Compression vs. Leakdown

[axl911]

How do you interpret the result if:

1. Compression=good but leakdown=bad?

2. Compression=bad but Leakdown=good?


Can you do one w/o the other? Which test is more preferred?

I've heard the leakdown numbers are generally unreliable for bigger enginer due to piston size.

---
anthony

[Grady Clay]

Anthony,

Both tests are worthwhile.

The advantage of a cylinder leak test is it gives you more diagnostic information. Not only does it give you a quantitative measurement relative to other cylinders and to other engines, it tells you where to pay attention to possible problems. It is a static test at TDC compression.

Cranking compression is a dynamic test. It gives you a quantitative measurement over several entire cycles of the engine.

There are several variations that can gain useful information. The most important is to not rely on only one test. If possible, go out and drive (freeway & power) the 911 and then re-test. Next, squirt oil in the cylinder, crank, and re-test. If it seals up there may be a ring sealing issue. If there is ANY leakage past a valve, check the valve clearance first.

There is no substitute for regular measurements so you know a base-line. I do a cylinder leak almost every time I change plugs.

To answer your question #1 you want to know what is bad: rings, valves, or head sealing. You would seldom find case #2.

I do a reliable leak test on our 460CID Ford. Leak down is a reliable test on 3 liter aircraft cylinders. I don’t think piston size has much to do with it.


Best,
Grady

[ckissick]

Both are useful and should be done. Leak down is nice, because, as Grady said, you get more diagnostic info from a leak down test.

I'll explain: When you do the leak down, you force pressurized air into the cylinder at TDC. There should be very little leakage, about 2-5% is good. If there's more, you can listen to three places where it's leaking. If you hear air hissing through the air filter/intake, then it's a leaky intake valve. If you hear air from the exhaust pipe, it the exhaust valve. If you hear air inside the engine, through the oil tank filler, it's the rings.

You can do each cylinder and make note of where the noise is coming from in order. ie: "Cylinder #5: 70% R,E,I" means in the #5 cylinder, there is 70% leakage, with most of the air blowing past the rings, then the exhaust valve, then the intake valve. The rings are shot, and the exhaust valve may be burnt.

I had these numbers on my car, along with compressions in the 80-110 range. (And lots of oil leakage) That's why I am currently installing a new 3.0. (Funny thing is, the car ran strong and fast; just messy and smelly. These engines are tough.)

Charlie
Montara, CA
1970 911E 3.0
1950 VW

[304065]

Hmm, 3-liter aircraft cylinders? Which one has those?

Wright R-1820, as seen on the B-17, Grumman Albatross and the T-28 . . .

1820 cubic inches divided by 64 cubes/liter =28 liters, divided by nine cylinders = 3.15 liters per cylinder!

Wright R-3350, B-29, 2.90 liters!

Imagine a 3-liter soda bottle!

Anyway, the reason I was going to post is that TCM (Teledyne Continental Motors) has a technical publication out that says that only 80 psi, as used in the standard leakdown, does not properly seal the rings and so may be unreliable, they recommend a compression test also.

I thought that was pretty interesting that an engine manufacturer would say that. But then again, they recommend rich-of-peak operation. . .

[GrindingGears]

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-2.htm

Wayne has a great write-up on this. Check it out.

[Grady Clay]

John,

It was the Wright Cyclone at 2908 cc per cylinder times 18 I was thinking of. Twin turbo and 2200+ HP gets anyone’s attention. It is amazing how compact that engine is. A rotery 930 times three on steroids.

Just like this thread, both cylinder leak and cranking compression yield useful information. I’ll bet anyone with four R-3350s does these tests before every flight. I lost a good Porsche friend when one of his Allison V-1710s hiccupped on takeoff from SLC in about ’74. I was going up for some stunts the following weekend in the P-38.

Best,
Grady

[john70t]

Minor tangent.
Just for the sake of the neubies : There's no comparison of junkyard auto fixers (like myself) and the aircraft mechs/pilots on the board.
Having a self-rebuilt engine fail means a short tow to the garage to you and me.
In their case it is the loss of a $million+ aircraft and passenger/freight.

Measure twice, cut once.

[304065]

Grady,

Nice to see you are an aircraft engine buff as well! I'm convinced that the height of piston engine technology was in the postwar period. I remember going to the chief pilot's office with my old man in the mid-'70s, and seeing a HUGE piston used as an ashtray on his desk. I have one on mine, although it's only out of a GO-480, but cigars are technically forbidden. . .

[Grady Clay]

John,

When I was still an undergraduate and many of my friends were buying airplanes, I made the decision to play with cars. Cars are less expensive … well, in most cases. You can use them every day, true unless you want to commute daily from SF to Reno. Otherwise they are the same – cool toys.
Do I like fast airplanes, YOU BET. When I was being recruited for Navy Aviation (’66), one of my friends got permission to take me for a minor excursion – in an F-4. We flew from Pittsburgh to a test area SE of Long Island. WOW.
Unfortunately my eyesight kept me out of the military altogether.

Back to the thread at hand. The techniques of aviation are readily applicable to our daily Porsches, cylinder leak and cranking compression included. These just arn't applicable to a General Electric J79 in a Phantom

Best,
Grady

[palle7688]

Nice to meet others from the industri here, we also have a Dutch pilot.
I do maintenance on commercial jets mostly Boeing (727-737-757-767)