Cylinder leak down test

[daren007]

What do you use as a base pressure.

[bbrocksr]

Originally Posted by daren007

What do you use as a base pressure.

80 pounds if you are using an aircraft style leakdown tester, dual gauges with an orifice between them.

If you are just checking for leaks line pressure is okay.
Bill

[daren007]

Line pressure could be 120#?

[bbrocksr]

Originally Posted by daren007

Line pressure could be 120#?

Yes.

[pbishop]

I agree with 'bbrocksr'. It just depends on the 'head' you are using and or if your head has been significantly trimmed (surfaced). Looked it up in 'Mikes Affordable' catalog. For a standard head it should be at 76 psi across the cylinders. If you have trimmed (surfaced) the head it should a read a little higher. My 'standard head' has had around .100" taken off.



Thanks for the question. I have not checked my compression in some time. Think I will do that this weekend.

[BRENT in 10-uh-C]

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This is getting to be confusing as it appears there is two different comments going. In leak-down testing, you are using percentages as your guide. Generally you will calibrate your tester with a line pressure that is easily divisible by 100. I typically use 100 pounds first. If you inject 100# at TDC and your secondary gauge shows 85#, then you have a 15% leakage rate. If your line pressure is 110# and the secondary is showing 87#, it is much harder to calculate in your head.

It is worth noting that using too high of primary pressure can give a false reading, -especially if the piston gland rings are worn as the pressure can get in behind the ring and force it outward causing the ring(s) to seal better than normal.

One other note, a leak-down test is really only beneficial as a comparison gauge over a series of tests. We use it with our race engines generally just after the engine has been built as a baseline. Then we will do another test after we have finished dyno pulls or after a night of racing. This lets us compare numbers to see if it has sealed up. From there we may check it again after four or five races and compare numbers again. With a Model-A engine, you really need several similar readings to have an idea of the engine's condition. Using it one time really only gives an idea if you have a major issue such as a blown gasket, leaky seats, or excessive blow-by.

[Patrick L.]

The fellas have pretty well covered it. But, my question is, what are you looking for ?

If a compression test indicates that something may be wrong and you don't have a real leak tester, then a simple one can be rigged to simply inject some air in a hole [ 50-100#] and look/ listen for the leak to see what is wrong.

[Jim Brierley]

A compression test will tell you if something is wrong or weak. A leak-down will tell you exactly where the problem is.

[bbrocksr]

Originally Posted by Jim Brierley

A compression test will tell you if something is wrong or weak. A leak-down will tell you exactly where the problem is.

Best answer!

[daren007]

I have been doing leak down/compression test on aircraft for 45 years I was looking for a base number for the Model A as I have heard several different answers. I used 100 psi and had air in the radiator on cylinder number two. All cylinders held 90 including cyl. #2. There is no water in the oil nor water going through the combustion chamber. I removed the head and found absolutely nothing. The engine is back together and runs just fine. I was wondering if too high a number as a base could get by the gasket on the test but the engine run ok otherwise.

[SeaSlugs]

Originally Posted by daren007

I have been doing leak down/compression test on aircraft for 45 years I was looking for a base number for the Model A as I have heard several different answers. I used 100 psi and had air in the radiator on cylinder number two. All cylinders held 90 including cyl. #2. There is no water in the oil nor water going through the combustion chamber. I removed the head and found absolutely nothing. The engine is back together and runs just fine. I was wondering if too high a number as a base could get by the gasket on the test but the engine run ok otherwise.

its possible - seals and gaskets only hold a finite amount of pressure before leaking. I generally just put whatever the compression thats generated by the piston should be into the cyl. If you have a problem it will show up well before you reach your compression number.

You may have to do one test cold one test hot as it allows cracks to open up and things to flex. Especially true on cars that are cold run great till things start to heat up then they die or misfire.

If they all held 90 psi thats more than the engine itself can generate (if its stock) so id go for a drive, get everything nice and hot then redo the test and see hat happens.

[Flathead]

The cylinder pressure created when the spark plug fires will be greater than you will get from your shop air compressor.