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Compression/leakdown test results

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  • Clem Z.
    Expired
    • January 1, 2006
    • 9427

    #31
    Re: Compression/leakdown test results

    Originally posted by Timothy Barbieri (6542)
    Gary,

    What type of pcv breathing does the motor have. Is the pcv baffled to prevent oil from being sucked up into it, this is more important than people think.

    I once built a 350 that had the same problem and then re-ringed and still the same. I now believe the problem was a combined high volume oil pump and poor pcv arrangment. So much oil up top and the baffles could not keep it away from the suction of the pcv.

    The cranking compression pressure numbers sound very good to me..
    i have seen the same thing incorrect PCV

    Comment

    • Domenic T.
      Expired
      • January 29, 2010
      • 2452

      #32
      Re: Compression/leakdown test results

      Originally posted by Clem Zahrobsky (45134)
      i have seen the same thing incorrect PCV

      Tim & clem,

      I agree, I saw this happen twice reguarding the PVC.

      The first was a PVC line clogged at the carb and the second was a stuck (closed) PVC.

      In both cases the crankcase pressure built up and forced oil past the rings.

      The good news was that the rings didn't wear with all that oil but the combustion chamber was filthy.

      In my early days I pinched my cain oiler/ crankcase breather off with a tie wrap on my Brit bike and when I got on the freeway it smoked 2 lanes out instantly.

      The PVC valve operation IS more important than people think.


      DOM

      Comment

      • Gary R.
        Extremely Frequent Poster
        • April 1, 1989
        • 1796

        #33
        Re: Compression/leakdown test results

        Hi Guys,
        Yeah I guess I just tacked onto this thread since it was pretty similar to what I'm seeing and I don't think the was an answer to this thread?

        The 350 is the original 72 2 bolt short block I had punched over 020". I could have made a mistake installing the rings but I'm pretty anal with this stuff plus I file fit the rings.

        I'm using LT1 covers with the baffles, the hose from the pvc has at best a very very light film in it. Nothing I would expect to see with the amount of oil it's using, probably about a qt/250-300 miles. The plugs were starting to foul on 5-7, the lowest readings at 180.

        I stopped at the leak down because the hose crimp was leaking and it's PITA installing it into the heads with headers on the LH side. I got cut up on my right forearm playing around. My thought was if the leakdown was anywhere near 40% the compression test wouldn't be in the 180-205 range. I could be wrong there.

        I have two thoughts, 1- the WP heads have open stud holes into the intake runners and I didn't seal the stud threads, or 2- the intake gasket shifted or the head to intake angle is off. The heads and intake are new.

        I have a buddy who chased a similar problem and finally pulled the engine, honed it with a ball hone/drill and used cast iron rings. That stopped it for him, I'm not sure that's my problem.

        Before I got back to it the Drivers window mounts broke,so I ended up pulling the door apart!! I like rebuilding diff,arms,and boxes in the machine shop better!!!

        Comment

        • Domenic T.
          Expired
          • January 29, 2010
          • 2452

          #34
          Re: Compression/leakdown test results

          Gary,
          With file fit rings you should be good in the ring dept.

          When you said the problem was #5 & #7 it struck a nerve as a comon problem that I am sure you dont have, but those 2 cylinders are the MOST comon to have their wires crossed at the distributor or plug.

          DOM

          Comment

          • Timothy B.
            Extremely Frequent Poster
            • April 30, 1983
            • 5177

            #35
            Re: Compression/leakdown test results

            Gary,

            The 350 engine I had problems with I also re-ringed with stock cast iron rings and it did not solve my problem. I also had the Lt1 aluminum valve covers with the baffle. Do you have a high volume oil pump and what's the oil pressure after warm up at idle etc.

            I have no idea what pcv valve I installed but I think the LTI valve is a different part # FWIW. The camshaft installed was L-82 along with the high volume high pressure pump. I fouled plugs like you at 250-350 miles, nasty looking plugs when I took them out..

            You could very well be overwhelming the PCV baffle and valve guide seals. Some race engine builders build a restriction in the passages from the rear camshaft annulus to the lifter oiling passages to prevent over oiling the top end. The old SHP solid lifter small blocks used a lifter called edge orifice to limit oil through the lifter to the top end..

            Start the engine use some duct tape and block off the PCV and all openings like oil fill tube etc. and attach a vacuum gauge to the dipstick tube and see if the crankcase has zero pressure or very slight pressure (maybe 1/4-1/2 lb). This is normal, but if the crankcase develops vacuum then you know the intake is leaking. I know about burning hands on headers

            Comment

            • Gary R.
              Extremely Frequent Poster
              • April 1, 1989
              • 1796

              #36
              Re: Compression/leakdown test results

              Tim
              It is a new Melling M-55 oil pump, which I believe is the oem replacement not a high volume or pressure pump. However running 15w40 oil it does pump out 70 psi on the mechanical gauge.

              Here are the plugs with 800 miles on them. Some are 1/2 white- lean and 1/2 fouled. The carb is a Holley out of the box 670 Avenger, I didn't get to start tuning it.

              #1


              #2


              #3


              #4


              #5



              #6


              #7


              #8

              Comment

              • Rich W.
                Expired
                • March 13, 2011
                • 146

                #37
                Re: Compression/leakdown test results

                That's a good bit of oil burn..I would think that if the PCV valve is sucking oil in all the plugs would look the same..3 actually looks decent..kinda..compared to the others..I would get a real leak down test and get good numbers..

                Comment

                • Timothy B.
                  Extremely Frequent Poster
                  • April 30, 1983
                  • 5177

                  #38
                  Re: Compression/leakdown test results

                  Gary,

                  The plugs look nasty, mine looked even worse. If you can do a leakdown test it's probably the best thing to do as it will point you in the right direction if there are problems in the valves or rings. When you get a few minutes try the test for intake manifold leak like I described above.

                  If the motor is making 70 lbs. oil pressure that's alot of oil upstairs. Pulling the pan and changing the pump to a 45 psi relief is easy compared to taking things apart that you so carefully put together. I still say look at the easy things first and that much oil whiped up in the top end could very easily be sucked up by the pcv valve.

                  The mains and rods are only going to let so much oil pass because of the bearing clearance. The path upstairs is through the lifter bores and lifters and I don't think there is much restriction there.

                  I am not saying that's your problem but after my experence I would look carefully at the breathing and pcv along with that oil pressure. I remember using my 68 with this engine at track day at Summit Point and watching the oil gauge drop to 0-10lbs when going into a corner, all the oil up top.. It's a wonder the engine did not seize. Not that I am any kind of a racer but I did run the s--t out of that car..

                  In this post there are pics of the M-55 pump I used on my 67. It's a older pump and the casting is the heavy duty like the GM pumps but for some reason the relief springs are set 10-15lbs higher.

                  https://www.forums.ncrs.org/showthread.php?t=82068

                  Comment

                  • Gary R.
                    Extremely Frequent Poster
                    • April 1, 1989
                    • 1796

                    #39
                    Re: Compression/leakdown test results

                    Thanks Tim.
                    My biggest problem is getting the time to work on my own cars. Last weekend I had the time to start to test it and rip the power window apart.
                    If I get time again I'll at least get the window back together and fix the leak tester. I'll also do the tape test.

                    Gary

                    Comment

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