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Thread: Rear Fan Ideas

  1. #1
    David 1 FAST VE's Avatar
    Join Date
    Jun 2005
    Southaven, MS

    Rear Fan Ideas

    Keep in mind this IS NOT an RXP but an X-4 hull but the principals should be the same. The X-4 has a down pipe similar to the RXP mounted under the seat that draws air through the seat for incoming air. Some people were mounting fans in the tube blowing air into the hull to get more fresh air into the hull. Some claimed that it helped some said it was a waste of time. So I tried it....

    A brief description of the ski will help though. This was a custom built VE 920 built and designed by George Grabouski. It used 90mm Honda CR 500 pistons for a displacement of 930cc or somewhere around there.

    I was running a Spec II Factory Pipe that was extruded honed and ceramic coated, Novi 46mm carbs, the intake was matched to the cases by HPT. There was no expense spared on the design of this engine down to custom designed sleeves for the cylinders.

    For those that do not know when you tune a 2 stroke the idea is to get the expansion chamber hot enough to just start to make water drop sizzle when dripped on the pipe from a WOT run.

    This boat was tuned as much as could be, piston wash was perfect; egt's were 1065 & 1050 degrees.

    Now I decided to add the UMI setup and figured that I would make some changes. The fan blowing air into the hull did defiantly help on the bottom end some but no gains in the upper rpms.
    When I changed the steering I decided to cut a hole in the bottom of the hood, cut out from behind the mirrors and drill holes in the front storage bucket to get more air flow into the hull.

    This was a definite improvement from before and I picked up about 50 RPMS which I had to compensate with a high speed adjustment.

    From there I decided to try something else. I removed my egt probes and fixed them into the hull one in the front of the engine, and one in the rear. I found that it was cooler in the front of the ski which I attributed to there being way more air flow in the front than the rear. The temperature difference was a little over 10 degrees from front to back and overall temperatures were 30 degrees over ambient.

    So my big idea that some think is well not that good of an idea was to turn the fan around and pull air from out of the hull causing air to flow from front to rear. Using the egt gauges I found what was pretty interesting. The hull temperatures were now within 5 degrees of ambient front to back and I gained another 100 rpms over where I was before.
    I had to go up a jet size to compensate for the new found rpm's.

    This ski truly ran 70 mph on a stalker radar gun and would hole shot 99% of the skis out there.
    I am not saying it was the fastest X-4 out there but there were only a hand full of skis around that were built anywhere like this one.

    I am attaching some old pictures, sorry they are not the greatest but you will get the idea, I hope to try something like this on the RXP very soon, and I will let you guy know the outcome.
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  2. #2
    GTXX's Avatar
    Join Date
    Jan 2006
    NW, GA
    when you had the fan blowing in there was no place that air was going out, not easily anyway. When you opened up the front, that let more air in from that end but the fan was working against any air trying to go out. turning the fan around to pull air out was the right thing to do.

  3. #3
    ccassone's Avatar
    Join Date
    Nov 2005
    Branford, Connecticut
    Can you post more pics of that xp? I am really interested in it. Post as many pics as you can of the motor pump, everything! Thanks!!! Charlie

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