinders?

Same question for leak down test? At what point do you say reduild?

I know this should be on the archives, but I've had no luck finding it so far.

I asked this question recently myself. As for compression general wisdom held that the numbers be fairly close to one another rather than a certain value, say 180. So its better to have a C test showing 180 to 185 across all six cylinders instead of 170 to 200. Didn't ask about leakdown but, I suspect it would be the same.

You may know that leakdown is a measurement of % of air pressure, which when fed into the cylinder at top dead center (valves closed) that leaks out. So if you feed 100 psi of air into the engine and 99 psi remains in the engine, leak down % = 1%.

Perfect leak down is 0%, though it is unlikley achievable and maybe not any better than 1%. That said, lower is obviously better. I just did a leak down on my rebuilt engine (300 to 400 miles).

It was 1% on a four cylinders and then 2% on one and 4% on the final. These are good marks. But....the engine also has 400 miles on it. If I were very anal, I'd take a look at the 4% and see if I could make sure that there was nothing between the valve and the valve seat. The smallest amount of carbon could cause leakage. I did mine warm (2 hours after running), but not hot. That could change the results, too.

Unless I'm racing in unlimited (budget) class racing, I don't think this engine has issues (at least compression issues).

I would say to 6% is good. Below is an indication of an issue, especially if done hot. As an example, I'd be concerned if I the numbers were:

example 1:
1%, 2%, 1%, 2%, 25%, 2%.
OR
example 2:
10% 12% 15% 11% 12% 10%
example 2

I'd be concerned example # 1 has an issue in one cylinder. It could be as simple as a poorly adjusted valve OR as bad as a burned valve or broken ring(s).

In example 2, I'd suspect a well worn engine.

Note, when the test is performed, you can actually hear to where the air is leaking. These are the possibilities: 1. into intake (via intake valve). 2. into exhaust (via exhaust valve) 3. into crankcase (via rings)

Doug

Post your results and we can help you evaluate.

Doug

Compression numbers (value) are not as important as having them being nice and similar. Note similar, within say 10% of each other or less. Many things effect the actual number you will see on the gauge. Was the motor hot or cold? Was the throttle open or closed? Have the valves been adjusted recently? How fast does the starter spin the motor over? How high above sea level are you located etc etc etc....

Doug gives a good explanation of a leakdown. you can do a leakdown at either TDC or BDC. I prefer to do a leakdown only if I have varying compression test numbers so I can pinpoint where each cylinder is leaking.

Cheers

5% variance or run

Just as a further point of reference, I did a leakdown on my turbo motor last week. I had planned on doing it hot, but was interrupted after getting the intercooler and air filter assembly off. I ended up doing it cold the following morning with these results @100psi and TDC:

#1 - 8%
#2 - 18% - leaking at exhaust and intake - probably burnt valves
#3 - 4%
#4 - 3%
#5 - 2%
#6 - 5%

This motor has 108K currently and hasn't been rebuilt in the 70K miles I've owned it and receipts from previous three owners show no major work. Car runs smooth with an ocassional miss at idle when cold and LM1 shows normal a/f ratios. I plan to do a hot leakdown in a couple weeks for comparison.

If you find ever find a problem with the leak coming from the rings (take off the oil cap and listen for a hissing sound coming from the oil tank) try moving the piston in and out to make sure the rings are firmly seated. Do this while the cylinder is still pressurized (it's not real easy).

If the leak is coming from a valve, try tapping on the stem of the valve to see if it's possible to dislodge any carbon that might be causing a slightly open valve. I use a plastic mallet to hit the valve stem.

We do this with aircraft motors as a normal routine during 100 hour & annual inspections. It's a lot easier to move the pistons on an aircraft while the cylinder is pressurized by moving the prop a few degrees, just make sure you have a firm grip otherwise 100psi will spin the prop when you get a little off TDC.