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Thread: Max compression for 93 oct on LS7 with 244/254 .650 cam?

  1. #1

    Max compression for 93 oct on LS7 with 244/254 .650 cam?

    what is a safe compression ratio for an ls7 with 244/254 .650 lift cam?

    want to ultimately be able to safely run 93 but want more compression for e85

    I'm thinking 12-12.2:1 what does everyone else think?


  2. #2
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    Nice
    Thanks,

    Dustin

  3. #3
    Advanced Tuner Montecarlodrag's Avatar
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    12:1 is on the edge for 93 octane but it will work with reasonable timing for a daily driver.
    I wouldn't use 93 with that CR for racing. You will lose all the advantage of the raised CR because you will be forced to use much less timing.

    If you want to use 93 only for a while until you are ready for E85, you'll be ok.

  4. #4
    thank you for the response :-)

    i'm not that up on everything yet, but i'm reading and learning

    I remember reading about how having a larger cam reduces the effective compression ratio.

    IIRC something to do with the valves being open more and longer.

  5. #5
    Advanced Tuner Montecarlodrag's Avatar
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    Yes, thats called dynamic compression ratio.
    The static compression ratio is calculated measuring engine's parts and it's always higher than dynamic CR

    The more overlap the cam has, the less dynamic CR

  6. #6
    Ah dynamic yes that's what i was thinking of.

    I couldnt remember if it was static or dynamic that was affected.

    I'll do a search for a dynamic compession calculator for curiosities sake.

    Thanks

  7. #7
    For anyone else wondering about static and dynamic here is a good read
    http://cochise.uia.net/pkelley2/DynamicCR.html

  8. #8
    Advanced Tuner vette_c6's Avatar
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    for pump gas , i wont hit 12

    specialize in: V8 Tuning (Kuwait)
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  9. #9
    Thanks for the feedback everyone

  10. #10
    11.4 is as high as we go on pump gas..

    Now E85 is a different story!
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  11. #11
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    Just built a LS7 for a customer that had 11.9:1 static CR. The cam was very similar to yours, I had the combustion chambers in the factory heads cleaned up and CC'd to 66cc. It runs perfect with no KR activity wot starting at 32 degrees and finishing with timing at 26 degrees all done with crappy Socal 91oct. AFR was from 12.4:1 - 13.0:1 richest at peak torque of 5500 rpm's.

    Remember a big duration cam will scrub off cr in the low rpm range (or become less efficient) while increasing efficiency towards the mid and upper rpm range. Also the engine will ask for quite a bit more timing as well in the ranges where it is most inefficient.

  12. #12
    Quote Originally Posted by HoP View Post
    Just built a LS7 for a customer that had 11.9:1 static CR. The cam was very similar to yours, I had the combustion chambers in the factory heads cleaned up and CC'd to 66cc. It runs perfect with no KR activity wot starting at 32 degrees and finishing with timing at 26 degrees all done with crappy Socal 91oct. AFR was from 12.4:1 - 13.0:1 richest at peak torque of 5500 rpm's.

    Remember a big duration cam will scrub off cr in the low rpm range (or become less efficient) while increasing efficiency towards the mid and upper rpm range. Also the engine will ask for quite a bit more timing as well in the ranges where it is most inefficient.
    Great info, thank you for the response!

    That's pretty impressive for socal 91 which is about like EC 89 octane