compression

[opitica]

i was just wondering how do you check compression

[sracing]

With a compression tester, or better yet a Leakdown gauge set. However with a leakdown gauge set you need an air compressor.

With a compression checker, you open the throttle and crank the engine with the tester in a sparkplug hole . (make sure the ignition system is crippled so the engine doesn't start.) The readings indicate the relative merit of each cylinder.

With a leakdown, you lock the engine and make sure the valves are closed on the cylinder you are checking. Then air is pumped into the cylinder while the gauge set indicate the amount of leakage.

A leakdown test is the better method.

Jim
SR Racing

[duplox]

Neither is really 'better', as they check completely different things. They're somewhat related, but not really. Leakdown checks ring seal and valve sealing, compression checks compression.

[sracing]

Originally Posted by duplox

Neither is really 'better', as they check completely different things. They're somewhat related, but not really. Leakdown checks ring seal and valve sealing, compression checks compression.

What? Are you following me around? <g>

No, both do identical things in a different way. They both check compression. (However it IS much easier to determine what the problem is when using a leakdown tester. IE. intake valve vs exhaust valve, vs rings, vs a scored cylinder, etc.

Can you cite an example of what could be done with a compression checker that can't be done with a leak down tester and better? Except for possibly a broken crankshaft? <g>

The problem with the compression tester is that there are many variables. Crank speed, condition of the battery, throttle position, number of compression strokes that are done during the test. (it will go up each time). So it's OK for a relative comparison test between cylinders ONLY. EG. If one reads 75 and the others read 110, you can assume you have a problem in the low one.

With a leakdown test you are dealing with absolute values and you are checking all the above.

But COMPRESSION is what you are checking.

BTW, we have a compression checkers here at the shop, but never use them except to make a very quick and dirty test on an engine in the car that seems to have one or more cylinders not producing well.

For serious diagnostics you have to use leakdown testers.

Now stop being argumentive and read a book...<g>

Jim
SR Racing

[curtis73]

Do you mean, "find the compression ratio," or do you mean "test to see if a used engine still has good compression" ?