Compression readings

Big Block A body Tech

  1. Steve welder

    Steve welder Well-Known Member

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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
     
  2. fishmens67

    fishmens67 FABO Gold Member FABO Gold Member

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    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.
     
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    • Steve welder

      Steve welder Well-Known Member

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      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
       
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      • krazykuda

        krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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        Your readings vary less than 10%, you're fine...
         
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        • Jim Lusk

          Jim Lusk Well-Known Member

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          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.
           
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          • Bewy

            Bewy Well-Known Member

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            One head could have been surfaced more than the other....or the block deck height is different side to side.
             
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            • Steve welder

              Steve welder Well-Known Member

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              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
               
            • 69_340_GTS

              69_340_GTS Well-Known Member

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              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.
               
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              • mbaird

                mbaird mbaird

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                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
                 
              • dano

                dano Evil Handy Man

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                Was the test done with the engine warmed up fully?
                 
              • Steve welder

                Steve welder Well-Known Member

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                No, I cant do it on a hot engine.
                carb butterflies opened wide, all plugs out, fully charged battery
                 
              • MOPAROFFICIAL

                MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                "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.
                 
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                • Steve welder

                  Steve welder Well-Known Member

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                  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
                   
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                  • MOPAROFFICIAL

                    MOPAROFFICIAL FABO Gold Member FABO Gold Member

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                    I would.
                    Perf or commuter, but I'm a stickler for some things being equal
                     
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                    • oldkimmer

                      oldkimmer FABO Gold Member FABO Gold Member

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                      Your rings may not be seated yet. Run it for another thousand miles. Kim
                       
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                      • 512Stroker

                        512Stroker We are all here because we are not all there.

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                        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.
                         
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                        • RustyRatRod

                          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                          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?
                           
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                          • Steve welder

                            Steve welder Well-Known Member

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                            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
                             
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                            • AJ/FormS

                              AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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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?
                               
                              Last edited: Nov 1, 2021
                            • Steve welder

                              Steve welder Well-Known Member

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                              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
                               
                              Last edited: Nov 1, 2021
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                              • Jim Lusk

                                Jim Lusk Well-Known Member

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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...
                                 
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                                • RustyRatRod

                                  RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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                                  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.
                                   
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                                  • Steve welder

                                    Steve welder Well-Known Member

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                                    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 ass 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
                                     
                                  • AJ/FormS

                                    AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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                                    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.
                                     
                                  • fishmens67

                                    fishmens67 FABO Gold Member FABO Gold Member

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                                    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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