Post overhaul compression check
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,037
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From: The Corner of Moncks
St. Jude Donor '07
Post overhaul compression check
I recently overhauled the 327 in my '68. I've been through the break-in and I've put about 50 road miles on it, yesterday I decided to go back over it and check fastener torque, etc. During the course of this I decided to do a compression check, I have rebuilt a few engines but I've never done a post rebuild compression check before and I'm surprised by the results, Cylinders #1, 3 & 8 are reporting 140 psi, #6 is at 150, #2 & 7 are 160 and #3 & 4 are a whopping 170. My build should have yielded about 10:1 compression so I was expecting 145-150 psi per cylinder. The engine runs really good, throttle response is what I expected, idle is smooth, but these numbers seem crazy. Anyone else experience anything like this?
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From: At my Bar drinking and wrenching in Lafayette Colorado
Compression tests are not useful for good engine health diagnosis. Do a leakdown test - this will give you an accurate assessment of engine health and any potential issues with sealing.
Lars
Lars
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From: Cool Northern Michigan
Sometimes these compression tests variances are simply a battery getting tired. (The very last cylinders tested). Always keep a charger on it.
You are allowed around ten pounds leeway per cyl. You have more. Sometimes that is just a poor fitment of the tester in the sparkplug hole. (leakage)
Next, you look for a pattern of adjacent cylinders reading low. You have 1 & 3 side-by-side reporting low. That could be a head gasket issue between those two, but not likely.
Sometimes valve-lash, more specifically, improper valve-lash will affect comp test. Remember, to compress air, the cyl has to have air to compress. If a valve is not opening all the way or a valve is not closing all the way, then that "lash" will show up on your test. This is where a leak down test is superior.
You are allowed around ten pounds leeway per cyl. You have more. Sometimes that is just a poor fitment of the tester in the sparkplug hole. (leakage)
Next, you look for a pattern of adjacent cylinders reading low. You have 1 & 3 side-by-side reporting low. That could be a head gasket issue between those two, but not likely.
Sometimes valve-lash, more specifically, improper valve-lash will affect comp test. Remember, to compress air, the cyl has to have air to compress. If a valve is not opening all the way or a valve is not closing all the way, then that "lash" will show up on your test. This is where a leak down test is superior.
Le Mans Master
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A leakdown test can also identify a leak at the intake valve....hearing air in intake manifold, exhaust....hearing air at exhaust ports....or rings,...hearing air in engine case / breather. Another factor with all of this is is the engine cold or hot?? That changes the outcome.....as welll as how fast the battery is spinning the crank.
As for leakdown test.....you can change the numbers by rocking the crankshaft back and forth....which basically can seat the rings better, and thus get better numbers.....which does NOT mean anything but a good feeling, because what matters is how the rings seat when engine is operating, not what you can make it do by jacking the crank back and forth. I done lots of compression tests on propeller driven airplane engines, and some mechanics will make the numbers better by moving the prop (i.e. the crankshaft) back and forth.....still does not make it right, correct or legal. In airplanes they have to be less than 20% leakage or the cylinder is unairworthy.
Lastly.....give the engine some time to fully break in. If its running good and not burning oil, I would not panic over the whole thing. I know, because I feel the same way, that we wish numbers would all be very close, and right where we want them to be.....so I understand.
As for leakdown test.....you can change the numbers by rocking the crankshaft back and forth....which basically can seat the rings better, and thus get better numbers.....which does NOT mean anything but a good feeling, because what matters is how the rings seat when engine is operating, not what you can make it do by jacking the crank back and forth. I done lots of compression tests on propeller driven airplane engines, and some mechanics will make the numbers better by moving the prop (i.e. the crankshaft) back and forth.....still does not make it right, correct or legal. In airplanes they have to be less than 20% leakage or the cylinder is unairworthy.
Lastly.....give the engine some time to fully break in. If its running good and not burning oil, I would not panic over the whole thing. I know, because I feel the same way, that we wish numbers would all be very close, and right where we want them to be.....so I understand.
Thread Starter
Melting Slicks
Joined: May 2006
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From: The Corner of Moncks
St. Jude Donor '07
So, I'm basing my assumption on an online calculator that tells me my cylinder pressure should be ~150 psi, this is based on the cam spec I found on the internet and the pivotal number is "Inlet valve closes ABDC" apparently the number I found for the cam I used is erroneous. Wonderful. So obviously my mission has now shifted to finding the lost pounds in the low cylinders since the high cylinders are capable of 170 psig. Yay.
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I've built a lot of engines. Wouldn't even consider doing a compression test until 500 miles. Generally if the rings aren't seated by 200 miles, there not going to. But 50 miles? Geezz.
give it a bit to run in Mate!
give it a bit to run in Mate!
Instructor
Joined: Nov 2023
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From: Western new York
I recently overhauled the 327 in my '68. I've been through the break-in and I've put about 50 road miles on it, yesterday I decided to go back over it and check fastener torque, etc. During the course of this I decided to do a compression check, I have rebuilt a few engines but I've never done a post rebuild compression check before and I'm surprised by the results, Cylinders #1, 3 & 8 are reporting 140 psi, #6 is at 150, #2 & 7 are 160 and #3 & 4 are a whopping 170. My build should have yielded about 10:1 compression so I was expecting 145-150 psi per cylinder. The engine runs really good, throttle response is what I expected, idle is smooth, but these numbers seem crazy. Anyone else experience anything like this?
Thread Starter
Melting Slicks
Joined: May 2006
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From: The Corner of Moncks
St. Jude Donor '07
Safety Car
Joined: May 2001
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From: Fairfax Virginia
I agree with the continue to break in the engine for another 500-1000 miles crowd.
Having built many engines I don't ever recall having a need to do a compression test on a "non-broken" in engine. After you have some miles on it then you can test it if you still feel the need. The compression test is a great test if it is done correctly. It does require the fuel shut off and the carburetor/throttle body to be wired wide open along with a fully charged battery that has been charged ahead of time and is attached during the duration of the testing.
The Leak Down system is used a lot on private piston powered aircraft. It can tell a mechanic so much more than just the compression. It also requires a good source of pressurized air to perform the test.
Having built many engines I don't ever recall having a need to do a compression test on a "non-broken" in engine. After you have some miles on it then you can test it if you still feel the need. The compression test is a great test if it is done correctly. It does require the fuel shut off and the carburetor/throttle body to be wired wide open along with a fully charged battery that has been charged ahead of time and is attached during the duration of the testing.
The Leak Down system is used a lot on private piston powered aircraft. It can tell a mechanic so much more than just the compression. It also requires a good source of pressurized air to perform the test.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 2,037
Likes: 36
From: The Corner of Moncks
St. Jude Donor '07
I agree with the continue to break in the engine for another 500-1000 miles crowd.
Having built many engines I don't ever recall having a need to do a compression test on a "non-broken" in engine. After you have some miles on it then you can test it if you still feel the need. The compression test is a great test if it is done correctly. It does require the fuel shut off and the carburetor/throttle body to be wired wide open along with a fully charged battery that has been charged ahead of time and is attached during the duration of the testing.
The Leak Down system is used a lot on private piston powered aircraft. It can tell a mechanic so much more than just the compression. It also requires a good source of pressurized air to perform the test.
Having built many engines I don't ever recall having a need to do a compression test on a "non-broken" in engine. After you have some miles on it then you can test it if you still feel the need. The compression test is a great test if it is done correctly. It does require the fuel shut off and the carburetor/throttle body to be wired wide open along with a fully charged battery that has been charged ahead of time and is attached during the duration of the testing.
The Leak Down system is used a lot on private piston powered aircraft. It can tell a mechanic so much more than just the compression. It also requires a good source of pressurized air to perform the test.
Pro
Joined: Aug 2000
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From: Vancouver B.C.
Don’t think it was mentioned, but I generally keep carb throttle blades open when doing a compression test. Might make some difference. But as mentioned, I’d wait until 500 miles of break in to retest. Report back. Good luck.