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Few compression test questions

pearljam724

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These are few questions directed to those that have maybe rebuilt or worked on more than one LA engine small block.
What is generally the largest difference you would want to see performing a dry vs. wet compression test ? I understand that it’s important that each cylinder be within 20 psi give or take. But what about dry vs. wet ?
I understand LA factory engines are pretty low compression engines. Having said that, what is a minimal psi a good condition La engine would be ? My understanding is any reading over 100 and all cylinders being within 20-25 psi of one another is important. A better question might be. What is compression psi on a freshly rebuilt La engine that hasn’t had the compression bumped up from stock ?
Lastly. Should an engine with a mildly larger cam have a little more compression than a stock cam. A slightly larger cam with stock heads, stock pistons.
 
seat of the pants its been i think 110 -130 for the smoggers and 150 -180 for the hicomps.
this is wet and based on memories...id be interested to know the differences dry.

the second question we need to know which stocker you have,
a smogger or an earlier hicomp,late 60s 318 230hp etc etc.
plus whats the cams operating range?
 
72-360. First production year. Im not sure that year is a smog engine or not. Unknown cam. My over all compression is good I believe based on LA model with only a small cam modification. Everything else is stock related to compression. Stock matching numbers heads, etc. But, a couple cylinders might be showing slight wear due to dry vs. wet. I’m not sure how acceptable the differences are. But, overall no cylinder tested under 115 dry. My logic is, a lower compression ratio engine should have lower compression psi compared to an engine that has a higher compression ratio. But, that is theory.

D0AFF222-1A57-4C47-800B-B24A22EC1147.jpeg
 
one of these other guys is gonna know exactly what percentage is bad but i suspect those are nearing the edge.
cylinder 6 and 8 is the worst and also in line.
all the cylinders show various readings from loss of 5 -thru-35lbs of compression from wet to dry.
id say the motor is ready for a rering or rebuild or whatever you have planned.
just my opinion.
 
The only time I ever did a wet reading was to determine if a cylinder with a very low reading was the result of bad rings or a bad valve. If adding some oil brought the compression up, it was a bad ring. No change in compression was a bad valve.

Now doing a compression test, cold verses warm is what I'm used to doing. The thing is by time you get all of the plugs out, especially if you have headers, the block is cooling down. And by the time you get to the last cylinder it's cooler yet.

Your "dry" readings are really close to one another. I don't know how accurate taking a "wet" reading is between cylinders if they wouldn't get coated the same.

Are you having a power loss or blowing smoke? Are you doing the test with the throttle plate wide open?
 
When I did compression test on my 68 340 (in a 72 RR), I only did a dry test and got these numbers
1 -- 170
2 -- 168
3 -- 168
4 -- 160
5 -- 167
6 -- 159
7 -- 165
8 --- 151
I then did a leakdown test and 8 (16% leak), 4 (12.5%), 5 (11.25%) and 7 (11.25%) were all over 10%. The other cylinders were under 5%. So I had compression on three cylinders that were within 10 points of my max (170), but were still above 10% on leakdown (which is no good). I pulled motor and had it refreshed. But I think you need to do a leakdown test to get a true read. From my experience, the compression numbers can be high and relatively close to each other and still not have a truly tight engine.
 
The only time I ever did a wet reading was to determine if a cylinder with a very low reading was the result of bad rings or a bad valve. If adding some oil brought the compression up, it was a bad ring. No change in compression was a bad valve.

Now doing a compression test, cold verses warm is what I'm used to doing. The thing is by time you get all of the plugs out, especially if you have headers, the block is cooling down. And by the time you get to the last cylinder it's cooler yet.

Your "dry" readings are really close to one another. I don't know how accurate taking a "wet" reading is between cylinders if they wouldn't get coated the same.

Are you having a power loss or blowing smoke? Are you doing the test with the throttle plate wide open?
No problems. Engine runs very well. Yes, throttle open during cranking. No blowing smoke etc. was just curious as to where the compression was. Yeah, I agree on the engine cooling down. It takes about an hour to do a dry and wet test. By that time, four remaining cylinders are completely cooled off by the time you get to them.
 
When I did compression test on my 68 340 (in a 72 RR), I only did a dry test and got these numbers
1 -- 170
2 -- 168
3 -- 168
4 -- 160
5 -- 167
6 -- 159
7 -- 165
8 --- 151
I then did a leakdown test and 8 (16% leak), 4 (12.5%), 5 (11.25%) and 7 (11.25%) were all over 10%. The other cylinders were under 5%. So I had compression on three cylinders that were within 10 points of my max (170), but were still above 10% on leakdown (which is no good). I pulled motor and had it refreshed. But I think you need to do a leakdown test to get a true read. From my experience, the compression numbers can be high and relatively close to each other and still not have a truly tight engine.
Do you know your compression ratio on that engine ?
 
Many years ago I had a stock 77 360 that I did a compression test on all were around 165 PSI.
The engine had around 150,000 miles on it at the time.
It was a very good runner.
You may want to do a check on your gauge against a 100 PSI air supply, just to verify it's accuracy.
Or try another compression tester to be sure.
Otherwise you could possibly be considering a rebuild based on false information.
 
I'm sure your's runs real well.
Not to be critical but I've seen some pretty wiped out motors which ran smoothly.
We had a 283 once that was real smooth, but "couldn't pull itself out of a paper bag".
But that one also used a quart every 500 miles which is almost always a pretty good indicator of wear with older engines.
 
I'm sure your's runs real well.
Not to be critical but I've seen some pretty wiped out motors which ran smoothly.
We had a 283 once that was real smooth, but "couldn't pull itself out of a paper bag".
But that one also used a quart every 500 miles which is almost always a pretty good indicator of wear with older engines.
ive seen them so bad we could take the cam bearings out by hand,and the timing chain too.
still ran smooth and a little power.
 
When I did compression test on my 68 340 (in a 72 RR), I only did a dry test and got these numbers
1 -- 170
2 -- 168
3 -- 168
4 -- 160
5 -- 167
6 -- 159
7 -- 165
8 --- 151
I then did a leakdown test and 8 (16% leak), 4 (12.5%), 5 (11.25%) and 7 (11.25%) were all over 10%. The other cylinders were under 5%. So I had compression on three cylinders that were within 10 points of my max (170), but were still above 10% on leakdown (which is no good). I pulled motor and had it refreshed. But I think you need to do a leakdown test to get a true read. From my experience, the compression numbers can be high and relatively close to each other and still not have a truly tight engine.
It also depends on what ring package you have. I have this discussion with Mahle engine components, i use their pistons and metric plasma moly ring package and i have 13% leak down exactly on all 8 cylinders after 400 mile in my newly built 493 stroker, but it also show 200 psi on all 8 cylinders dry and cold engine with a compression test and no change with oil in. They say they use leak down numbers as secondary information especially on a metric ring pack that relies on gas preassure for sealing rather than mechanical tension. He says my compression test numbers is really great with my camshaft(231/237@050) and my engine is as good it can be. My plugs looks great and no oil consum or blow by what so ever, the engine runs great with huge power. Just something to think about when people say the leak down test is the only way to go.
20200306_195007.jpg
20200306_195007.jpg
 
Dry readings all within 5 psi. Those readings wouldn't scare me at all. Run it.
Doug
 
145 psi factory so looking at tests u did it indicates a well worn engine.
 
as to your other question
larger cam with later intake closing usually results in lower compression
but with the factory cams and the long closing ramps
you can have a bigger cam that closes earlier
 
Well worn engines seldom wear to equal compression numbers. The number itself is subjective. Cranking speed, cam both are big contributors. Equal numbers mean its happy.
Doug
 
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