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  1. #1

    Not sure whether to do a gprxp conversion or keep the gp1390 lowell big bore

    Trying to decide whether to keep my single piped gpr1390 big bore or go ahead and do the 4tec gprxp conversion.

    My gpr atm is fast but not that fast. I often ride my mates rxp215 and it just seems to have so much more of an arm pulling sensation even though when we drag, the gpr eats it.

    The fuel economy of the gpr1390 sucks, majorly sucks, I get around an hour of heavy use before its beeping at me, Im guessing the gprxp would be alot better in fuel economy?


  2. #2
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    What MPH on the GPS are you getting on the GPR?

  3. #3
    Prism's Avatar
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    Just keep in mind... you'll need a 4 stroke engine ALMOST twice as big as a two stroke to run about the same speed OR the same size of each with the 4 stroke needing about 10 to 12psi of boost to compete (everything else being equal.) It still takes the same amount of horsepower to move that craft a certain speed regardless if it's 2 or 4 stroke (physics doesn't care). Horsepower comes from fuel and oxygen burned. The advantage is not so much a giant increase in fuel efficiency from the 4 stroke, (especially against a 2stroke with the proper tuned exhaust) but the fact that the boost can be turned up even higher...which makes more power, which requires more fuel and so on. If you decide to "follow the boost" I may take the big bore off your hands if you want to sell lol.
    Last edited by Prism; 04-09-2015 at 01:34 PM.


  4. #4
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    Hey I want dibs on the motor too!

    This forum will give people a false sense of reality it makes it seem like every ski on the water is doing 85 mph. Out on the water if you're doing 75mph you're beating 99.999% of the skis you'll ever run into. Don't forget it's not terribly difficult or expensive to have an 80mph 1300r (and still more reliable than a 4-tec). All you need is a $600 port job and a fine tuned hull.

  5. #5
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    If you are running 70 you are beating 95 percent or more of the PWCs on the water. I am in the process of having a new motor built for my triple piped Polaris. The way it is now it beats my friend's FZS out of the hole, but he is 2 MPH or so faster on top. Not for much longer though.

    THe power band is totally different on a 2 stroke. You get power off idle and a good snap down low. The 4 strokes come on at a higher RPM and pull harder in the mid and upper ranges, when you get into the boost. To me, nothing beats that low end "hit" you get from a 2 stroke. I don't think the bigger 2 strokes are really any worse on fuel than the SC 4 strokes, unless you can stay out of the boost on them. But what fun is that?

  6. #6
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    Everyone is chasing the 4-stroke boost bandwagon. Faster? Sure if you throw $30k at the motor in these insane build threads. Real world mods? A $600 port job on a 1300 will smoke everything on the water except flashed and modded FZRs... and not all of those will beat the 1300.

    My buddy sold his modded GP1200r for an RXP 215 to go faster. After all his time/money/work it's doing 72 GPS. I'll fly by that thing once I have Jim mill the head on my ski. Crap, I think I can get at least 71 out of mine with more ride plate tuning on the stock motor.

  7. #7

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    Quote Originally Posted by daniellib View Post
    Trying to decide whether to keep my single piped gpr1390 big bore or go ahead and do the 4tec gprxp conversion.

    My gpr atm is fast but not that fast. I often ride my mates rxp215 and it just seems to have so much more of an arm pulling sensation even though when we drag, the gpr eats it.

    The fuel economy of the gpr1390 sucks, majorly sucks, I get around an hour of heavy use before its beeping at me, Im guessing the gprxp would be alot better in fuel economy?
    I have a 1300 (Tommy Jordan's; 2fast4u) old ski, and it runs 82 to a best of 83.8 consistently. I also have a gprxp now. Holy s#!! the difference. I am running consistent 88mph and even 90.??mph sitting on the seat a couple of times. I have only had my gprxp for a couple months and still working on the ride plate setup. My recommendation to you is do the conversion, you will NOT regret it. I messed up by riding a buddies gprxp and I then owned one the next week.

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  9. #8
    powerstroke specialist mikegp's Avatar
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    [QUOTE=riick370;2533187]Everyone is chasing the 4-stroke boost bandwagon. Faster? Sure if you throw $30k at the motor in these insane build threads. Real world mods? A $600 port job on a 1300 will smoke everything on the water except flashed and modded FZRs... and not all of those will beat the 1300.

    dude ,seriously??? i quess you have not ride a gprsho with just a plain unmolested sho engine, they go for 82/84 w/o any mods.
    so how you came out with $30k in mods?

  10. #9
    powerstroke specialist mikegp's Avatar
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    daniel , you should w/o any doubt do the conversion, 2 years ago i was in your position , than a sho came into my world, oh man im glad i got rid of my 1300r .they were good , fun , beautiful , but are a thing of the past as now.
    4 stroke is the way to go this days.

  11. #10
    Moderator beerdart's Avatar
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    Agree my bone stock 255 GPRXP was 82mph all day the new owner wicked it up some and she is running low 90's.
    Quote Originally Posted by mikegp View Post
    daniel , you should w/o any doubt do the conversion, 2 years ago i was in your position , than a sho came into my world, oh man im glad i got rid of my 1300r .they were good , fun , beautiful , but are a thing of the past as now.
    4 stroke is the way to go this days.

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