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  1. #1
    Hydrotherapy's Avatar
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    Wink difference in compression 05 1300r

    Hi guys
    new thought for the day..... I am aware that the compression on number 3 cylinder is lower than that of cylinders 1 and 2 with a stock head. If I go for a higher compression ratio, should I raise the compression on 3 at the same time? Does anyone know why number 3 has lower compression from new and what is the knock on effect of having all three cylinders with the same compression.
    The riva heads are 145 - 150 psi on all 3 cylinders, are these heads making the cylinders all the same since the heads I belive can go over any cylinder. I am assuming that the difference in the compression on a standard head is due to the area of the dome in the head. Milling the stock head I believe leaves the difference in compression on number 3. Can more performance be had by having equal compression across all cylinders, as a smaller bang on one third of the cycle must cause a slightly unsmooth revolution? Will the riva heads bolt straight on without any squish issues?
    Interested in the theory more than who can supply worked heads at the moment as Jerry's can supply riva heads at a competitive price, just need to know how and why.


  2. #2

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    No. 3 fails more often on 3 cylinder ski motors, some say this is due to crank flex advancing the timing on 3. But most tuners run differently/richer jetting on no3 when running carbs. This is difficult to do on a efi engine as all 3 are mapped the same. So for EFI engines its common to lower the compression on no.3 so that this one in not run so close to the edge.

  3. #3
    txgp1300r's Avatar
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    I cut all three of my riva heads myself on our clausen lathe and all were cut the exact same number to yeald an exact 150 psi on all three cylinders(one layer gasket) with that in mind ,I ran it this way on premium fuel all summer with my efi settings on 6.6.3.6. with no problems.....BUT..when winter got here i went out to ride and had a lean siezure on number 3.(air 55,barometer30+).this occured at 5000 rpm steady cruise..i think if i had added fuel i would have been ok...but why did 3 burn up and #1,and #2,are perfect...i can only assume that this is the power take off point and works harder,it crapped out at 117 hours.....to fix this im going to use one layer of head gasket on #1 and #2,and two layers on #3.

  4. #4
    mtrager's Avatar
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    TXG,

    Where would you have added the fuel ?

  5. #5
    Happily Self-Employed WFO's Avatar
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    Do yourself a favor and leave the compression on #3 lower. You cannot richen #3 independently. It will save the piston in the long run. Get yourself the R&D stage II heads. They have staggered compression.

    Also the 05-06 head gasket has larger holes for added cooling.

    I am using 45cc heads on #1 & #2 and a 46.5cc head on #3.

  6. #6
    Happily Self-Employed WFO's Avatar
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    Or mill the stock head

  7. #7
    I am planning to have the stock head milled also. What PSI should I shoot for if Im going to run 92 octane and still need it to be reliable?

    Also If it ends up needing to be lower to fit these requirements like 140 psi, is it worth the effort? would I notice a difference between stock and 140?

  8. #8
    I have recently check the stock compretion for my 2005 GP1300R and then results where as follows

    cylinder 1 140 psi
    cylinder 2 140 psi
    cylinder 3 135 psi but took litle time to reach 135.

    i was thinking that stock is 120 to 125 psi.

    can anyone explain the problem why 140 psi on stock heads.

    i experience problems inserting in the cylinder but i dont know in which.

  9. #9
    KaFF's Avatar
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    I'm also looking to shave my stock head. I was planing to increase compresion to 160. will that be too much? I dont want to use race gas

  10. #10
    Happily Self-Employed WFO's Avatar
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    Quote Originally Posted by KaFF View Post
    I'm also looking to shave my stock head. I was planing to increase compresion to 160. will that be too much? I dont want to use race gas
    nope, as long as #3 is lower

    EDIT:
    unless its an 05-up... then stick with 150
    Last edited by WFO; 08-18-2008 at 08:50 PM.

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