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  1. #1
    just GOT TO DO IT! great lakes gpr 1200's Avatar
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    1390 groupk motor HELP?

    Hey I have a 1390 gk and the comprestion is 140? If I removed the head an took one of the three layers off head gasket off that would give me about 10psi right? That Is where I want to be with stock timming and 91 (93 all the time) octane right? What Tipped me off to the comprestion was I ran some 105 octane gas (that was 110 mixed with 93 following the chart) and gained NOTHING AT ALL. so I wanted to get up to max compression for 93 octane fuel?
    Did any of this make since?


  2. #2
    Moderator Insanediego Joe's Avatar
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    IMO, Leave the oem gasket alone. If you want higher compression get your head milled or buy an aftermarket head.

  3. #3

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    Quote Originally Posted by Insanediego-RXP
    IMO, Leave the oem gasket alone. If you want higher compression get your head milled or buy an aftermarket head.

    That's right! Guys will mess with them all day long and be fine. BUT there are that .1% that you'll be part of if you mess with then gasgets! Go with a milling of the head! Gourp K will do for you and Lowell will also ~$170. That's much cheaper then replacing pistons, heads, shoot the whole dammmmm engine if something fail big.

    IMHO!

  4. #4

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    you'll get a little more compression BUT squich big factor make it to tight
    and you may start poking hole in piston
    like that BB not fast enought?

    later cd

  5. #5
    OCFZR's Avatar
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    You really wont gain anything with the higher Octane because of your compression. The higher octane will burn slower allowing for higher compression and less detonation. You should try several different compression guages. You will find quite a variance between different manufacturues of guages.

    tim

  6. #6
    GPRPITMAN's Avatar
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    You can get the head milled to run more compression to run higher pitch prop getting more speed. But I am at 140 across with 93 octane I would leave it alone play with the advent box with some curves and it will pull like a train You know where I am at with my boat

  7. #7
    just GOT TO DO IT! great lakes gpr 1200's Avatar
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    I'm sure glad I asked. It seemed like a good idea. I'l just file that in the file marked "dumb ass ideas".

    Thanks guys.

  8. #8

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    not really,may people are adjusting squich by adjusting multi head gaskets
    you just will have to ask group K what the sweat spot is on head setup
    your think right ,just keep tweeking

    later cd

  9. #9
    just GOT TO DO IT! great lakes gpr 1200's Avatar
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    well I just got out from under my ski, I was working on the pump shoe depth. It just didn't look right. I found my inserts pulling thru and small cracks around them.DANG IT! well it's going on the rack in the garage. I'm going to have to ride the 2004 stock 1300r, there is not much ridding left. I might as well just start saving money, and getting it ready for next summer.
    It ain't that bad. the 1300r only has 15 or so hours on it. so it's like ridding a new ski for a while. but I won't be beatting any RXPs for a while! ... I guess I can just laugh it off.

  10. #10
    philip_gpr's Avatar
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    there are a lot of us that have run with only 1 layer of the gasket

    it is always better to use the gasket as originally designed but sometimes by simply splitting the gasket you can get the squish and compression right

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