Leak Down Test Acceptable Values
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Melting Slicks
Joined: Nov 2000
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From: Bogart GA
Leak Down Test Acceptable Values
I checked my engine last night and 7 cylinders came in at approx 6% and the one was at 9%. The cylinders were also relatively dry. Car has been sitting for about 6 months. I have been spraying fogging oil into them however. I did not index the pistons.
Race Director
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From: Plano TX
If you sprayed oil into the cylinders the test won't tell you much except that your valves are sealing. You should run the engine up to temp and then do the test again with the engine hot (or atleast warm if you don't like 2nd degree burns). If the leakdown still comes in under 10% you're good.
Le Mans Master
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From: Ann Arbor Michigan
If you sprayed oil into the cylinders the test won't tell you much except that your valves are sealing. You should run the engine up to temp and then do the test again with the engine hot (or atleast warm if you don't like 2nd degree burns). If the leakdown still comes in under 10% you're good.
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
I think I'd use some white-out on that gauge... A 30% leakdown rate would indicate serious engine problems. In fact, if' you're seeing leak rates above 15%, there are issues with the engine that should be addressed. We just tore down a 406 that was showing 17% on a couple of cylinders, and we found problems with cyinder bore out-of-round and rings that had failed to seat on those cylinders. The engine passed compression testing, but was a "fail" on the 17% leakdown on 2 cylinders. The other cylinders were under 10%. A well-built engine will easily be in the 5-7% range.
Team Owner
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I think I'd use some white-out on that gauge... A 30% leakdown rate would indicate serious engine problems. In fact, if' you're seeing leak rates above 15%, there are issues with the engine that should be addressed. We just tore down a 406 that was showing 17% on a couple of cylinders, and we found problems with cyinder bore out-of-round and rings that had failed to seat on those cylinders. The engine passed compression testing, but was a "fail" on the 17% leakdown on 2 cylinders. The other cylinders were under 10%. A well-built engine will easily be in the 5-7% range.
If your have a motor that uses oil and has visible smoke they will LD test in the 10-12% By 15% you are talking a quart every tank or so and the motor will have a noticable power loss.
The modern 1/16th racing rings on forged pistons doing RPM are usually gone by 20,000 miles. 3-4% leak down can be done on well built motors.
KJL - Are you saying that you did not index the rings? End Gap and index are important.
Last edited by gkull; Jun 5, 2007 at 04:55 PM.
Thread Starter
Melting Slicks
Joined: Nov 2000
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From: Bogart GA
No, I know folks who check leak down with all pistons at the same stroke position in the cylinder for consistency because cylinders wear differently due to varying lateral loads imparted on the cylinder walls through the stroke.
I am interested to compare the "cold" values to a warm engine.
I am interested to compare the "cold" values to a warm engine.
Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
You have to test the cylinders with all pistons at TDC - if not, you'll have open valves. Also, if each piston is not at TDC, the pressure you put in the cylinder will rotate the crank. You cannot perform a leakdown test unless you put each piston in exactly the same position at TDC.
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Team Owner
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From: Reno Nevada
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When you ran the check did you listen near the block to see if the leak was all ring blow by? Lars brought up a good point on TDC. Any valve leakage in modern SS valves is not from valve or seat erosion. It is a problem of miss adjustment or even bent valves. Let me take this back. You really need to do a leak down test right after assembly and again after break in. Keep these records along with all your clearance information as a future refference
Race Director
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From: Plano TX
You have to test the cylinders with all pistons at TDC - if not, you'll have open valves. Also, if each piston is not at TDC, the pressure you put in the cylinder will rotate the crank. You cannot perform a leakdown test unless you put each piston in exactly the same position at TDC.
When my motor was shot it tested 16% at TDC. Bad, but not horrible... but when I tested 15 dgr BTDC it was 92%!!!
Race Director
Joined: Sep 1999
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From: Plano TX
Not always. You can have good oil control rings and bad compression rings. My engine only used 1Q/1500 miles, no smoke. Yet leakdown was 16% at TDC, 92% 15 dgr BTDC. So although it didn't burn oil, it pressurized the crank case and pushed oil through the PCV and fouled the plugs. But still only used 1Q/1500 miles.
Thread Starter
Melting Slicks
Joined: Nov 2000
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From: Bogart GA
I know my valves were closed because I did it with my valve covers off. I turned the crank until each cylinder was in the compression cycle and the valves were closed then I put it in gear.
My blow-by was deff through the rings. The heads are new AFR's. I am not to concerned about them.
My blow-by was deff through the rings. The heads are new AFR's. I am not to concerned about them.
Tech Contributor
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From: Connecticut, USA
This is a good thread as I have to do the same thing with my 350. It has 1200 miles on it and used about 2 qts already. I haven't had time to do anything to it other then pull the plugs. #1,5,7 2,6, and 8 were oil fouled. The insulator was only fouled 180* on most. No leaks but I can smell oil burning under load. Oil film on bumpers. I file fit the rings and the cylnders were supposed to be honed for the supplied rings. I haven't pulled the PVC line to see if there is any oil in it. I was going to do a leak test then compression test. Any other thoughts?
Le Mans Master
Joined: Mar 2001
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From: Ann Arbor Michigan
I think I'd use some white-out on that gauge... A 30% leakdown rate would indicate serious engine problems. In fact, if' you're seeing leak rates above 15%, there are issues with the engine that should be addressed. We just tore down a 406 that was showing 17% on a couple of cylinders, and we found problems with cyinder bore out-of-round and rings that had failed to seat on those cylinders. The engine passed compression testing, but was a "fail" on the 17% leakdown on 2 cylinders. The other cylinders were under 10%. A well-built engine will easily be in the 5-7% range.
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
I don't know how that's possible unless you have bent valves or a defective gauge. At that kind of massive leakage, you will hear the air whistling out of the leak source: You'll hear air whistling out the valve covers if the rings are bad, out the carb if the intake valves are bent, and out the exhaust pipe if the exhaust valves are bad. But there is no way you can have a defect-free engine with a 30% leakdown rate unless the guage is bad or the test is being done with the valves open.
Lars
Lars
Le Mans Master
Joined: Mar 2001
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From: Ann Arbor Michigan
I don't know how that's possible unless you have bent valves or a defective gauge. At that kind of massive leakage, you will hear the air whistling out of the leak source: You'll hear air whistling out the valve covers if the rings are bad, out the carb if the intake valves are bent, and out the exhaust pipe if the exhaust valves are bad. But there is no way you can have a defect-free engine with a 30% leakdown rate unless the guage is bad or the test is being done with the valves open.
Lars
Lars
