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  1. #1

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    What NOT to do with an RXP grate (experiment results)

    After reading up on intake tunnel pressures and how that slows you down, I decided to do an experiment. I have a couple of stock grates sitting around, so I modified one. I fab'd up a plate that blocks off the rear 3" or so of the tunnel opening. It essentially extends the rideplate forward and is flush with the hull. The theory is that it will sacrifice some holeshot and reduce tunnel pressure and thus, drag, at top speed.

    It turns out that "reduced holeshot" is an understatement. It cavitates terribly and would barely get on a plane. You have to gingerly feather the throttle while using body english just to get it out the water. Once you get on a plane, it seems to accelerate fine.

    As for top end, for reference this ski is a Stage 1+ that goes 74 mph in the winter stripped down and 71-72 in the summer. Today, I GPS'd 66 mph 2-up with a full tank. Not totally comparable, I know, so I raced one of my friends ski's. His is identical to mine except he has an XS intercooler and is not running correctly, only taching ~7700 rpm. He beat me, but just barely. So, the conclusion is that it either slowed down or added a very small amount of speed (<1 mph).

    The bottom line is that the stock RXP tunnel opening must be sized pretty optimally. Blocking off enough to KILL the holeshot didn't add any significant speed.

    Mike
    Last edited by engineermike; 06-08-2008 at 09:27 PM.


  2. #2
    05GTXLTD's Avatar
    Join Date
    Jan 2007
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    McKinney, TX
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    Quote Originally Posted by engineermike View Post
    After reading up on intake tunnel pressures and how that slows you down, I decided to do an experiment. I have a couple of stock grates sitting around, so I modified one. I fab'd up a plate that blocks off the rear 3" or so of the tunnel opening. It essentially extends the rideplate forward and is flush with the hull. The theory is that it will sacrifice some holeshot and reduce tunnel pressure and thus, drag, at top speed.

    It turns out that "reduced holeshot" is an understatement. It cavitates terribly and would barely get on a plane. You have to gingerly feather the throttle while using body english just to get it out the water. Once you get on a plane, it seems to accelerate fine.

    As for top end, for reference this ski is a Stage 1+ that goes 74 mph in the winter stripped down and 71-72 in the summer. Today, I GPS'd 66 mph 2-up with a full tank. Not totally comparable, I know, so I raced one of my friends ski's. His is identical to mine except he has an XS intercooler and is not running correctly, only taching ~7700 rpm. He beat me, but just barely. So, the conclusion is that it either slowed down or added a very small amount of speed (<1 mph).

    The bottom line is that the stock RXP tunnel opening must be sized pretty optimally. Blocking off enough to KILL the holeshot didn't add any significant speed.

    Mike
    So what would happen if you were to say......add a bit of horizontal to the grates at an angle that would encourage water flow into the impellar but still protect enough as to not suck in rocks, sea-weed, etc?

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