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  1. #1

    Jet pump venturi tuning help

    With the addition of the stuff in my sig and fine tuning everything I have been able to bring my 1900 lbs boat from a stock 46 MPH to a modded 51 - 54 MPH depending on the air and water temp.
    Now what I want is to improve the bottom end take off as much as possible but be able keep the speed at least the stock 46 MPH. So on the low side of performance, when the weather is hot, I have 5 MPH to play with.
    Can anyone with expierence tuning venturi sizes, or perhaps a math wiz tell me how much I should bore out the venturi and steering nozzle? The venturi measures 84mm stock and the steering nozzle measures 87mm. I assume that I would bore each part the same amount?

  2. #2

    Join Date
    Jan 2007
    Lakeland, FL
    I hope you get better help than from me, but you know some trail and error will be involved, since your pump tunnel and intake are different from other applications here. Used venturis can be had fairly cheap, maybe buy an extra or two and bore them out each a step larger. Each mm should gain you 50-80rpms, so keep that in mind with your prop. I personally use the stock 84mm, but throw a 83mm on when it gets colder (need a 82mm too). I wouldn't bore the steering nozzle out as much either, to keep the steering nice and snappy.

  3. #3
    My RPMS right now are just shy of 8000 so if I went +2mm on the venturi I assume that should bring me to 8100 or slightly above (hate the analog guages).
    Perhaps a better question may be how much can I bore out the venturi without risking it blowing up. 86mm, 87mm or more?
    Also how much can a 14/19 be bent? I am currently at +2mm would +3 or 4 be pushing the limits if I had to?
    Last edited by cfnnslsq; 08-06-2008 at 05:46 AM.

  4. #4
    If you left your setup as it is and tried a 13/18 3 blade skat trak impeller you will loose a bit of top speed and will gain more bottem end. Maybe try that first and then fine tune it from there.

  5. #5

    Join Date
    Jan 2007
    Lakeland, FL
    I was running a 14/19+3mm successfully before, but +4mm pretty much ruined the prop, so I wouldn't try for much over 3mm. On the venturi, I just measured out 87mm on one, and it seems it would leave it dangerously thin. I'd say 86mm is the limit. Here's a thread of what BRP had to do to the "X" venturi to get it to 87mm (post #2):

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