checking compression question

On 01 Jun 2004 01:17:10 GMT, snipped-for-privacy@aol.comnojunk (Braukuche) ran around screaming and yelling:

only if you want to see *IF* it *could* build pressure...you won't be able to turn the engine over fast enough to overcome the pressure that naturally escapes past the piston ring gaps....you need to turn it over with the starter...(car in neutral, and throttle wide open) JT

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No.

In fact, the question sez you've misunderstood the purpose of a compression check.

The purpose of a compression check is to gain an approximate figure of merit for the relative wear BETWEEN the rings and the valves of a RUNNING engine. For the results to have any value you must first fulfill the basic requirements so as to impose a rough standard on the results.

The basic requirements are for the engine to be warm, in the sense of having been recently run. Then you pull all of the plugs, block the throttle fully open, screw the compression tester into a cylinder and spin the engine AT LEAST ten revolutions with the starter. You do that for each cylinder, recording the results.

That's the dry test.

Then you do it all over again except you squirt a fixed amount of oil into each cylinder before turning it over.

That's the wet test.

With the results of both tests right there on the paper you now have a relative measure of merit that gives an indication of the GENERAL condition of the rings & valves.

That's ALL the test does.

The method isn't very precise. The quality of compression gauges varies wildly. Nor is it very accurate, with different mechanics getting different readings even when testing the same engine with the same gauge.

The real joke is the fantastic conclusions some people draw from the data when in fact the only conclusion you may draw with a reasonable level of assurance is that if a reading more than 25% below the average of the other cylinders, then you MAY have a problem.

A leak-down test provides more definitive information but it too assumes a running engine. With a leak-down test your are comparing the engine to a known standard (ie, the inlet air pressure). With a compression tester the best you can do is look for gross differences between the cylinders. If all of the cylinders have the same amount of wear & leakage a compression test is relatively useless unless you know the history of the engine and its compression ratio.

-Bob Hoover

PS -- A compression tester uses the engine to literally 'pump up' the section of hose between the gauge and the fitting in the spark plug hole, a Schrader valve being fitted to allow the pressure to be released for the next text.

Instead of specifying a given number of revolutions, some manuals say to simply spin the engine until there is no change in the indicated pressure.

... and

Wouldn't performing either test on a recently run engine ensure that the results from the first to last cylinder tested are not comparable? From the moment it is shut down each component is colling down and will be at a different temp during each cylinder's test. This would also be the reason valve lash is set with the engine cold instead of at operating temp.

Max

Perhaps "less" comparable. That said, I suppose one could test all 4 at once with 4 gauges, if your starter would handle the load! For the comparison that needs to be made, probably the most important aspect of it is to do the comp checks on a regular basis like every spring and every fall and to duplicate procedures as closely as possible each time. That way you have a true reference point in case you feel something has gone amiss as well as keeping track of the normal wear of top end components. I have never done leak-down tests but obviously I should have been doing them!

Several years back I needed a Datsun 1600 for my pickup. I located one in a wrecking yard. It ran and sounded pretty good. I asked for a compression check even though there was no audible loping. The guy helping me grabbed the compression gauge and headed back out to do the test. He held the gauge very close like he was preventing me from seeing it, so of course I made every effort to catch a glimpse... They had removed the glass so they could position the needle anywhere they wanted! He was sure surprised when that engine had 270psi cranking compression! I was not interested in how high the number was but in how close together the four of them were. They were close. I bought it. I still laugh about that gauge. -BH

the sense of

block the throttle

but it too assumes

comparing the engine to

ensure that the

comparable? From

will be at a

also be the reason

operating temp.

Well, if it can spin the engine for 5 seconds or so, that's enough.

I've now done several leak-down tests on the new engine (all cold). The first one was on the long-block, the next following the initial

20-minute run and two more at 100 and 500 miles. #1,3 & 4 have all settled at 3-5%. #2 is still at 10% for some reason.

Max