My depression compression test

Well since I've got the motor out of the car I decided it would be easy to do a compression test. I've read in past posts about wet and dry compression test. Anyways this is how I did mine.

The motor hasn't ran in about 3 weeks. I took all the plugs out and inserted the compression tester in the head one at a time.
I jumped the stater and let the motor spin about 5 times. heare are the results.

#1 - 105
#2 - 90
#3 - 103
#4 - 115

So did I do the test right or should I have added oil to each cylinder first?

Could it be that the motor hasn't broken in yet? FYI -I recently took the heads off and freshened up the gaskets and looked at the motor. I have no history on the motor.

Any help appreciated.
TIA

[J P Stein]

The only thing missing from your description was : "I had the throttle wide open".

If you didn't have it open, the test results are bogus.

[Joe Ricard]

I was always under the impression that you get more consistent results with a warm motor. I f you had the cylinders off it is quite possible that the rings have not gotten thier seat yet. Did you do anything to the heads. ie. lap the valves?

[Va914]

Can you run the engine on the ground (out of the car)? I have been wondering this myself, so maybe someone will chime in here.

This may sound WT but if you were to set the engine in a old tire (for vibration reasons) you could get the engine up to a nice temp before the tire started to melt.

[ejm914]

JP and Joe are both correct. Ideally a compression test is done at normal cranking speed on a warm engine with the throttle fully open. If the test reveals there may be a problem a "wet" test is sometimes done. This involves squirting some oil into the cylinders to help the rings seal and repeating the test. This test is of limited value on a flat engine, since the oil will run to the bottom of the cylinder. A better test is a cylinder leakdown test. The piston is brought to TDC and compressed air is put in though the spark plug hole. A pair of gauges on the tester indicates the psi going in and how much the cylinder retains. From this you calculate the percentage of leakage. During the leakdown test you get a good idea of where the problem is, you can hear the air escaping through the valves or into the crankcase. Under the right conditions Shawn's compression numbers would be low with too much variation between cylinders.

It's quite possible (and sometimes exciting)to run an engine out of the car. It's easier with a carbed motor, FI is a bit more involved. Plan on the motor moving around, especially when you rev. it and keep a fire extinguisher handy.

Yeah, the whole intake was off, so Throttle wide open is an understatement. BTW Thanks JP for the tip about the distributor clamp - makes good sense.

I did lap the valves and basically just remove all the carbon and oil buildup.

Well, I've throw too much cash and time in this thing to fool with rebuilding it right now. I need to drive it for a few months just to get enough motivation to spend more money. Let's say that the compression test is correct, what could it do to my motor besides just run with less hp? I'm assuming that it would put unnecessary stress on the bottom end - but besides that-- what would could it do?

Thanks-

[Dave at Pelican Parts]

A few things: I don't know what altitude you're at. I'm guessing Alabama would be close to sea level, but if you're on top of a really tall (for AL) mountain, you would expect lower numbers than the "typical" ones you'll see listed.

Second, it is possible that the valves haven't completely worn in yet--but unlikely if you've done a good job of lapping them in. You didn't say you re-ringed it, so you aren't seeing blow-by from rings that haven't broken in yet.

So you're probably seeing blow-by from a tired, nearly worn-out motor. 100 PSI (at sea level) is very low, right near the ragged edge of "rebuild it now". A nearly 25% variation between your highest and lowest cylinders is usually a symptom of a lot of wear.

You should expect low cylinder pressures and relatively low power. You should also expect lots of blow-by, so you may have a bunch of oil vapor coming out of your breather. And if the top end is this tired, chances are the bottom end is also pretty tired, so if drive on this for a few more years you may spin a bearing or some such. Finally, very worn rings can score the cylinders beyond saving, or can cause the pistons to break, which can be very bad for the rest of the engine.

Risks of this are probably relatively low, but they are there. I wouldn't drive it for years like that...

--DD

[mejulihn]

Geez, Dave -- don't sugarcoat it. Just tell it to him straight!

[Dave at Pelican Parts]

I don't want to be real doom-and-gloom... It is possible that everything will get better when you run the car. Or it's possible that the battery was down on charge, or the connections through the starter weren't as solid as they could have been, so the starter may have been cranking slowly.

But it's also possible that this motor is still really tired...

--DD

DD-
It's 500 feet above the beach where I'm at. Shelby county - sinkhole capitol of the world. The starter does crank really slow - this might be the reason for the low compression - well maybe.

I hooked my battery to the starter and the ground strap. Then I shorted the starter out to crank the motor. I've read in past posts that people say to check your ground strap for a bad ground. Can the ground strap go bad without it tearing? I took mine and cleaned it up. It's probably the original one. Can the strap get resistance in it over time and need replacing?

I appreciate the California-Sunshine-State-got-Flowers-in-your-hair version instead of the doom-and-gloom one.

[karlp]

I get the feeling that you were looking for reassurance that your comp. readings were O.K., but it sounds like you need a warm engine to test
I just finished putting rings in mine and ran it on the bench. I stacked three cinder blocks under each side of the front mounting bar, put a 2X6 on top of the cinder blocks and ran lag bolts through the mounting bar holes into the 2X6. I stacked three more under the tranmission end and strapped the tail of the trans. to the top one.
The motor is carbed so fuel was not a problem. Gas tank out of the lawn mower. Electrical is straight forward.
I didn't bother checking compression warm anyway, because the rings are new and I can't run it this way enough to seat them. Besides, regardless of readings, it is going back in because it sounds good. New rings cold read 110-105 on all 4.
If you have the time and inclination, I would set up a way to run it out of the car. It sure is easy to adjust valves and look for oil leaks and fix tin and all that. Besides, It's real cool to show your buddies.

Karl