Compression question

Discussion in 'Small Block Mopar Engine' started by Calipag, Dec 7, 2018 at 9:02 PM.

  1. Calipag

    Calipag FABO Gold Member FABO Gold Member

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    I have a stock 1974 Dart 318 LA engine with around 76,000 miles on it and it is a donor engine for my sons 1970 Dart. I did a compression test and came up with these results:
    #1 = 105
    #2 = 120
    #3 = 0
    #4 = 120
    #5 = 110
    #6 = 75
    #7 = 110
    #8 = 105

    Surprisingly the engine does run.....not well but it runs. I did the test with the throttle closed. The donor car did sit for some time before I got it.

    My question is: How can I tell if I have a head/valve problem or if the engine is tired and has some piston ring issues?

    I'm sort of hoping that its a head/valve issue at this point.

    Thank you for your help!
     
  2. nothingbutdarts

    nothingbutdarts Well-Known Member

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    Get #3 cylinder up at TDC and put air into the cylinder, listen to where the air is blowing out. If it comes out the exhaust, it's a bad exh. valve, if it comes out the intake, than intake valve, out the crankcase than hole in piston with as low as the reading is.
     
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    • Calipag

      Calipag FABO Gold Member FABO Gold Member

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      We had the intake off and the intake lifter of the (I believe) #3 cylinder keeper was gone and I had a bent push rod. I rotated the engine by hand and checked the valve train to make sure it was moving and all looked fine. But I did not check the compression at that time. The car is back firing out the carburetor. I will fill the cylinder with air tomorrow and let you know.

      I know #3 is my biggest concern but how about #6 it seems very low also.
       
    • Dana67Dart

      Dana67Dart Most undignified way to get to Colorado!

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      If a valve keeper is missing wouldn't the valve be hanging open and posably be hitting the piston?
      Backfiring out the carb would sugest intake valve open.
      Was the missing keeper on the intake of #3?
      Bent push rod sugests valve hit piston. OR valve is stuck in its guide and pushrod bent trying to move it.

      Good luck. Hope it is an easy repair
       
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      • Calipag

        Calipag FABO Gold Member FABO Gold Member

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        I hope it is the head and not the piston. But I did this to myself, once I saw that bent push rod I should have checked the compression in that cylinder and dug deeper into a know issue! I'm feeling like a real horses rear end now!
         
      • RustyRatRod

        RustyRatRod Just another dumbass. FABO Gold Member

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        Pull it and repair properly with a rebuild.
         
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        • Calipag

          Calipag FABO Gold Member FABO Gold Member

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          I wish that was in the budget. I guess I'll be looking into what year 318 I can use and look for a running cut out or part out engine.
           
        • RustyRatRod

          RustyRatRod Just another dumbass. FABO Gold Member

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          By the time you get another bad engine, you'll wish you had built the one you have.
           
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          • MOPAROFFICIAL

            MOPAROFFICIAL Well-Known Member

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            What you probably have there is an engine that sat with water in the cylinder
             
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            • Calipag

              Calipag FABO Gold Member FABO Gold Member

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              I'm trying to learn as I go along. How would you know that that buy those readings? I thought bad rings, bend valves, bad valve seats but not that water sat in the cylinders.


              Thank you
               
            • MOPAROFFICIAL

              MOPAROFFICIAL Well-Known Member

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              Or a valve has a piece missing.
               
            • AJ/FormS

              AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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              The numbers are all so low Ima thinking the cylinders and rings were super dry. No matter, the heads need work for sure, and the rings smell like toast.
              Could it be made to run? sure but at what sacrifice?
              I'm with Rusty/post6

              Grab a Magnum somewhere; she's a 9.5 engine with pretty good heads.
               
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              • nm9stheham

                nm9stheham Well-Known Member

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                I think he may mean the 'clip' in the top of the lifter was missing; that would allow the lifter to pump up, push the valve into the piston, and bend the pushrod. If this happened, OP, the #3 intake valve could now be bent and would indeed cause the 0 compression and the intake backfiring.

                As for #6, it certainly could be valve's badly sealing. Or it could be the rings. Go back and compression test #6 again. Then put 1/2 to 1 tsp of oil into #6 via the spark hole, and test compression again. If the numbers rise with the oil added, then you probably have a ring issue; if they stay the same with the oil, then look at the head/valve area.

                Sounds like the heads are coming off regardless of where this goes. At that time, the cylinders can be inspected/ measured.
                 
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                • Dana67Dart

                  Dana67Dart Most undignified way to get to Colorado!

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                  These were my dry, cool, wide open throttle numbers from my 67 273 (after 180000 miles on a rebuild)

                  #1 110
                  #2 107
                  #3 112
                  #4 120
                  #5 107
                  #6 110
                  #7 110
                  #8 112

                  most of yours are in the same range..
                   
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                  • Calipag

                    Calipag FABO Gold Member FABO Gold Member

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                    You are correct it was the clip that was missing.

                    I think from all the input, that I need to start looking into rebuilding what we have or replacing it as @AJ/FormS suggested.

                    Time to kick the Camaro out of the garage again and bring the Dart back in for a new/another motor.

                    Thank you to everyone for all the help.
                     
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                    • Calipag

                      Calipag FABO Gold Member FABO Gold Member

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                      What year Magnums should I be looking for a direct bolt in?

                      Thank you
                       
                      Last edited: Dec 8, 2018 at 10:46 PM
                    • RustyRatRod

                      RustyRatRod Just another dumbass. FABO Gold Member

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                      Any of them.
                       
                    • Calipag

                      Calipag FABO Gold Member FABO Gold Member

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                      No problem bolting up to the automatic transmission?
                       
                    • RustyRatRod

                      RustyRatRod Just another dumbass. FABO Gold Member

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                      No.
                       
                    • stixx

                      stixx FABO Gold Member FABO Gold Member

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                      OP wrote this above... Shouldn't a compression test be done at WOT (as Dana67 did)?
                       
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                      • Aaron65

                        Aaron65 Well-Known Member

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                        It should, but in this case it won't help that dead hole. Opening the throttle blades introduces air to compress, although many say that removing all the spark plugs introduces enough air into the manifold to do the same thing, regardless of throttle position. I always open the throttle anyway.
                         
                      • AJ/FormS

                        AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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                        In my experience , testing with closed throttle just requires more compression cycles to peak the pressure. So as long as you cranked until you had two consecutive same/similar-reading pulses; then the test should be valid.
                        With dry cylinders this could be many many pulses, and the numbers are always lower than expected.
                        When oiling them up, the oil has to be given time to climb up all around the perimeter of the rings. I usually put too much oil in ,let it sit for a bit, and then crank the engine for about 20 seconds to blow out the excess, which makes a bit of a mess. The excess has to come out, else it will skew your numbers higher than what is true.Some oil will always stay in there in the crevice and on the top of the piston, so your results will always be slightly high. This is not a big deal if you know this. Furthermore, you are not really looking for an absolute number, but comparative between cylinders.
                        In your case, you found 2 bad cylinders so it doesn't make a difference; the zero pressure in the one hole can only be made in one of two ways, and both require the head to come off.
                        The 105 to 120 is a large spread indicating the engine has other problems. But you rarely see a smog-teen with more than 130/135 wet tested, so the best of your dry test is 10 to 15 psi low, about what I'd expect. But your low cylinders indicate probably bad valve sealing.
                        So even if your heads were all fixed up, you won't know about the other problems .... because of only having done the dry-ring test.
                        But if you now do a wet test and all the numbers jump up a similar amount, say 10 psi. then your best holes are looking good. And the others, because they also jumped up 10 psi, then (except for 3/6) one could assume the rings are reasonably good. Leaving 3 and 6 as obviously head problems. So now, one could expect a fair engine after the heads are fixed.
                        The second witness to that would be a Leakdown test. No point in testing 3/6 but if the other 6 holes all read low leakage and similar numbers (after the oiled compression test) then I would just fix the heads and drive it.
                        In your case however, you cannot go straight to the LD test without having done an oiled test, or at least oiling the rings, because with dry rings or rings with dryspots , the LD test-results will be ambiguous. When you put 80 psi in there, it will just leak out the dryspots..
                        In your case, I would remove the rocker gear and do the LD test with the pistons at the bottom of the bores, where the least amount of wear is likely to have occurred. Also, with the pistons at the bottom, you are free to tap the valves and watch the numbers improve. And so, by the time you are done, maybe all the numbers (except 3/6 of course) may be similar, indicating possibly, that at least the short block is usable as-is.
                        As to the tapping of the valves;
                        sometimes carbon gets stuck between the valves and the seats, and they don'r close properly. Tapping smashes the carbon and the numbers improve. Same with rust on the seats or sometimes the valves stick in the guides and sometimes the guides are baggy and the valves don't center themselves. Rising numbers with tapping may mean there is something wrong with the heads. In any case if the numbers come up, that might indicate that the rings are ok.
                        As to the LD results;
                        The higher the test pressure, in my experience, the larger the percent leakage will be read. IMO this is a good thing, more accurately reflecting the running pressures which could be over 200psi at times. So I test at 100psi with the pistons at the bottom.
                        The industry standard seems to be 80psi. And so, with a test pressure of 80, I like less than 4%, less than 2% even on a fresh engine. Some will say up to 8% on a used engine will be ok. And on a hi-mileage DD that might be true. But an engine with 8%(or more) LD, will not win any awards for efficiency, nor for power; but it will locomote the vehicle it is in..
                        Happy HotRodding
                         
                        Last edited: Dec 9, 2018 at 9:01 PM
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                        • MOPAROFFICIAL

                          MOPAROFFICIAL Well-Known Member

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                          There's at least a 5 psi difference between testing throttle closed and wide open.

                          OP. I'm glad it was just a clip, that's a rarity with a used mystery motor. Most of the time it's worse. I had a 340 I picked up that looked cherry.... upon tear down the crank was broke in half at the no.2 journal and already maxed bore.... funny stuff.
                           
                          Last edited: Dec 9, 2018 at 11:01 AM
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                          • nm9stheham

                            nm9stheham Well-Known Member

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                            That's about what I have seen too... but a weak/dead cylinder still shows up weak/dead.
                             
                          • MOPAROFFICIAL

                            MOPAROFFICIAL Well-Known Member

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                            He's simply explaining the best procedure is all.
                             
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