Compression readings

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Steve welder

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383, fresh rebuild .....The heads were done previously for a block which turned out to have a rod knock. At the time that ending was assembled it was discovered one of the heads had a crack, so a used head was sourced and had hardened exhaust seats and all new valves/.springs, just like the old cracked head.
The now rebuilt heads were installed on another rebuilt block
I think the engine runs to rough at idle, has no hesitation, otherwise runs ok
I suspect a vacuum leak but here is the compression readings
#2-175
#4-180
#6-173
#8-175

#1-190
#3-185
#5-190
#7-190
Could the one head have been resurfaced so many times over the last 50 60 years and it now has a higher compression ratio?
How would a engine like this run?
All the plugs with the exception of #3 and #7 look fine
#3 insulator has a rust color covering it
#7 is fouled , looks slightly wet......even though the wires are new and good quality maybe a bad wire or plug?
Is it possible that intake would not align properly if the heads are of different heights ?
Whats up with such a wide range of compression readings
thanks for all and any thoughts
 
the readings seem awful high for a stock 383, should be closer to 150- 160.
what heads do you have ? closed chamber or open chamber.
 
the readings seem awful high for a stock 383, should be closer to 150- 160.
what heads do you have ? closed chamber or open chamber.
Exactly what im thinking......All I know is the original head was off a 290 HP engine.... What head the rebuilder used to replace the cracked one, I dont know
Tomorrow I take off valve covers
My only thoughts are was this head milled and the rebuilder never picked that up.
The other head would be more in line with a stock 383
 
One could be closed chamber, small valve while the other head is open chamber, large valve. Really won't know much until you open it up or stick a camera in a spark plug hole.
 
One head could have been surfaced more than the other....or the block deck height is different side to side.
 
Your readings vary less than 10%, you're fine...
If it was two four cylinder engines id agree, but two heads with such a difference in compression
The block was decked. I doubt any issues with block, since one head was replaced this is what concerns me......I would have liked to think the engine builder rechecked those heads when he assembled the engine but you knows.........Valve covers coming off later today
Thank you for your help
 
Do the testing again. Usually you'll get some different numbers. Then compute an average. +/- 10% is the acceptable range. Right now it looks like your average is 182. The highest you show is 190 which is only 4% higher, the lowest is 173 which is only 5% lower. No matter how you look at it your well within the "acceptable" range.
 
Within range but something is off about them all being on the same bank . At least they offset each other in the firing order.lol
 
383, fresh rebuild .....The heads were done previously for a block which turned out to have a rod knock. At the time that ending was assembled it was discovered one of the heads had a crack, so a used head was sourced and had hardened exhaust seats and all new valves/.springs, just like the old cracked head.
The now rebuilt heads were installed on another rebuilt block
I think the engine runs to rough at idle, has no hesitation, otherwise runs ok
I suspect a vacuum leak but here is the compression readings
#2-175
#4-180
#6-173
#8-175

#1-190
#3-185
#5-190
#7-190
Could the one head have been resurfaced so many times over the last 50 60 years and it now has a higher compression ratio?
How would a engine like this run?
All the plugs with the exception of #3 and #7 look fine
#3 insulator has a rust color covering it
#7 is fouled , looks slightly wet......even though the wires are new and good quality maybe a bad wire or plug?
Is it possible that intake would not align properly if the heads are of different heights ?
Whats up with such a wide range of compression readings
thanks for all and any thoughts
"It" -being the head...doesn't have a compression ratio..but we know what you meant.lol

It's safe to assume one head had maybe .015 more milled from it than the other .
You should always cc a single replacement head. If they aren't a pair from the factory, and sometimes even if they are, they will be different. I've had one chamber by itself be 4cc smaller than the other 3 with a set of J heads. It shouldn't really run rough per say cause of that...but maybe the vj leaks or it's a drastic flow difference..some do more than others at machine shops. Some just touch the 45 and send it, others might do a deshroud.. who knows.
 
"It" -being the head...doesn't have a compression ratio..but we know what you meant.lol

It's safe to assume one head had maybe .015 more milled from it than the other .
You should always cc a single replacement head. If they aren't a pair from the factory, and sometimes even if they are, they will be different. I've had one chamber by itself be 4cc smaller than the other 3 with a set of J heads. It shouldn't really run rough per say cause of that...but maybe the vj leaks or it's a drastic flow difference..some do more than others at machine shops. Some just touch the 45 and send it, others might do a deshroud.. who knows.
What is was concerned with is intake manifold to head alignment, could that be a issue.
I would say your correct one head was milled more than the other, especially that I had to replace one which was cracked, which I found before the engine was even started.
My only question is would you pull the heads and send them out to get CC all the same and have heads checked for differences in resurfacing?
I really feel the none of the valves is leaking, its just a difference with milling
Thanks
 
What is was concerned with is intake manifold to head alignment, could that be a issue.
I would say your correct one head was milled more than the other, especially that I had to replace one which was cracked, which I found before the engine was even started.
My only question is would you pull the heads and send them out to get CC all the same and have heads checked for differences in resurfacing?
I really feel the none of the valves is leaking, its just a difference with milling
Thanks
You could take the intake off and the readings would not change.
They would have had to mill it a bunch to make that much of a difference.
I'm with Kimmer run it.
You wont know the difference in the seat of your pants.
 
Hot, cold, throttle plates open, closed.....it just doesn't matter a BIT as long as you do all cylinders the same. I would assume (I know, dangerous), that the bank with the higher readings is the replacement head, correct?
 
Thanks for the replies
I took off both rocker arms,,,,removed the carb, blanked off intake.....Put a small amount of air pressure into intake about 5 psi....Wont hold any pressure not even a few seconds..... There is a bad leak, I can hear the air escaping and it sounds like its coming from under the intake manifold.
Originally I had a bad vacuum leak, I had removed the intake and was unable to use any other gasket except the valley pan, the bolts would not line up
So I used RTV over the entire sealing surfaces and that made a difference but it still didn't run right and the vacuum gauge indicated a possible leak
Which took me to were I. am now
Im thinking the one head with higher compression has been been resurfaced to much, raising compression but giving me a misalignment issue with the intake manifold
Regardless I do know valves should be good and something is leaking.......I will be doing a smoke test later this week and to be on the safe side im going to do a leak down on the head with lower compression
 
Put a small amount of air pressure into intake about 5 psi....Wont hold any pressure not even a few seconds.
I'm trying to visualize this ,lol
You know that there are 4 chambers connected to one side of a dualplane intake right? Two on the left and 2 on the right. I guarantee you that not all 4 intake valves will ever be closed at the same time on one leg of the dual plane. Not even two. Now I suppose you cold find the open one and back off the rockers as you did on the one, but how will you guarantee that the other three are all three also closed?
 
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I'm trying to visualize this ,lol
You know that there are 4 chambers connected to one side of a dualplane intake right? Two on the left and 2 on the right. I guarantee you that not all 4 intake valves will ever be closed at the same time on one leg of the dual plane. Not even two. Now I suppose you cold find the open one and back off the rockers as you did on the one, but how will you guarantee that the other three are all three also closed?
I removed both rocker arm assemblies. All the valves are closed both sides
When I pressurised one of the cylinders in question while I know there will be air leakage past the intake valve, its leaking way to much
 
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Shouldn't be any air leaking past a closed valve. Sounds bent. Worked on a 340 last year that had a weird idle issue and definite miss. Compression check was not pretty, but I was surprised that a cylinder with a bent exhaust valve would build 90 lbs of pressure AND there were two of them...
 
The air in the cylinder test is a simple one. Although too many people just use maximum shop air and expect to find it, a lot of times, full shop pressure is too much and can throw off where it's actually coming from, so here's how I do it. I regulate the air down to 10PSI and do the test on each cylinder.

Air out the exhaust pipe is burned or improperly seating exhaust valve, ir something keeping it open. Maybe a guide too tight.

Air out the intake manifold at the carburetor mount pad, same thing with an intake valve.

Air coming up into the valve cover/rocker arm area will be bad or broken rings, hole in a piston or "some such".

Regulating the air pressure is important, as it will help you pinpoint the problem faster.

I agree with Jim. You shouldn't have any air coming past valves under any circumstances. The rings however, will have a little leakage, but not bad.
 
Shouldn't be any air leaking past a closed valve. Sounds bent. Worked on a 340 last year that had a weird idle issue and definite miss. Compression check was not pretty, but I was surprised that a cylinder with a bent exhaust valve would build 90 lbs of pressure AND there were two of them...
Jim bare with me as old age creeps up, im forgetting
We use to pour liquid down each port when we had a head off to see how long it would take it to leak around any given valve.....If I recall 15 seconds before you saw wetness around the valve in combustion chamber? But id think you might always get some leakage around a valve if you kept pressure on it long enough
Leak down tests show what percentage air is from escaping combustion chamber and to what point its either acceptable or not
My heads were not done at the same time as I found one of the heads cracked before I even started the engine......So machinist sourced a head and rebuilt it.....What I think is happening now as I research and think about it is there is no cracks in this head, no blown head gasket and all the problem may be is the intake valves aren't seated well on that cylinder head
When I put shop air into the head in question it leaked a small amount of air past the intake valve into the manifold and other head ports and that might be expected
What I have to do is a leak down test on that head and see how bad the intake valve is leaking
The other head is tighter than a frogs *** so when he did the head that is the problem maybe he was having a off day......
While im not happy with the differences in compression between the two heads and performance could suffer, this may have no impact on how the car was idling
A leak down test and a smoke test to check the intake manifold to head ought to shed light to if there is a problem
 
Thanks for the replies
I took off both rocker arms,,,,removed the carb, blanked off intake.....Put a small amount of air pressure into intake about 5 psi....Wont hold any pressure not even a few seconds.....

I read this,
I took off both rocker arms
and didn't put it together, that you meant,
both rocker arm assemblies.
In this case it's an easy find.
 
The cylinder heads need to be surfaced the same amount.
If not your intake will never seal for very long, I don't care how much sealer is put on, it won't seal.
I'd be interested to know what pistons and heads on it to register such high cranking #'s.
I just can't see those numbers with an open chamber head.
Look for a leaking intake manifold causing the rough idle.
 
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