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Technical Weird cylinder pressure numbers

Discussion in 'The Hokey Ass Message Board' started by Ralphies54, Dec 30, 2015.

  1. Ralphies54
    Joined: Dec 18, 2009
    Posts: 712

    Ralphies54
    Member

    Thought I would take a little time today to see why my 305 in my avatar truck is puffing a little smoke on startup and I started with a compression test. Surprised to find all 4 rt side cylinders at 158- 162 and all left sides at 118 -122. Puff is from left pipe also. Question is why would all 4 be so different from rt. Squirted oil in cylinders but made only 2 lb difference. Truck burns no oil between changes to measure. All 8 plugs were coated with soot when I pulled them. I'm thinking the puff may be fuel?? Real question is why the difference in pressure. I'm hoping someone can offer some help. Thanks Ralphie
     
  2. If you oil it and the pressure didn't come up that suggests a head problem as opposed to a ring problem.

    Loose guides or bad seals will cause one to puff on start up but bad seals won't cause low compression where loose guides will.

    I would try a leak down test I think you will find that the left side doesn't hold compression. if it works out to be heads do both and not just one because if one is going the other is not likely to be far behind.

    SBC are bad to loose the left bank first by the way. I have no idea why, it is just a quirk that they have always had.
     
    stimpy likes this.
  3. ^^^ this
    /thread
     
  4. Budget36
    Joined: Nov 29, 2014
    Posts: 3,259

    Budget36
    Member

    Is the heat riser thingy on the same side?...Could be partially closed.
     
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  5. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 24,616

    Jalopy Joker
    Member

    bad headgasket?
     
  6. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,068

    Rusty O'Toole
    Member

    I wouldn't question what Beaner said about one side wearing out first. But, is it possible 1 head was replaced some time in the past, or a pair of mismatched heads installed?
     
    stimpy, falcongeorge and wedjim like this.
  7. Ralphies54
    Joined: Dec 18, 2009
    Posts: 712

    Ralphies54
    Member

    Thanks for the replys so far. If I pull the covers and can see a head # are L&R heads the same#. Are they interchangeable?? I didn't say but I guess you figured out this is a SBC. Thanks Ralphie
     
  8. I always thought that the SBC ran inherantly hot on the left side because of the bypass in the stock water pump to the right side into the head. In time that wears out the left bank rings first.

    #5 and #7 always seem to run hottest and wear out first, because of the unbalanced water flow favoring the right side. #5 is usually the first one to ping or detonate when the timing is too far advanced or the fuel is not enough octane. Don't ask how I know...

    Edelbrock and Stewart have balanced flow (no bypass) water pumps that seem to correct this problem.

    Plus, 305's are known to have soft castings and wear out the rear cylinders first, also compounded by the unbalance of the stock water pump. My 2 cents...
     
    Last edited: Dec 31, 2015
    falcongeorge likes this.
  9. wedjim
    Joined: Jan 1, 2014
    Posts: 420

    wedjim
    Member
    from Kissimmee

    I would oil those cylinders and do a wet compression test. If the numbers improve significantly, motor. If not, head.
     
  10. Ralphies54
    Joined: Dec 18, 2009
    Posts: 712

    Ralphies54
    Member

    I did a oiled test ,only gained 2 or so lbs, so I'm thinking Gasket or ??
     
    wedjim likes this.
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 45,280

    squirrel
    Member

    When I worked in the machine shop years ago, we had a fixture to deck blocks off the crank centerline, and I did a few 350s...I was amazed at how far off they were, from the factory. Maybe one deck is way taller than the other?
     
  12. SBC heads will swap from one side to the other no problem.

    Monday and Friday blocks are the worse. LOL I have seen as many on here have some real atrocities on assembly line motors, 4 bolt blocks with 2 bolt caps, motors with one larger bolt than the rest in the line up, soft plugs glued in because the core holes were too big, decks so far out of align that I could not figure how the intake ever sealed.

    Oh and who can ever forget the over bored 396s, uh I mean 402s. LOL

    Jim could be onto something here for sure.
     
    Last edited: Dec 31, 2015
  13. ottoman
    Joined: May 4, 2008
    Posts: 280

    ottoman
    Member
    from Wisconsin

    As said above... do a leak down test first
     
  14. did you do the test with wide open throttle and all the plugs removed?
     
    falcongeorge likes this.
  15. Bent Wrench
    Joined: Jun 18, 2009
    Posts: 19

    Bent Wrench
    Member

    350 head on low side
     
    Montana1 likes this.
  16. 305's also have light weight heads and are prone to warp and crack easily from running hot.
    Sorry to be the bearer of bad news about 305's.
     
  17. squirrel, we ran into the same problem with our 377 dirt modified engine one side was .010 higher than the other bank. It could be that the right side head was rebuilt at one time. Working in a machine shop we were always surprised when someone brought in one head of a V8 to rebuild.
     
  18. squirrel
    Joined: Sep 23, 2004
    Posts: 45,280

    squirrel
    Member

    I was thinking about the head being milled also...and maybe a shim gasket on one side, composite on the other..you never know
     
  19. wedjim
    Joined: Jan 1, 2014
    Posts: 420

    wedjim
    Member
    from Kissimmee

    IMO, if a wet test made little difference, diagnosis is only going further with the heads off for comparison and inspection.
     
  20. Ralphies54
    Joined: Dec 18, 2009
    Posts: 712

    Ralphies54
    Member

    I had a chance to pull the covers, both heads have same #, but that's all I've had time for. Thanks again Ralphie
     

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