LOWER (not low) compression on one cylinder
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Joined: Sep 2024
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From: Richmond, Virginia
LOWER (not low) compression on one cylinder
So my 1980 L48 engine compression test shows driver side is consistently 160psi, the passenger side has one cylinder (6) has 165psi but the others (2,8) are in the 150's with number (4) being 140psi. So I don't think 140 is low enough to say cylinder (4) is bad but (I think 120psi or less is bad) but should I worry? I am about to put a CAM in her and really don't want to remove the head but I could while I am working on her but want some input first.
Thanks in advance.
Ken
Thanks in advance.
Ken
Burning Brakes
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I’d have to say it depends entirely on you and your goals for the car.
Cylinder 4 probably has worn rings, or it could be valve related.
Either way, more camshaft is not going to help the situation. A leak down test would give you more information as well. do you have a borescope camera? You could take a peak inside the spark plug hole.
if your goal is just drive, have fun, fix it when it breaks, then go for it.
a bigger cam is not going to suddenly grenade your engine(in my opinion)
Cylinder 4 probably has worn rings, or it could be valve related.
Either way, more camshaft is not going to help the situation. A leak down test would give you more information as well. do you have a borescope camera? You could take a peak inside the spark plug hole.
if your goal is just drive, have fun, fix it when it breaks, then go for it.
a bigger cam is not going to suddenly grenade your engine(in my opinion)
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Are you pulling the engine to do the cam swap or doing it in place....if you are going to pull the engine, do it all at the same time, bearings rings, hone, etc. If you want more power then get a flat top or domed piston while you are at it and save up for heads next year
1967 Pedal Car Champion
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Common advice is that the difference between the high and low should not be more than 10%. With 160-165 that would mean under 144-148 is out of normal variability. A leak-down test would help isolate where compression is being lost. (Do not get the Harbor Freight tester - it's inaccurate.) You could have broken rings from detonation in #4 and if that's the case, best to find it now.
Safety Car
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From: Fairfax Virginia
I agree with 67:72 on this one!
I would get someone to do a good Wet and Dry Compression test with battery, fully charged on a charger with the carburetor wired wide-open. The Leak Down test can probably show you where the leak is occurring inside the engine. When done carefully the Leak Down is one of the better tests that you can do on an engine as it can show specific issues.
Can you stick something in there and look at the condition of the cylinder. A good visual always helps. Have you tried to do the same test with some oil in the cylinders first? The wet side tells a bit and is very useful.
I would get someone to do a good Wet and Dry Compression test with battery, fully charged on a charger with the carburetor wired wide-open. The Leak Down test can probably show you where the leak is occurring inside the engine. When done carefully the Leak Down is one of the better tests that you can do on an engine as it can show specific issues.
Can you stick something in there and look at the condition of the cylinder. A good visual always helps. Have you tried to do the same test with some oil in the cylinders first? The wet side tells a bit and is very useful.
Thread Starter
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From: Richmond, Virginia
Update
Performed a WET test and determined the rings are causing the lower compression. Since the compression is 140psi and no lower than 120psi and the vette doesn't smoke I am going to resist the temptation of yanking the engine and just slap the Doug's header/sidepipes and CAM in her and go.
I want to thank all of the supporters who came to this link and answered my questions. Great group here.
I want to thank all of the supporters who came to this link and answered my questions. Great group here.
Old Pro Solo Guy
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From: Marlton NJ
And you really do not want to run much cam in a 1980 8:1 CR L48.Best to stick to 212-216* dur @ .050"
Which really isn't very big.
If you want bigger, it will get very soggy, due to low DCR, so it's better to do heads first.
I got tired of going thu all the details on how to wake up a low CR L48, so I wrote this thread:
https://www.corvetteforum.com/forums...e-numbers.html
The low compression in one cylinder could be either, weak rings, bad cam lobe, or leaking valve seat. A leak-down test would help you pinpoint that.
Which really isn't very big.
If you want bigger, it will get very soggy, due to low DCR, so it's better to do heads first.
I got tired of going thu all the details on how to wake up a low CR L48, so I wrote this thread:
https://www.corvetteforum.com/forums...e-numbers.html
The low compression in one cylinder could be either, weak rings, bad cam lobe, or leaking valve seat. A leak-down test would help you pinpoint that.
Last edited by leigh1322; Dec 16, 2024 at 12:02 PM.
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From: Richmond, Virginia
Thanks. Did a WET test and it is definitely the rings. Using a Comp Cams 268H which isn't a handful of cam and probably the most deployed corvette cam out there. I can't wait to read your link.
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What surprises me is that any of your cylinders are above 140 on a 8-1 compression engine. And also that no one else pointed this out.
Old Pro Solo Guy
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From: Marlton NJ
Yep. Good point. I would want to be sure it is rings before tearing into it. If the rings are tired, you need new ones.
If not, maybe you could get by. Big difference.
Buy a leak down tester, and it will check ring seal. Best ~$100 you'll ever spend. The wet test can fool you. The air leaking past ? will let you almost positively determine if it is rings (crankcase hissing) or valves seal (intake or exhaust hissing). Then you will know if the lower end is strong enough to add HP. Or you could even just rig up an air chuck into the spark plug hole.
Worn flat tappet cam lobes can also give you false low or high readings. Check them too.
The Comp 268H is a great choice for an old 8:1 CR engine. The original cam, not the extreme one. But I would go no larger.
If not, maybe you could get by. Big difference.
Buy a leak down tester, and it will check ring seal. Best ~$100 you'll ever spend. The wet test can fool you. The air leaking past ? will let you almost positively determine if it is rings (crankcase hissing) or valves seal (intake or exhaust hissing). Then you will know if the lower end is strong enough to add HP. Or you could even just rig up an air chuck into the spark plug hole.
Worn flat tappet cam lobes can also give you false low or high readings. Check them too.
The Comp 268H is a great choice for an old 8:1 CR engine. The original cam, not the extreme one. But I would go no larger.
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From: Virginia
I had similar compression in two LS engines. The leakdown test showed it was valves. I could hear the exhaust valve leak in one case, and see it with a borescope.
Two quick valve jobs later, compression is back where it should be. Never touched the bottom end.
Two quick valve jobs later, compression is back where it should be. Never touched the bottom end.
Safety Car
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From: Fairfax Virginia
Be sure to watch a couple good videos regarding using the Leak Down Tester. I learned to do it on a Cessna Aircraft and it is an awesome test to perform regularly in order to know when the engine needs to be over hauled. I was taught to use 100 psi with a larger air compressor to supply to air and then we would test each cylinder on the engine. With the results you will start to see patterns and this will help you know when you need to rebuild.
A couple Valve jobs is clearly cheaper than rebuilding the bottom end of the engine. If a particular cylinder is bad, maybe it has a broken piston or broken ring lands (which break during detonation). All you need to you drop the oil pan and pull the appropriate cylinder head then pull the piston and replace it and the rings and you are off to the races again. I have done it before and it worked just fine.
Depending on how often you will be using this tool should help you remember to buy it someplace like Harbor Freight where it shouldn't be as expensive as the ones professional mechanics would choose. The leak Down testers vary in range so don't "over-buy" unless you have an piston powered airplane next to your Corvette.
A couple Valve jobs is clearly cheaper than rebuilding the bottom end of the engine. If a particular cylinder is bad, maybe it has a broken piston or broken ring lands (which break during detonation). All you need to you drop the oil pan and pull the appropriate cylinder head then pull the piston and replace it and the rings and you are off to the races again. I have done it before and it worked just fine.
Depending on how often you will be using this tool should help you remember to buy it someplace like Harbor Freight where it shouldn't be as expensive as the ones professional mechanics would choose. The leak Down testers vary in range so don't "over-buy" unless you have an piston powered airplane next to your Corvette.
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Melting Slicks
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I sure would NOT trivialize the job of replacing a piston - "talk is cheap".
For someone who has little experience doing engine work and most likely does not possess all the required tools and "accessories" every step of the way is fraught with challenges.
Sounds easy peasy but carrying out is a whole different "ball game" and not particularly fun especially when you screw something up
For someone who has little experience doing engine work and most likely does not possess all the required tools and "accessories" every step of the way is fraught with challenges.
Sounds easy peasy but carrying out is a whole different "ball game" and not particularly fun especially when you screw something up
Last edited by QIK59; Dec 20, 2024 at 12:51 PM.
Melting Slicks
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Hey OP: quick questions for you. Is it smoking? Does it use a lot of oil? Is the power okay for the engine/combination? If you had never checked the compression would you be thinking of a partial/whole rebuild?
Sometimes the mind can play tricks (mine does) when we 'know' we have a potential problem. If the only 'problem' is slightly 'out of spec' compression readings maybe this is the proverbial mountain out of a mole hill? If it smokes, power is down, etc. then pay no attention to my stream of unconsciousness.
I had a Hurst/Olds W30 that had compression readings of 20% off high/low. Really bugged me. Until it didn't. Didn't smoke. Didn't use oil. Had good power. So I decided to just enjoy it. Drove it that way for another 5 years and then decided to check the compression again. All within about 10%.
I have another thought. If you're getting readings that high on an 8:1 engine, are you sure you don't have some cylinders that are carbon-ed up and reading higher than others? Just a fleeting thought.
Best of luck! Paul
Sometimes the mind can play tricks (mine does) when we 'know' we have a potential problem. If the only 'problem' is slightly 'out of spec' compression readings maybe this is the proverbial mountain out of a mole hill? If it smokes, power is down, etc. then pay no attention to my stream of unconsciousness.
I had a Hurst/Olds W30 that had compression readings of 20% off high/low. Really bugged me. Until it didn't. Didn't smoke. Didn't use oil. Had good power. So I decided to just enjoy it. Drove it that way for another 5 years and then decided to check the compression again. All within about 10%.
I have another thought. If you're getting readings that high on an 8:1 engine, are you sure you don't have some cylinders that are carbon-ed up and reading higher than others? Just a fleeting thought.
Best of luck! Paul
Melting Slicks
Joined: Aug 2011
Posts: 3,167
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Hey OP: quick questions for you. Is it smoking? Does it use a lot of oil? Is the power okay for the engine/combination? If you had never checked the compression would you be thinking of a partial/whole rebuild?
Sometimes the mind can play tricks (mine does) when we 'know' we have a potential problem. If the only 'problem' is slightly 'out of spec' compression readings maybe this is the proverbial mountain out of a mole hill? If it smokes, power is down, etc. then pay no attention to my stream of unconsciousness.
I had a Hurst/Olds W30 that had compression readings of 20% off high/low. Really bugged me. Until it didn't. Didn't smoke. Didn't use oil. Had good power. So I decided to just enjoy it. Drove it that way for another 5 years and then decided to check the compression again. All within about 10%.
I have another thought. If you're getting readings that high on an 8:1 engine, are you sure you don't have some cylinders that are carbon-ed up and reading higher than others? Just a fleeting thought.
Best of luck! Paul
Sometimes the mind can play tricks (mine does) when we 'know' we have a potential problem. If the only 'problem' is slightly 'out of spec' compression readings maybe this is the proverbial mountain out of a mole hill? If it smokes, power is down, etc. then pay no attention to my stream of unconsciousness.
I had a Hurst/Olds W30 that had compression readings of 20% off high/low. Really bugged me. Until it didn't. Didn't smoke. Didn't use oil. Had good power. So I decided to just enjoy it. Drove it that way for another 5 years and then decided to check the compression again. All within about 10%.
I have another thought. If you're getting readings that high on an 8:1 engine, are you sure you don't have some cylinders that are carbon-ed up and reading higher than others? Just a fleeting thought.
Best of luck! Paul
As long as the engine doesn't make any concerning sounds - drive on.
I get my engine oil analyzed once a year just for giggles - lead content comes back a bit high - bearings ? or leaded gas contamination from the previous 40 years ?
"Thread" it !! Nothing I can do anything anyway so drive on !
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Joined: Sep 2024
Posts: 82
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From: Richmond, Virginia
Hey OP: quick questions for you. Is it smoking? Does it use a lot of oil? Is the power okay for the engine/combination? If you had never checked the compression would you be thinking of a partial/whole rebuild?
Sometimes the mind can play tricks (mine does) when we 'know' we have a potential problem. If the only 'problem' is slightly 'out of spec' compression readings maybe this is the proverbial mountain out of a mole hill? If it smokes, power is down, etc. then pay no attention to my stream of unconsciousness.
I had a Hurst/Olds W30 that had compression readings of 20% off high/low. Really bugged me. Until it didn't. Didn't smoke. Didn't use oil. Had good power. So I decided to just enjoy it. Drove it that way for another 5 years and then decided to check the compression again. All within about 10%.
I have another thought. If you're getting readings that high on an 8:1 engine, are you sure you don't have some cylinders that are carbon-ed up and reading higher than others? Just a fleeting thought.
Best of luck! Paul
Sometimes the mind can play tricks (mine does) when we 'know' we have a potential problem. If the only 'problem' is slightly 'out of spec' compression readings maybe this is the proverbial mountain out of a mole hill? If it smokes, power is down, etc. then pay no attention to my stream of unconsciousness.
I had a Hurst/Olds W30 that had compression readings of 20% off high/low. Really bugged me. Until it didn't. Didn't smoke. Didn't use oil. Had good power. So I decided to just enjoy it. Drove it that way for another 5 years and then decided to check the compression again. All within about 10%.
I have another thought. If you're getting readings that high on an 8:1 engine, are you sure you don't have some cylinders that are carbon-ed up and reading higher than others? Just a fleeting thought.
Best of luck! Paul