Anyone ever do a leak down test on their cylinders? I am getting conflicting info on how to interpret the results.

One school of thought is the amount of difference between your two gauges isn't a big deal, so long as all cylinders are within a few percent of each other, while the other thing I've read is that you should see no more than 10% lose between your two gauges.

I tested my engine I'm working on, and with 72 lbs going in, 3 of 4 cylinders are reading in the 48 to 52 range. While the 4th is only 40 and is obviously leaking in the exhaust port. Not even sure what to do about #4 cylinder at this point, but at the same time I'm wondering about the other 3, the plus is they are pretty consistent, but if I'm only suppose to see a 10% loss then the whole thing sucks.

Thoughts?

I am not sure for the 4 cylinder but all those numbers seem plenty low. My 2.8 was 135 compression across all of them. I believe the rule is no more than 10% difference in cylinders

First do compression dry test then maybe wet test.

87 FSM:

quote COMPRESSION TEST ! Important Disconnect the "BAT." terminal from the HEI distributor or ignition module. To determine if the valves or pistons are at fault, a test should be made to determine the cylinder compression pressure. When checking cylinder compression, the throttle and choke should be open, all spark plugs removed, and the battery at or near full charge. The lowest reading cylinder should not be less than 70% of the highest and no cylinder reading should be less than 689 kPa (100 PSI). This should be done with four "puffs" per cylinder. Normal - Compression builds up quickly and evenly to specified compression on each cylinder. Piston Rings - Compression low on first stroke, tends to build up on following strokes, but does not reach normal. Improves considerably with addition of oil. Valves - Low on first stroke, does not tend to build up on following strokes. Does not improve much with addition of oil. [For wet testing] Use approximately three squirts from a plunger type oiler.

Closer the numbers the better. Many books allow 15-20psi range for top cyl to bottom cyl.

Percentage Change

To get close to "normal" compression specs... (I think) 15 x compression ratio (V6=8.9, L4=9) − 15.
15 is close to Absolute(atmospheric) pressure at Sea Level. Real values depend on local weather and altitude.
− 15 is because most gauges do not use Absolute pressure. They zero at normal atmospheric pressures.

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[This message has been edited by theogre (edited 04-20-2015).]

Riddick,
I am doing a leak down test, not a compression test. Engine is not in the car. I am trying to check it's "health " before putting it in. Compression test to come later.

quote Originally posted by edfiero: I am doing a leak down test, not a compression test. Engine is not in the car. I am trying to check it's "health " before putting it in. Compression test to come later.

Note that a non-running engine can cause weird/false result for many tests because rings are very to totally dry, engine have a bit of dirt/rust issues, etc, and comparing # is very hard or impossible. Ideally you want all cyl to get close to same #, say ~20% for example. If all cyls leakdown get a passing test then is likely best you get.

All you need is any flywheel w/ good teeth, starter and whenever to hold that in place for compression testing.

directions leakdown test
https://mobiloil.com/en/art...o-do-a-leakdown-test
https://www.youtube.com/watch?v=ofSiTGeLXvc is one ok video to watch.

How it works (wiki) http://en.wikipedia.org/wiki/Leak-down_tester