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  1. #1

    Compression on a 155NA GTX - Compression too high?

    I have a 2006 155NA that I bought broke. Found a 2005 engine and used the block, pistons and rings(well within spec) from the 2005 and matched with my 2006 crank, heads, etc. All new bearings and gaskets. I had the machine shop hone it for me and check the specs on everything and everything was good. I just got it back together and put 3 hours on it. It seems to run great. Tonight I did a compression check and I got results of

    1-180
    2-180
    3-150

    Put the plugs in 2cyls and gauge in the other and ran the check again

    1-180
    2-180
    3-160

    So #3 is low. If I really tried hard, I could get all of them within 10% of each other.
    Reading done with a cold engine, throttle wide open.
    The machine runs upto 7400 RPMS. (I think these things have a limit of 7600 RPM?)


  2. #2
    I shortened up the post to get to the meat....

    1) Are these numbers OK to run on 87 octane as the ski is rated? Or do I need to retard the cam timing?
    2) Is there any inherent issue with having 1 cylinder slightly lower?

    My thinking is that if I retard the timing and reduce compression that these numbers may come closer into alignment.

  3. #3
    83.0 @ 8400 jhill19's Avatar
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    I would retard the timing to get the compression within factory spec to run on 87 octane.

  4. #4
    Quote Originally Posted by jhill19 View Post
    I would retard the timing to get the compression within factory spec to run on 87 octane.
    What is the factory spec? It is not listed in the service manual that I could find.

    Thanks,
    Greg

  5. #5
    83.0 @ 8400 jhill19's Avatar
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    on the scic motors (rxp) it's around 150

  6. #6
    Quote Originally Posted by jhill19 View Post
    on the scic motors (rxp) it's around 150
    So since this is a non-supercharged engine using the stock higher compression pistons is it reasonable to think my numbers OK?

  7. #7
    I found this post which tells me, I am probably OK with compression that high. http://www.greenhulk.net/forums/show...ht=compression.

  8. #8
    83.0 @ 8400 jhill19's Avatar
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    hmm, probably be ok then.

  9. #9
    boost junkie skidoochris's Avatar
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    I bet they should be closer to 200 psi when perfect for NA

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