Compression Test
Thread Starter
Pro
Joined: Apr 2012
Posts: 534
Likes: 21
From: PA
Compression Test
Just took a compression test on a 66 327/350hp, we are having trouble with our rebuilt motor with about 1,2000 miles on it. The motor smokes and we are trying to figure out whats making it do that. I might add this is the second rebuilt in the same amount of miles and it's doing the same thing. We took a compression test this morning and here are the results: We did not take a compression test on the first one. Here are the results:
c-1 160 c-2 170
c-3 155 c-4 170
c-5 160 c-6 150
c-7 150 c-8 150
Would you consider these readings correct for that motor, or is something not right with the internals? My main concern is the 20 lb and the 15 lb difference in some of the cylinders or is exceptable? All responses are welcome Thank you
c-1 160 c-2 170
c-3 155 c-4 170
c-5 160 c-6 150
c-7 150 c-8 150
Would you consider these readings correct for that motor, or is something not right with the internals? My main concern is the 20 lb and the 15 lb difference in some of the cylinders or is exceptable? All responses are welcome Thank you
Last edited by maggiesadie8525; Aug 29, 2013 at 11:01 AM.
Team Owner
Joined: Mar 2002
Posts: 44,642
Likes: 1,827
From: Huntsville AL
Not sure what the "official" reading should be from a +/- standpoint but those numbers look fine to me.
Found this;
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.
Found this;
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.
Last edited by Mr D.; Aug 29, 2013 at 11:12 AM.
Safety Car
Joined: May 2007
Posts: 3,677
Likes: 646
From: Indiana
2025 c3 ('68-'73) of the Year Finalist - Unmodified
2025 Corvette of the Year Finalist - Modified
2024 C1 of the Year Winner - Modified
2023 C1 of the Year Finalist - Modified
2023 C3 of the Year Finalist - Unmodified
2022 C1 of the Year Finalist - Modified
2020 C1 of the Year Finalist - Modified
2019 C1 of Year Finalist (performance mods)
2018 C1 of Year Finalist
I don't think you have a compression problem. You didn't say if it smokes only during startup or all the time. If it smokes all the tme I would think you have a broken oil ring on the piston.
Thread Starter
Pro
Joined: Apr 2012
Posts: 534
Likes: 21
From: PA
Safety Car
Joined: May 2007
Posts: 3,677
Likes: 646
From: Indiana
2025 c3 ('68-'73) of the Year Finalist - Unmodified
2025 Corvette of the Year Finalist - Modified
2024 C1 of the Year Winner - Modified
2023 C1 of the Year Finalist - Modified
2023 C3 of the Year Finalist - Unmodified
2022 C1 of the Year Finalist - Modified
2020 C1 of the Year Finalist - Modified
2019 C1 of Year Finalist (performance mods)
2018 C1 of Year Finalist
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
It's possible that there's a problem with 1 or more oil scraper rings. That will cause oil burning and still show good compression.
Also, ask the engine builder what he used for piston to bore clearance. I assume he used Speed Pro L2166F pistons.
Another possibility is intake port seal problems. This will show more dramatically during closed throttle higher RPM coasting (very high manifold vacuum).
Pull all 8 plugs, photograph and post.
Also, ask the engine builder what he used for piston to bore clearance. I assume he used Speed Pro L2166F pistons.
Another possibility is intake port seal problems. This will show more dramatically during closed throttle higher RPM coasting (very high manifold vacuum).
Pull all 8 plugs, photograph and post.
Melting Slicks
Joined: Aug 2004
Posts: 3,078
Likes: 101
From: Riverhead New York
If you are using Synthetic Oil in your new engine this could cause your problem. These older engines need Organic Oil during the Break In period. Synthetic is too slick to allow the Rings to Seat properly. I'd change your oil to Dino stuff and take the car out and Beat the Snot out of it for a little bit. Maybe 50-100 miles. This "May" Seat the Rings and cure your problem. Also, put some Marvel Mystery Oil, (MMO), in the gas at 1 ounce per two Gallons of gas to lubricate those new Guides in the Head. Al W.
Burning Brakes
Joined: Oct 2004
Posts: 1,210
Likes: 1
From: fighter pilots make movies, bomber pilots make history
Maury,
This is a very rare situation, but since it occurred on your TWO rebuilds, please consider. It happened to me, my aluminum intake manifold had a crack on the bottom of the intake. It was hidden by the the oil plate on the bottom of intake manifold.
It would suck oil mist from the gallery and foul the plugs after several hours runtime. Some intermittant smoking. I welded and continue to use with no problems.
Joe
This is a very rare situation, but since it occurred on your TWO rebuilds, please consider. It happened to me, my aluminum intake manifold had a crack on the bottom of the intake. It was hidden by the the oil plate on the bottom of intake manifold.
It would suck oil mist from the gallery and foul the plugs after several hours runtime. Some intermittant smoking. I welded and continue to use with no problems.
Joe
Le Mans Master
Joined: Feb 2008
Posts: 6,460
Likes: 1,886
From: Folsom CA
Compression on any new rebuild should be equal, post what rings were installed, Moly, Chrome or Cast Iron. Moly rings will seat right away, usually by the end of a first pull on a dyno. Ask the machine shop what final hone was used, he should describe it exactly.
Start up smoke does point to valve stem seals. 1200 miles should have everything seated in, unless you drive it with no load pampering it all the time.
Start up smoke does point to valve stem seals. 1200 miles should have everything seated in, unless you drive it with no load pampering it all the time.
Thread Starter
Pro
Joined: Apr 2012
Posts: 534
Likes: 21
From: PA
Maury,
This is a very rare situation, but since it occurred on your TWO rebuilds, please consider. It happened to me, my aluminum intake manifold had a crack on the bottom of the intake. It was hidden by the the oil plate on the bottom of intake manifold.
It would suck oil mist from the gallery and foul the plugs after several hours runtime. Some intermittant smoking. I welded and continue to use with no problems.
Joe
This is a very rare situation, but since it occurred on your TWO rebuilds, please consider. It happened to me, my aluminum intake manifold had a crack on the bottom of the intake. It was hidden by the the oil plate on the bottom of intake manifold.
It would suck oil mist from the gallery and foul the plugs after several hours runtime. Some intermittant smoking. I welded and continue to use with no problems.
Joe
Thread Starter
Pro
Joined: Apr 2012
Posts: 534
Likes: 21
From: PA
Compression on any new rebuild should be equal, post what rings were installed, Moly, Chrome or Cast Iron. Moly rings will seat right away, usually by the end of a first pull on a dyno. Ask the machine shop what final hone was used, he should describe it exactly.
Start up smoke does point to valve stem seals. 1200 miles should have everything seated in, unless you drive it with no load pampering it all the time.
Start up smoke does point to valve stem seals. 1200 miles should have everything seated in, unless you drive it with no load pampering it all the time.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
Not sure what the "official" reading should be from a +/- standpoint but those numbers look fine to me.
Found this;
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.
Found this;
When checking your cranking compression you should be looking for inconsistencies between cylinders. Don't focus simply on how high or low your figures may be, unless all the cylinders are extremely low, (less than 100 psi), which could indicate you're doing the test incorrectly, or your engine is about 7 cylinders shy of a full power plant. A good thing to watch for is repeatability between cylinders. That means that with same number of starter revolutions, each cylinder should build pressure within about 5 percent of the others.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh