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  1. #1
    Chine Walker
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    Compression increase vs. HP gain

    A stock 1300 '03 GPR has a compression of @ 120, 120 & 118.psi. By increasing to 150; what HP gains can you expect with all thing being equal?
    Thanks,
    J.P.


  2. #2
    philip_gpr's Avatar
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    hp

    typically when the compression is raised the squish is also lowered - both act to raise HP across the complete powerband

    R&D claims a 1 to 1.5 mph gain with their head, that would be approx 8 to 12 hp

    some of us will raise compression even higher and lower the squish even more than than an off the shelf aftermarket head would provide for further gains

  3. #3
    Chine Walker
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    Philip:
    R&D cast heads seem to come with lower compression ratios for cyl. 2 & 3; I guess this is a safe route.
    Does it pay to machine the stock head just a little to obtain @ 135-140 psi?
    This would give a greater detonation safety margin, but would the HP increase justify?
    j.p.

  4. #4
    philip_gpr's Avatar
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    I am probably not the person to answer this. I run carbs with the 1200 ignition so I have a lot of control over fuel delivery and timing.

    If this helps I changed the jetting to delivery more fuel throughout the rpm range. Then I reduced the timing in the mid and high rpm range. I always use Sunoco 94 fuel. My compression is 170 and my squish is .040.

    If you raise compression you would have to use an EFI controller and you would also need to use higher octane fuel.

    What I don't know is how much you can safely raise the compression on an EFI ski. (the answer to this will depend on the fuel octane you use)

    Maybe someone more familiar with using the EFI ski can comment.

  5. #5
    Chine Walker
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    The highest octane fuel here on L.I. is 93. That's why I was thinking of doing a mild compression increase. One thing that I question is the staggered comp. ratio on the GPR motor; doesn't this create different HP figures within the 3 cylinders, thus creating high stress on the crank?? Just a thought...
    j.p.

  6. #6
    philip_gpr's Avatar
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    lower hp on 3 does not have any negative effect on the crank , actually it reduces the stress on the crank

  7. #7

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    I don't think you would have any problems at 145-150lbs if you run 93 octane. You can just have your stock head milled, but you would gain even more if you had the squish band cut also.

    When my ski was EFI I ran 160lbs on all 3 cylinders and a 3 degree advanced and ran fine even with the sound suppression still installed. BTW the pump gas riva heads on my ski with a stock double layer head gasket was right at 145lbs.

  8. #8
    THE PLATE MAN JIM'S PERFORMANCE's Avatar
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    Hello j.p, You can get the stock head milled with no problem. The stock head is great because number 3 cylinder is staggered and keeps that cylinder from detonating. If interested in getting your head milled give me a ring. Thank you, Jim

  9. #9
    Chine Walker
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    Thanks Jim:
    Then I guess as others mentioned, I'll need to get the EFI controller. I've read that every ski is different so there's no starting point.
    Thanks,
    J.P.

  10. #10
    The controller will come with instructions and a couple starting points depending on your mods. then you can fine tune it from there. Do a search and you will see what alot of others are set at.

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