compression test

[mikey]

Just did a compression test on my '70, 1.7 liter, and all cylinders came out around 115 lbs. Is this low, high, or in between for a 1.7 liter??? I am not blowing smoke, so I am not too worried. I also ran the test with a warm engine. I was told it should be done with a hot engine, but dont you risk stripping head threads when pulling plugs out of a hot engine?? Also, is there much variance between compression gauges? I used a screw in type.

Mike

[oredith]

115 sounds low (but don't quote me, wait for someone like -DD's answer). i think they're suppose to be 130 and up, not sure if that's for a 2.0 or all the motors. screw in is usually a bit more accurate since it's a bit hard to get the "hold-in" types to seat correctly through the engine sheetmetal. are you sure you're not blowing smoke?, or just not smoking at idle?.. mine's got similar compression, and i blow smoke when i accel hard, and when i let up the throttle at above 4k.

[BenI]

Ive heard that, due to variables such as gauge calibration, engine temp, outside air pressure..., the important thing to look for when comp testing is standardization between cylinder readings. If your engine feels strong and all the cyls are within 5psi of eachother, Id say your good.

[Germain]

Thats true, what Beni said. Of course if there all at 50 instead of 120 or what have that would prob be bad. What you looking for is all of them to be close to spec +/- 10 percent, and then all of them with in 10 percent of that.

So for example
spec is 120, 10% is 12 so 108-132 is acceptable (never heard anyone complain about high compression before though )

No say the readings are 110, 102, 108, 105. The 110 and 108 fall in the 10% of spec, and the 102 and 105 are within 10 percent of them. If these numbers match up thats more important then falling on or above spec.

I would say if those were the numbers I got on my 2.0l I would think the engine is getting tired and it is about time to think of a rebuild. If one of those numbers was in the 90's, I would park the car and do my rebuild now.

Well, thats my interpretation on the whole issue.

[JP Noonan]

Considering the difference in gauges, accounting for sea level, and the engine must be warm with the battery fully charged all the plugs out and the throttle stuck wide open...your numbers sound fine for a high milage engine.

What they tell you is all the cylinders are worn about the same so there are no broken rings, burnt valves, or dropped seats. If a cylinder ever drops below 70-90 the valve is probably burnt and about to drop a seat. I have had a motor with 70 that turned out to be rust pitting on the cylinder though.

Another thing about comp testing. Use the same number of revolutions per cylinder. the cylinders should not only match in max compression but should "pump-up" in about the same incriments. something like 40 lbs first stroke, 70 2nd, 90 3rd, then slower increment until it stops. A bad cylinder, pumped enough, can come close to a good one. 5-8 compression strokes is good I think.

Two motors I have written down are a fairly fresh 2.0L with 145-150 and a tired 150,000+ mile 1.7 with 115-120-110-70.

[Jim Fraser]

Hmmm....I have a '72 1.7l. Did the compression test, all plugs out, engine warmed up, 10 revs per cylinder. I forgot one thing though...to open the throttle valve!! DOH! My readings were 122, 133, 135 and 125, cylinders 1 thru 4. Will opening my throttle valves change my readings much? I guess from these numbers everything is fine though. I just wanted a bench mark.

Jim 'too much time in the sun' Fraser

[John Rogers]

Ten cranks should give a pretty close reading even with the throttle shut and the engine sucking through it. The reading would go a little higher so you might want to try it again to see what the reading will do.

[AlexO]

I'm glad this topic came up. I did a compression test on my '74 2.0 a while back. Readings were in the 110 range after 4-5 cranks, plugs out, throttle open but on a cold engine. The readings were even but I was curious how much they should go up on a warm engine.

[mikey]

with a warm engine, the readings should go down......right??? The metal heats up and expands, and parts fit looser. Compression slips by the rings easier, as well as any expanded gap.

[JP Noonan]

Well... The metals expand, but the Jugs are cast iron so they expand less then the steel rings and much less then the aluminium pistons. The tolerences get smaller the hotter the motor. This is also why so many of the bolts on air-cooled motors are so low. Head bolts on a Type-4 are 22-23 ft lbs vs. a Ford 2.3L of 80 ft lbs. The alum VW motor expand so much that the torque at operating temp is equal to the Ford. So if you over torque a VW head 5 lbs cold it may be 20 lbs overtorqued at temp causing the stud to pull from the case (numbers are approx, not sure what the thermal expansion numbers are, but you get the idea).

This raises another problem, the aluminum threads of the head expand into the steel threads of the plugs when hot. So removing hot plugs makes it easier to strip the plug hole in the head. Anti-seize compounds help but I've read can make the plug run hotter because it is insulated from the head and can cause grounding problems.

Catch 22. To get useable readings the motor needs to be warm. A warm engine strips plugs easier.

If the motor is cool enough to work on than it should be cool enough to reduce the striping. Like I said before if a plug starts to get tight on the way out you are starting to strip the threads. Screw it back in and out a few times until it isn't hard to remove. This helps but is no guarentee.


BTW anyone heard about using copper based anti-seize vs the usual lead based stuff? I've heard the lead stuff is bad, but what about the copper?