Is my compression acceptable?

[KDMatt]

Also, I'm assuming that the "engine warm" requirement has to do with getting oil in the cylinders (a "wet" condition). This will yield a different result from a dry one, where the oil pump has not gotten oil to the rings to help seal. I've heard of people squirting light oil into the plug holes to simulate this prior to the compression test.

Squirting oil into the cylinders is actually a very good way to determine if your rings are worn out. The theory being that if they're worn, the compression well go up as they meet oil.

I believe the engine warm requirement has a bit more to do with the fact that the engine will have expanded on a microscopic level, allowing for a better seal between the rings and cylinder walls.

[d240zx2]

Hey, Matt. Glad to see you around again.

[BrianL1987]

Thanks for all the advice(s). I checked compression again with all plugs out, engine warm & throttle pedal to floor. Now my results are:150-160 all cylinders. I suspect that this is closer to acceptable. A shot of WD40 into cylinders made no difference (motor oil most likely would have).

[ajmcforester]

Thanks for all the advice(s). I checked compression again with all plugs out, engine warm & throttle pedal to floor. Now my results are:150-160 all cylinders. I suspect that this is closer to acceptable. A shot of WD40 into cylinders made no difference (motor oil most likely would have).

I think you would be OK for now , but I would keep close tabs since you a little on the low end.

[Sailor Bob]

I learned the hard way that compression readings need to be considered with a number of other issues, the smoking from the tailpipe is a biggie. If there is alot of blue smoke (doesnt' matter why) then there is oil in your cylinders and that will significantly alter your readings in the + range.

My case I was hoping it was caused by valve seals so I wouldn't have to deal with bottom end work. Took the time to do the seals myself (great learning experience) to no availe - still had to do the bottom end. Compression was excellent due to oil in the cylinders - I now guess.

Long story short - didn't feel like spending the money to repair the L24 so I bought an old zx and rebuilt the L28.

[dclamm]

Interesting point Sailer Bob. I have compression readings around 170 (except one at 135, valve issue I think), and it blows allot of blue smoke all the time (idle, rev, constant rev, off throttle). I'm still looking at other issues (stuck/gunked up oil rings esp, also the ATF I had to put in the cylinders to unstick the valves after 16 years sitting). But could explain why the compression is decent (except one) and still shows telltale signs of worn rings...

[ajmcforester]

Interesting point Sailer Bob. I have compression readings around 170 (except one at 135, valve issue I think), and it blows allot of blue smoke all the time (idle, rev, constant rev, off throttle). I'm still looking at other issues (stuck/gunked up oil rings esp, also the ATF I had to put in the cylinders to unstick the valves after 16 years sitting). But could explain why the compression is decent (except one) and still shows telltale signs of worn rings...

I think you might need to take the head off and take a peak on what is going on. A frozen ring can cause a lot of damage so I'd look into that soon.

[Sailor Bob]

dclamm - went through all that, seafoam, wynns, ATF befor I did the stem seals. Still had to do a major job. Sometimes the long route is the shortest way.

Up side is I now have a good engine - virtually the same a new with more hp.

[StinkyCheese]

I did a compression test today on my '73. I didn't know I had to keep the throttle wide open... I got around 150 on all 6 cylinders. I know the manual says you want 170+, but I assume a 40 year old moter losing 20psi is pretty good.. I don't want to repeat the test (it was easy, but I'm lazy, what can I say..). Should I feel good about my 150? I have performance issues (hard to start, hesitates when accelerating from a dead stop, hesitates at around 4000 rpm if I'm stomping on it, back fires, etc, etc). But I don't think 150 compression is to blame, do you?

[esprist]

Compression test should be done "dry" and "wet"

Please make sure that higher compression does not simply mean it it good. It could loose some compression through rings or valves, but it might gain some from carbon biuilt up on the top of pistons.

Now what is more important is "variation" between cylinders should be within 10-15%.

After compression test is conducted and have some doubt, and then try leak-down test. Leak down test is the easiest and the most accurate test you can conduct on your engine.

By the way, compression test procedure is following:

1. Engine at NPT (Normal Operating Temperture

2. All the spark plugs out.

3. Battery fully charged

4. Keep throttle at WOT (Wide Open Throttle) position.(This cuts off fuel on Carb cars) (if it is a fuel injected car, remove the fuel pump relay or fuse)

5. crank 4-6 times for dry testing

6. Squirt oil into cylinder from spark plug each hole you are tesing and crank. (Wet test) This tell you if compression ring is good or bad.

Reading should be about 10-15% higher than reading from dry test. If reading goes way up, then the rings are worn.

If there is no change in # in wet test, but compression is lower than spec. Then check valve operation

**By the way, the valves should be properly adjusted, too.

[71Nissan240Z]

mine is between 180-190psi with the f54 p79 head mild cam

[Wally]

 

accident

 

 

Edited November 11, 2021 by Wally