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  1. #1

    question about compression

    Can anyone explain why it is possible to run high compression on a yamaha stand up ski (185 psi) on pump gas but on a kawasaki ultra 150 any compression above 155 psi will require race gas of 100 octane or better. Any help or suggestions would be appreciated. Thanks!!!!


  2. #2
    thesepaperwings's Avatar
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    Just be happy that you are faster then the stand up.

  3. #3
    It has to do with the Yamaha being a twin while the Kawasaki is a triple. I believe I read somewhere that its because the pipe for a twin is better tuned and allows equal pulses while the triple is not. If you look around all the twins run higher compression than the triples do.

  4. #4
    dhoad's Avatar
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    Quote Originally Posted by Shogun View Post
    It has to do with the Yamaha being a twin while the Kawasaki is a triple. I believe I read somewhere that its because the pipe for a twin is better tuned and allows equal pulses while the triple is not. If you look around all the twins run higher compression than the triples do.
    yup

  5. #5

    Thanks!!!

    Thanks!!!! Any more info? Can you run aftermarket exhaust like cofeman or factory and run higher compression and still run pump gas?

  6. #6
    jonny5's Avatar
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    Actually it has to do more with the timing then being a twin or tripple.

    On a 99gp1200 which we have run and raced forever we have ran it with 165psi on 87 for ever with riva's cdi's on rev B which I think is for mod. And also at 175-180 at 91 octane. Which running the lower octane fuel actually helps to advance the timing making it run a little hotter.

    I have asked this question my self because I have also run 155 on a gpr with 87 no problems ever but I think the stock kawi....ultra/stxr are already pretty close to the edge with the timing maps which is the reason they run pretty strong in stock form.

    Add any compression and it will start to deto with out a fuel change as in adding fuel or octane to it.

    The kawi's are just a little wierd when adding compression ...never been a problem for some other ski's....yam seadoo

    Any way hope this helps. If you want to add compression to 155 just use 91 and run it richer all over the range for a little bit and see what happens then start to lean it some ....by adding more fuel your cooling it but with it being so sensitive you have to pay attention to the sound and the way it runs before you loose a piston.

    I have not had a chance to test it this way yet but may get around to it. It's only a piston....

  7. #7
    I am also running coffman tripples with advent ignition which has timing curves for stock pipe, closed course, tripple race gas and tripple pump gas. Would I be able to run pump gas with higher compression if I used one of the stock timing curves? The ski is running 175 psi on 104 octane race fuel and I would like to run it on 87 or at least 91 without changing the heads if that is possible that way if I want to race it I could. Thanks for any advice!!

  8. #8
    btldigger's Avatar
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    I tried the caughmans with skiworx 42cc heads on 93 octane and burned 2 pistons. I would say you would need race gas with the raised compression and tripples no matter which timming curve you use. If you want to run pump gas then put stock heads on and run 93 oct.

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