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Old 04-22-2012, 04:22 AM
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wet compression test V8 oil sealing

My question is when performing a wet compression test with a spoon full of oil placed in the cylinders of a V8 engine wouldnt the oil just sit in the lower side of the cylinder and not help seal the full circumfrence of the cylinder ?
Also how much oil is too much or not enough to put in ?
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Old 04-22-2012, 05:08 AM
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Quote:
Originally Posted by theg
My question is when performing a wet compression test with a spoon full of oil placed in the cylinders of a V8 engine wouldnt the oil just sit in the lower side of the cylinder and not help seal the full circumfrence of the cylinder ?
Also how much oil is too much or not enough to put in ?

Do a leak down test at TDC as the will have more merit then a compression test
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Old 04-22-2012, 05:36 AM
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If you want to do a leak down or compression test get the engine and oil up to operating temp and then run your test. Don't put any oil in the cylinder. And a leakdown test is better for showing wear if you can do one.
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Old 04-22-2012, 07:31 AM
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Compression tests are useful in comparing one cylinder to another during the "diagnostic" phase. They're also useful to determine how the cam events are affecting performance, particularly at low engine speed.

Leakage tests are more specific and in some ways, more "accurate" than compression tests. FAA requires a leakage test every 100 hours. While it's "procedure" to do a leakage test at TDC, I've found moving the piston down the cylinder can unveil other problems not found at the "top". In one case, the piston pin had "walked", grooving the wall. At TDC, it was less than 5%. 1/2 way down, 90%.

When you squirt (with a small oil can, "pump type") oil into the cylinder and crank it, the pressure inside the cylinder will very nearly perfectly distribute the oil around the rings. It's important to do the "dry" test first, in order for the "wet" test to have meaning. If compression comes "up" more than 10-15%, it clearly shows the rings are worn.

A compression test will also give a clear "picture" of cylinder pressure and dynamic compression. A larger (overlap) cam and/or a lower static compression ratio will yield a lower reading. Readings in the 150-160 range are desirable for a "pump gas" engine with iron heads (typically).

There are many tools in our "arsenal" to determine what's "what" in an engine. Both of these are useful when the proper steps are taken and the data is correctly interpretted.

"Rule of thumb" is all cylinders should be within 10% of each other and in the "green" range for a "healthy" engine. A "dead" cylinder is usually a valve or a badly "burned" piston (a "gaping" hole somewhere). "Low" readings come from normal wear.

FWIW

Jim
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Old 04-22-2012, 06:02 PM
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Quote:
Originally Posted by Mr. P-Body

When you squirt (with a small oil can, "pump type") oil into the cylinder and crank it, the pressure inside the cylinder will very nearly perfectly distribute the oil around the rings. It's important to do the "dry" test first, in order for the "wet" test to have meaning. If compression comes "up" more than 10-15%, it clearly shows the rings are worn.



"Rule of thumb" is all cylinders should be within 10% of each other and in the "green" range for a "healthy" engine. A "dead" cylinder is usually a valve or a badly "burned" piston (a "gaping" hole somewhere). "Low" readings come from normal wear.

FWIW

Jim

Great info Jim . Thanks for your help

I found 2 cylinders 7 and 8 on a rover 3.5 v8 to be down on compression (reading 105 compared to 145 for the rest and am getting ready to do the wet test to try and identify either the valves or rings .
Unfortunately I dont have leak down test equipment .
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