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  1. #1

    general question about bore size and performance

    It sounds stupid, I know, but I've been wondering now for some time how much performance gain you get from boring a cylinder bigger. Obviously there's substantial gains going from stock on a 84mm bore to an 89mm bore, I get that. I'm just wondering if you rebuild a motor and go up .5, or 1.0, or 1.5 how much more hp we're really talking about. For arguments sake, lets just say that everything is stock on a GP1200 NPV motor, its actually 1131 cc. You bore it over 1.0 mm, now its 1156 cc. Thats 25 more cc. Can that somehow be related to a certain hp increase without actually doing a back to back comparison?


  2. #2
    Avi8tor's Avatar
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    I would imagine if you made 150 hp with a 1131cc engine.... 150/1131 = .13262 hp per cc ..... multiply that times 1156cc overbore size and you get approx 153 hp so I would say yes, you would have a small hp gain potential.

    On a stock gp1300r with 170 hp, going to 1390 could yield 11-12 hp just from displacement alone. Make that a ported 1390 and woohoo.

  3. #3
    ^^^OK, that seems like some valid logic. The actual hp spec for a gp1200 is 135 hp. So that makes .11936 hp per cc. So if you bore it to 1156 you get a modest 3 hp gain or so.

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