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  1. #1
    miagp21's Avatar
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    compression vs. timing??

    Hi guys, i'm wondering what combination will work better for top end? Would it be less timing with higher compression, more timing with less compression, or an even amount of both?? I'm currently running on my ported 1300R (NPV) 165psi compression with +3 timing. I was wondering wether to lower compression (150-155psi) combined with +4 timing or +2 timing with with 170 compression or just leave it the way it is??


  2. #2
    philip_gpr's Avatar
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    good question

    the fastest guy on the forum used to run very high compression (170)

    recently there have been others to run very well with lower compression (140 ish)

    with higher compression you want to reduce timing

    if its me I'd leave things as they are

  3. #3
    Happily Self-Employed WFO's Avatar
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    Quote Originally Posted by philip_gpr View Post

    with higher compression you want to reduce timing

    if its me I'd leave things as they are
    +1

    if you lower compression a bit, then you can run more timing

    oh, and you need a little lower compression on #3 or you can stick a piston

    I usually run about 10psi lower on #3 to be safe

  4. #4
    sleeplessnMS's Avatar
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    I'd like to know wutz the skinny on these cylinders? I hear #3 is the deto but #2 runs the hottest ect ect ..... I've stuck em all!!! LOL

  5. #5
    Moderator RX951's Avatar
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    when you talk about compression and timing, don't forget "octane" Thats a key variable as well.

  6. #6
    miagp21's Avatar
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    I run this combination (+3 timing/ 165psi comp) on 93 octane and so far i haven't had any issues, but i know i'm pushing it. The reason i brought this up is cause i've always though that more timing=better take off/mid-range with less top end.. so i thought maibe someone has tried different combinations of timing/comp on the same ski with same mods and noticed any difference. i'm thinking of trying lower timing with the 165psi comp i have now and compare the results.. Or leaving the combination i have now (+3/165psi) and run higher octane. What do you guys think?

  7. #7
    o uncola o's Avatar
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    Quote Originally Posted by miagp21 View Post
    I run this combination (+3 timing/ 165psi comp) on 93 octane and so far i haven't had any issues, but i know i'm pushing it. The reason i brought this up is cause i've always though that more timing=better take off/mid-range with less top end.. so i thought maibe someone has tried different combinations of timing/comp on the same ski with same mods and noticed any difference. i'm thinking of trying lower timing with the 165psi comp i have now and compare the results.. Or leaving the combination i have now (+3/165psi) and run higher octane. What do you guys think?

    I may be wrong, but seems like keeping what you have and running higher octane will not yield gains. It may be safer and less likely to predetonate, but the performance will be the same.

    I was under a similar impression with regard to the compression v. timing thing. More advanced timing seems to give it a kick all around, but with a focus on the mid and lower ranges. Higher compression seems to also give it a boost all around, but with a focus on the high mid and high ranges.

    DISCLAIMER: Once again, I have no data to back this up, just the impression that I have gotten from reading and some seat of the pants dynos in other vehicles, excluding the GPR.

  8. #8
    miagp21's Avatar
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    Quote Originally Posted by o uncola o View Post
    I may be wrong, but seems like keeping what you have and running higher octane will not yield gains. It may be safer and less likely to predetonate, but the performance will be the same.

    I was under a similar impression with regard to the compression v. timing thing. More advanced timing seems to give it a kick all around, but with a focus on the mid and lower ranges. Higher compression seems to also give it a boost all around, but with a focus on the high mid and high ranges.

    DISCLAIMER: Once again, I have no data to back this up, just the impression that I have gotten from reading and some seat of the pants dynos in other vehicles, excluding the GPR.
    Yeah, i've gone thru this with my last ATV (i'm sure it's different in a ski), i added timing and gained bottom end performance but lost top-end.. I'm going to try lowering the timing and compare results. Does any one know if lowering timing gain RPM's at top end??

  9. #9
    PolarisNut's Avatar
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    With a 2 stroke, you eventually reach a point of diminishing returns with compression and timing...Too much timing and too much compression will hurt peak and over rev hp. High rpm two strokes like to have the timing pulled back a bit on top...it "frees" them up a little and allows you to rev them a bit further past their peak hp point, without the power falling off like a stone. I have found though, that while the engine seems happier up top with a little less timing, it can be a bit slower to hit peak rpm, and to recover that RPM after a heavy load situation (i.e. going full throttle out of a turn).

    There is only one way to find out what your combination likes, and thats to test it. Different porting will change timing and compression requirements, as will exhaust back pressure (pipe design, silencer, stinger size, amount of water injected,etc). Dome design (squish width and clearance, bowl size and shape) also plays a role in what your actual combination will like.

  10. #10
    miagp21's Avatar
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    Quote Originally Posted by PolarisNut View Post
    With a 2 stroke, you eventually reach a point of diminishing returns with compression and timing...Too much timing and too much compression will hurt peak and over rev hp. High rpm two strokes like to have the timing pulled back a bit on top...it "frees" them up a little and allows you to rev them a bit further past their peak hp point, without the power falling off like a stone. I have found though, that while the engine seems happier up top with a little less timing, it can be a bit slower to hit peak rpm, and to recover that RPM after a heavy load situation (i.e. going full throttle out of a turn).

    There is only one way to find out what your combination likes, and thats to test it. Different porting will change timing and compression requirements, as will exhaust back pressure (pipe design, silencer, stinger size, amount of water injected,etc). Dome design (squish width and clearance, bowl size and shape) also plays a role in what your actual combination will like.
    Yeah, i guess it's going to get down to testing and trying different combinations for my individual setup


    Thanks to everyone for the advise..

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