Compression Check
Thread Starter
Advanced
Joined: Jun 2009
Posts: 74
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From: ord nebraska
Compression Check
Well I finally got around to checking the compression.
Engine is an L82 in a 1977 Corvette. Mileage on the engine is supposed to be 58,000.
Here is what I came up with.
1) 165
2) 160
3) 160
4) 163
5) 160
6) 170
7) 170
8) 168
I was going to rebuild the whole engine but if I could get away from doing so then it sure would make life a bit easier.
I want to put Brodix IK180's on with a cam change.
Do you engine guys think my compression is good enough to leave the bottom end alone?
Thank you, Marty
Engine is an L82 in a 1977 Corvette. Mileage on the engine is supposed to be 58,000.
Here is what I came up with.
1) 165
2) 160
3) 160
4) 163
5) 160
6) 170
7) 170
8) 168
I was going to rebuild the whole engine but if I could get away from doing so then it sure would make life a bit easier.
I want to put Brodix IK180's on with a cam change.
Do you engine guys think my compression is good enough to leave the bottom end alone?
Thank you, Marty
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
Well I finally got around to checking the compression.
Engine is an L82 in a 1977 Corvette. Mileage on the engine is supposed to be 58,000.
Here is what I came up with.
1) 165
2) 160
3) 160
4) 163
5) 160
6) 170
7) 170
8) 168
I was going to rebuild the whole engine but if I could get away from doing so then it sure would make life a bit easier.
I want to put Brodix IK180's on with a cam change.
Do you engine guys think my compression is good enough to leave the bottom end alone?
Thank you, Marty
Engine is an L82 in a 1977 Corvette. Mileage on the engine is supposed to be 58,000.
Here is what I came up with.
1) 165
2) 160
3) 160
4) 163
5) 160
6) 170
7) 170
8) 168
I was going to rebuild the whole engine but if I could get away from doing so then it sure would make life a bit easier.
I want to put Brodix IK180's on with a cam change.
Do you engine guys think my compression is good enough to leave the bottom end alone?
Thank you, Marty
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
although your numbers look fairly consistent, a leak down test is really the only way to determine a healthy ring seal....plus if you add a new set of heads on old rings you always run the risk of more oil consumption because of better vacuum than with the old heads
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
I agree about a leak down check. A compression check doesn't tell you much. Compression is nothing more than a function of cylinder volumn differences from BDC to TDC.
I found it easier to turn the engine with a wrench on the harmonic balancer and all the spark plugs removed. I started at #1 cylinder at TDC and checked each cylinder in the firing order as I rotated the engine. You have to hold it at TDC for a few seconds to check the reading because the pressure wants to push the piston down.
Check each cylinder at 90 psi. Even a decay of 20% is considered o.k. My '73 SB ranged from 9% to 12% loss.
I found it easier to turn the engine with a wrench on the harmonic balancer and all the spark plugs removed. I started at #1 cylinder at TDC and checked each cylinder in the firing order as I rotated the engine. You have to hold it at TDC for a few seconds to check the reading because the pressure wants to push the piston down.
Check each cylinder at 90 psi. Even a decay of 20% is considered o.k. My '73 SB ranged from 9% to 12% loss.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
I agree about a leak down check. A compression check doesn't tell you much. Compression is nothing more than a function of cylinder volumn differences from BDC to TDC.
I found it easier to turn the engine with a wrench on the harmonic balancer and all the spark plugs removed. I started at #1 cylinder at TDC and checked each cylinder in the firing order as I rotated the engine. You have to hold it at TDC for a few seconds to check the reading because the pressure wants to push the piston down.
Check each cylinder at 90 psi. Even a decay of 20% is considered o.k. My '73 SB ranged from 9% to 12% loss.
I found it easier to turn the engine with a wrench on the harmonic balancer and all the spark plugs removed. I started at #1 cylinder at TDC and checked each cylinder in the firing order as I rotated the engine. You have to hold it at TDC for a few seconds to check the reading because the pressure wants to push the piston down.
Check each cylinder at 90 psi. Even a decay of 20% is considered o.k. My '73 SB ranged from 9% to 12% loss.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Since you probably don't have a dyno in your hip pocket, do the leak-down test. Also, check the height of the "wear" ridge at the outer end of each cylinder. If the ridge is very slight and is even all the way around each cylinder, the rings should not be highly worn. It wouldn't hurt to pull a couple of the crank bearing caps to see what kind of wear the bearings have also. It would be 'unusual' to rebuild the upper part of the engine without doing the bottom end also. But, with less than 60K miles, good bearings and leak-down tests, you should be OK if you just bump the compression to 9.5 or a bit higher.
I'm not sure how much HP gain you will get by just switching out heads and cam for the L-82 stuff...they weren't too bad to begin with.
I'm not sure how much HP gain you will get by just switching out heads and cam for the L-82 stuff...they weren't too bad to begin with.
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Race Director
Joined: Oct 2000
Posts: 15,044
Likes: 79
From: central FL, near the beach.
the guy doesn't have a leakdown tester!
.
If it doesn't knock, smoke, burn oil, or blow oil out the breather, put on the new heads + cam. go have some fun.
Just ignore the salesmen that want to talk u into a rebuild.
.
If it doesn't knock, smoke, burn oil, or blow oil out the breather, put on the new heads + cam. go have some fun.
Just ignore the salesmen that want to talk u into a rebuild.
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Doing a rebuild on a 58,000-mile engine is rediculous unless you have a specific glaring problem.Lars
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
wow..before i spent money on a dyno test i would do the leakdown to see where the condition of the ring seal really was....after all the op's original question was if he had enough compression for a new stick and some after market heads..i suppose he could do the usual "bolt the thing together" and take a chance but then how much compression does he really have that is important for choosing a cam instead of an educated guess so he can pick the right cam??....if he checks the original mill specs. and checks with leakdown then it would make the choice a lot easier and he would know the health of ring seal or if he should change pistons and re ring or go for it all and do machine work..to each his own..jmo....
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
I know this isn't a leak down test thread BUT, I own this leak down tester and it's one of the best $40 tools I ever bought.
http://www.harborfreight.com/cpi/cta...emnumber=94190
Here is some interesting things you can learn from a leak down check.
If one or more cylinders are identified as having 15% to 30% greater compression
loss than the others, you can listen for escaping air pressure to diagnose the
problem. Air escaping at the following locations indicates a potential problem.
1. Oil dipstick tube indicates pressure escaping from the cylinder into the oil
jacket. This usually indicated stuck, burned or worn rings or cylinder walls.
2. Radiator filler opening bubbles or sound indicates pressure escaping into the
coolant jacket. This can indicate cracked cylinder walls or damaged head
gasket. In case of a damaged head gasket, often two or more adjacent
cylinders will show the same problem.
3. Adjacent cylinder indicates a damaged head gasket.
4. Tail pipe indicates burned, stuck or worn exhaust valve.
5. Carburetor or fuel injector intake indicates stuck, burned or worn intake valve.
http://www.harborfreight.com/cpi/cta...emnumber=94190
Here is some interesting things you can learn from a leak down check.
If one or more cylinders are identified as having 15% to 30% greater compression
loss than the others, you can listen for escaping air pressure to diagnose the
problem. Air escaping at the following locations indicates a potential problem.
1. Oil dipstick tube indicates pressure escaping from the cylinder into the oil
jacket. This usually indicated stuck, burned or worn rings or cylinder walls.
2. Radiator filler opening bubbles or sound indicates pressure escaping into the
coolant jacket. This can indicate cracked cylinder walls or damaged head
gasket. In case of a damaged head gasket, often two or more adjacent
cylinders will show the same problem.
3. Adjacent cylinder indicates a damaged head gasket.
4. Tail pipe indicates burned, stuck or worn exhaust valve.
5. Carburetor or fuel injector intake indicates stuck, burned or worn intake valve.
Racer
Joined: Mar 2008
Posts: 428
Likes: 0
No4n4me
Please note that people who post that a compression test is useless are not telling you the entire story. A healthy engine cylinder will build most of its pressure on the first stroke and then continues to build up to the fourth one for the test. If the needle doesn’t jump up on the first compression stroke, I would suspect a problem. So, do you remember if the needle jumped up on the first puff?
If you didn’t get the healthy numbers that you actually got and the first stroke produced weak pressure, I would recommend a wet compression test. Perform a wet test with a spoonful of new oil into the cylinder and spin the motor around, repeat the test. If the pressure jumps up significantly, either the rings, piston, cylinder walls or all three are worn.
Yes, the last word on compression testing for weak cylinders is the leak down test. I don’t consider your numbers weak. If you have access to a tester, it couldn’t hurt, but if it was my engine I would be happy with those numbers and orders some new toys. I am surprised nobody told you to do a cylinder power balance test and look under your bed for monsters.
Please note that people who post that a compression test is useless are not telling you the entire story. A healthy engine cylinder will build most of its pressure on the first stroke and then continues to build up to the fourth one for the test. If the needle doesn’t jump up on the first compression stroke, I would suspect a problem. So, do you remember if the needle jumped up on the first puff?
If you didn’t get the healthy numbers that you actually got and the first stroke produced weak pressure, I would recommend a wet compression test. Perform a wet test with a spoonful of new oil into the cylinder and spin the motor around, repeat the test. If the pressure jumps up significantly, either the rings, piston, cylinder walls or all three are worn.
Yes, the last word on compression testing for weak cylinders is the leak down test. I don’t consider your numbers weak. If you have access to a tester, it couldn’t hurt, but if it was my engine I would be happy with those numbers and orders some new toys. I am surprised nobody told you to do a cylinder power balance test and look under your bed for monsters.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
I still say buy an inexpensive leak down tester and forget the old fashioned compression test. If you have to put oil in the cylinder to temporarily seal the leak for another compression test why not just do the leak down test in the first place and determine the cause of the loss of compression by determining where the air is leaking to (exhaust, carb, crankcase, etc).
Thread Starter
Advanced
Joined: Jun 2009
Posts: 74
Likes: 0
From: ord nebraska
I don't have a leak down tester.
We put 2000 miles on the car before winter got here and the oil level stayed full. I never did notice any smoke from the exhaust that I know of.
Project80,
Yes the needle jumped right away. I tried to go 5 cranks on each cylinder. I also noticed that after 4 to 5 cranks that was all the needle would climb.
The spark plugs looked like they burned nice and clean. There was nothing to indicate to me that they have been burning oil.
We put 2000 miles on the car before winter got here and the oil level stayed full. I never did notice any smoke from the exhaust that I know of.
Project80,
Yes the needle jumped right away. I tried to go 5 cranks on each cylinder. I also noticed that after 4 to 5 cranks that was all the needle would climb.
The spark plugs looked like they burned nice and clean. There was nothing to indicate to me that they have been burning oil.
Thread Starter
Advanced
Joined: Jun 2009
Posts: 74
Likes: 0
From: ord nebraska
After I thought about it I could do a leak down test.
I will be pulling the engine anyway because we want to spice up the engine compartment and it will make it a lot easier to do the leak test.
I will order the one 08vycpe has linked to.
I never gave the leak down test any thought because I really did not care about the valves in the old heads.
The leak down test will be a sure way to check the rings then? Even if a valve leaks? Should any air escape the rings?
I will be pulling the engine anyway because we want to spice up the engine compartment and it will make it a lot easier to do the leak test.
I will order the one 08vycpe has linked to.
I never gave the leak down test any thought because I really did not care about the valves in the old heads.
The leak down test will be a sure way to check the rings then? Even if a valve leaks? Should any air escape the rings?
Racer
Joined: Mar 2008
Posts: 428
Likes: 0
no4n4me,
That jump on the first compression stroke is good news. The spec's call for 4 compression strokes, don't be surprised that it doesn't climb after that. The clean burning plugs are also a good sign. If you decide to do a leak down test, I would try to borrow one rather than buy one. Put the saved money towards your new heads.
Good Luck
That jump on the first compression stroke is good news. The spec's call for 4 compression strokes, don't be surprised that it doesn't climb after that. The clean burning plugs are also a good sign. If you decide to do a leak down test, I would try to borrow one rather than buy one. Put the saved money towards your new heads.
Good Luck
Drifting
Joined: Dec 2005
Posts: 1,360
Likes: 3
From: NORTHEAST
No4n4me
Please note that people who post that a compression test is useless are not telling you the entire story. A healthy engine cylinder will build most of its pressure on the first stroke and then continues to build up to the fourth one for the test. If the needle doesn’t jump up on the first compression stroke, I would suspect a problem. So, do you remember if the needle jumped up on the first puff?
If you didn’t get the healthy numbers that you actually got and the first stroke produced weak pressure, I would recommend a wet compression test. Perform a wet test with a spoonful of new oil into the cylinder and spin the motor around, repeat the test. If the pressure jumps up significantly, either the rings, piston, cylinder walls or all three are worn.
Yes, the last word on compression testing for weak cylinders is the leak down test. I don’t consider your numbers weak. If you have access to a tester, it couldn’t hurt, but if it was my engine I would be happy with those numbers and orders some new toys. I am surprised nobody told you to do a cylinder power balance test and look under your bed for monsters.
Please note that people who post that a compression test is useless are not telling you the entire story. A healthy engine cylinder will build most of its pressure on the first stroke and then continues to build up to the fourth one for the test. If the needle doesn’t jump up on the first compression stroke, I would suspect a problem. So, do you remember if the needle jumped up on the first puff?
If you didn’t get the healthy numbers that you actually got and the first stroke produced weak pressure, I would recommend a wet compression test. Perform a wet test with a spoonful of new oil into the cylinder and spin the motor around, repeat the test. If the pressure jumps up significantly, either the rings, piston, cylinder walls or all three are worn.
Yes, the last word on compression testing for weak cylinders is the leak down test. I don’t consider your numbers weak. If you have access to a tester, it couldn’t hurt, but if it was my engine I would be happy with those numbers and orders some new toys. I am surprised nobody told you to do a cylinder power balance test and look under your bed for monsters.
And any time you add that much oil to a cylinder the compression is always going to be higher,
Jackfits compression test was 180 to 200 but the leak down was up to 40 percent leak down no wonder the oil was getting black in a 100 miles.