[C2] Compression test numbers.
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 947
Likes: 250
From: Deshler Nebraska
Compression test numbers.
I have a 1966 with a sb. Engine not org. Did compression check and got these numbers. 1-130,3-165,5-155,7-140 2-152, 4-148, 6-152, 8-155. Pulls good through the gears. Get up in the 4,000 + range and can get an occasion back fire. Mech. thinks maybe bad cam lobe. ???
Planning on running it for now. Would prob. replace with a 350/390 hp Blueprint motor, if needed. Just a driver.
Planning on running it for now. Would prob. replace with a 350/390 hp Blueprint motor, if needed. Just a driver.
Le Mans Master
Joined: Nov 2015
Posts: 7,479
Likes: 2,650
From: Really Central IL Illinois
Compression should be within 10% or so on all cylinders. #1 and #7 are low. Squirt a teaspoon of oil in each of these two cylinders and repeat the test. If compression comes up, you have weak compression rings. If it does not come up, you may have a valve not seating or burnt seat or poor adjustment. The backfire could be a valve, but could be a loose exhaust manifold, and other issues. A leakdown test will tell you more, but doesn't look like it will get any better. A burnt valve should show as a fluctuation on a vacuum gauge at idle.
I have driven worse for many miles.
I have driven worse for many miles.
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 947
Likes: 250
From: Deshler Nebraska
Good idea about cylinder #1 and #7. I will try that today or tomorrow.
I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.
I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.
I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.
I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.
Team Owner
Joined: Feb 2003
Posts: 54,140
Likes: 9,447
From: Sitting in his Nowhere land Hanover Pa
2024 C2 of the Year Finalist - Unmodified
2015 C2 of Year Finalist
Good idea about cylinder #1 and #7. I will try that today or tomorrow.
I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.
I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.
I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.
I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Le Mans Master
Joined: Nov 2015
Posts: 7,479
Likes: 2,650
From: Really Central IL Illinois
Good idea about cylinder #1 and #7. I will try that today or tomorrow.
I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.
I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.
I am quite sure the exhaust manifolds are tight, but will check each bolt. Factory exhaust manifolds.
I don't think a bad cam would leave me sitting by the side of the road, will it ? Reliability is also my concern.
IF you have loose rockers and readjust them and they become loose again, you have lost a lobe or have bad lock nuts. Check the lift on the cam.
A leaking exhaust manifold donut gasket can also cause a backfire, but not as likely.
Ron
Le Mans Master
Joined: Feb 2008
Posts: 6,528
Likes: 1,939
From: Folsom CA
A leak down test will show where the compression is leaking. Intake Valve, Exhaust Valve, Piston Rings. Does not take any longer than a compression test.
It would be nice to learn where the problem is before getting out wrenches to replace an engine.
It would be nice to learn where the problem is before getting out wrenches to replace an engine.
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 947
Likes: 250
From: Deshler Nebraska
Burning Brakes
Joined: Apr 2020
Posts: 968
Likes: 360
When I was fighting my oil consumption problem I could foul a set of plugs in under 30 miles--to the point of misfiring.
The solution to my problem tured out to be NO thread sealant on the rocker arm studs. Once I applied the sealer all problems suddenly disappeared---clean plugs AND my static compression jumped from 140 to 180 AND my leakdown test improved from-30% to -3% and best of all my #1 and #7 cyls which were the worst became the best.
Eddie
The solution to my problem tured out to be NO thread sealant on the rocker arm studs. Once I applied the sealer all problems suddenly disappeared---clean plugs AND my static compression jumped from 140 to 180 AND my leakdown test improved from-30% to -3% and best of all my #1 and #7 cyls which were the worst became the best.
Eddie
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 947
Likes: 250
From: Deshler Nebraska
Wow. Good you could fix that problem. I have read about the "NO thread sealant" issue some place before.
I think the plugs looked normal. No heavy carbon or black on the plugs. A little from just being used, but I did not think any of them looked bad enough to misfire. I am going to take the car for a longer drive today. 50-100 miles and check things over again.
Interesting comment by R66 about, "a leaking exhaust manifold donut gasket can also cause a backfire". I actually did or do have some exhaust leak sounds coming from the right side. I remember cleaning the donut before installing it again. I found small pieces of carbon or dirt around the donut, probably should have left that on, but was thinking the donut will not be in the exact same place so maybe the deposits would make the fit worse instead of better so I removed those pieces.. I figured it was particles accumulated over time filling small air leaks. Never figured it would or could cause backfire, if it was not completely sealed. The leak was worse when first assembled, but less or gone now. I did tighten the 3 nuts a little more that hold the donuts. Maybe over time, hopefully shortly there will be no more exhaust leak and the backfiring will be gone.
Thanks for the information.
I will look into
I think the plugs looked normal. No heavy carbon or black on the plugs. A little from just being used, but I did not think any of them looked bad enough to misfire. I am going to take the car for a longer drive today. 50-100 miles and check things over again.
Interesting comment by R66 about, "a leaking exhaust manifold donut gasket can also cause a backfire". I actually did or do have some exhaust leak sounds coming from the right side. I remember cleaning the donut before installing it again. I found small pieces of carbon or dirt around the donut, probably should have left that on, but was thinking the donut will not be in the exact same place so maybe the deposits would make the fit worse instead of better so I removed those pieces.. I figured it was particles accumulated over time filling small air leaks. Never figured it would or could cause backfire, if it was not completely sealed. The leak was worse when first assembled, but less or gone now. I did tighten the 3 nuts a little more that hold the donuts. Maybe over time, hopefully shortly there will be no more exhaust leak and the backfiring will be gone.
Thanks for the information.
I will look into
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 947
Likes: 250
From: Deshler Nebraska
Question. Never have done valve work. Was the lack of sealant on the threads screwing into the head or the threads the on top that the nuts screw on to ? Or both ? What type or kind of thread sealers should be used ? How tight are the studs screwed into the heads ? Is the tightness of the studs screwed into the head measured in foot pounds ?
When doing or checking rockers and pushrods I am thinking that I need to have a piston at TDC on compression stroke when checking pushrod play and tightening of the rocker arm stud nuts. Just like when putting the motor together for the first time. (1) bring piston to TDC on compression stroke (2) tighten down rocker arm stud nuts until no play is felt of the pushrods (3) give nut an additional 1/2 turn. Done. Is this the correct procedure after applying thread sealer ?
When doing or checking rockers and pushrods I am thinking that I need to have a piston at TDC on compression stroke when checking pushrod play and tightening of the rocker arm stud nuts. Just like when putting the motor together for the first time. (1) bring piston to TDC on compression stroke (2) tighten down rocker arm stud nuts until no play is felt of the pushrods (3) give nut an additional 1/2 turn. Done. Is this the correct procedure after applying thread sealer ?
Le Mans Master
Joined: Feb 2008
Posts: 6,528
Likes: 1,939
From: Folsom CA
The leak Eddie is describing was on his Rocker Arm Studs and I believe it was with aluminum heads. If your heads are factory stock the rocker arm studs are pressed in and not threaded.
So before pursuing you need to see what heads are on your engine, It is common for studs to be installed when the factory heads are rebuilt so maybe your heads are threaded studs but you would need to check. Easy to see with valve cover off. Threaded studs all have a bolt face right where the stud bottoms in the head. Press in studs do not have a place to use a socket or wrench to remove them.
So before pursuing you need to see what heads are on your engine, It is common for studs to be installed when the factory heads are rebuilt so maybe your heads are threaded studs but you would need to check. Easy to see with valve cover off. Threaded studs all have a bolt face right where the stud bottoms in the head. Press in studs do not have a place to use a socket or wrench to remove them.
Thread Starter
Burning Brakes
Joined: Dec 2017
Posts: 947
Likes: 250
From: Deshler Nebraska
Got it.
Nice dry day. Took the car on a 80 mile drive. Steers nice, no vibrations, if it don't misfire I would trust this engine. Towards the end of the drive I down shifted to 3rd and slow brought up the rpm. Once you get in the 4000 to 4500 you get some backfiring. 4500-5000 you get a lot of backfiring. Hmmmm.
If, bad cam, hard to tell how long it has been bad. I figured it would prob need a timing chain and gear if I do the cam.
Also, don't I want to drop the pan and use Plastigauge to check bearing clearance ?
Heads are not alu. Heads don't have any tapped holes in the front side of the heads. Prob. Pontiac heads like I think the block is. Block # F0531HCH.
Nice dry day. Took the car on a 80 mile drive. Steers nice, no vibrations, if it don't misfire I would trust this engine. Towards the end of the drive I down shifted to 3rd and slow brought up the rpm. Once you get in the 4000 to 4500 you get some backfiring. 4500-5000 you get a lot of backfiring. Hmmmm.
If, bad cam, hard to tell how long it has been bad. I figured it would prob need a timing chain and gear if I do the cam.
Also, don't I want to drop the pan and use Plastigauge to check bearing clearance ?
Heads are not alu. Heads don't have any tapped holes in the front side of the heads. Prob. Pontiac heads like I think the block is. Block # F0531HCH.
Last edited by Nelag; Dec 18, 2024 at 03:47 PM. Reason: Adding info.
Burning Brakes
Joined: Apr 2020
Posts: 968
Likes: 360
Question. Never have done valve work. Was the lack of sealant on the threads screwing into the head or the threads the on top that the nuts screw on to ? Or both ? What type or kind of thread sealers should be used ? How tight are the studs screwed into the heads ? Is the tightness of the studs screwed into the head measured in foot pounds ?
When doing or checking rockers and pushrods I am thinking that I need to have a piston at TDC on compression stroke when checking pushrod play and tightening of the rocker arm stud nuts. Just like when putting the motor together for the first time. (1) bring piston to TDC on compression stroke (2) tighten down rocker arm stud nuts until no play is felt of the pushrods (3) give nut an additional 1/2 turn. Done. Is this the correct procedure after applying thread sealer ?
When doing or checking rockers and pushrods I am thinking that I need to have a piston at TDC on compression stroke when checking pushrod play and tightening of the rocker arm stud nuts. Just like when putting the motor together for the first time. (1) bring piston to TDC on compression stroke (2) tighten down rocker arm stud nuts until no play is felt of the pushrods (3) give nut an additional 1/2 turn. Done. Is this the correct procedure after applying thread sealer ?
MY issue was with Edelbrock E Street ALUMINUM heads. The holes for the oushrods are NOT blind holes as they are with the original cast iron heads. Edelbrock drilled them all the way through. The studs are threaded and Edelbrock makes a point of tightening the studs to 35lb/ft but makes absokutely no mention that these holes are drilled through INTO THE INTAKE STREAM and that sealant on those threads is VITAL. Shame on them!!!!
I used Permatex white hi temp thread sealant. and systematically went thorugh the entire engine. There was absolutely NO TRACE or evidence that anyone had ever applied sealant to the heads when they were assembled. So far, after better than 1000 miles I have no more issues. The compression has increased dramatically as noted above and I use virtually NO oil.
Eddie
Burning Brakes
Joined: Apr 2020
Posts: 968
Likes: 360
The leak Eddie is describing was on his Rocker Arm Studs and I believe it was with aluminum heads. If your heads are factory stock the rocker arm studs are pressed in and not threaded.
So before pursuing you need to see what heads are on your engine, It is common for studs to be installed when the factory heads are rebuilt so maybe your heads are threaded studs but you would need to check. Easy to see with valve cover off. Threaded studs all have a bolt face right where the stud bottoms in the head. Press in studs do not have a place to use a socket or wrench to remove them.
So before pursuing you need to see what heads are on your engine, It is common for studs to be installed when the factory heads are rebuilt so maybe your heads are threaded studs but you would need to check. Easy to see with valve cover off. Threaded studs all have a bolt face right where the stud bottoms in the head. Press in studs do not have a place to use a socket or wrench to remove them.
Eddie
.