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Thread: Evinrude 31.8ci Mod Advice

  1. #21
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    Interesting. If I'm looking at this right, with all reeds being the same, it already has a variable valve of sorts built into the design. The inner reeds being the ones that provide the most flow throughout the rpm. I'm curious as to how the motor's volumemetric efficiency would change if the incoming charge was redirected to the outside reeds at lower rpms and brought inward as the R's increased. There's always the area behind the reed retaining screw that I see as being problematic but with the right velocities, it's possible that you could get a stuffing charge that would be resistant by a millisecond to the pressure signal of the compressing primary charge. I would venture to guess the motor would be very throttle sensitive but I can't say for sure.

  2. #22
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    Quote Originally Posted by TorqueXrpm View Post
    Interesting. If I'm looking at this right, with all reeds being the same, it already has a variable valve of sorts built into the design. The inner reeds being the ones that provide the most flow throughout the rpm. I'm curious as to how the motor's volumemetric efficiency would change if the incoming charge was redirected to the outside reeds at lower rpms and brought inward as the R's increased. There's always the area behind the reed retaining screw that I see as being problematic but with the right velocities, it's possible that you could get a stuffing charge that would be resistant by a millisecond to the pressure signal of the compressing primary charge. I would venture to guess the motor would be very throttle sensitive but I can't say for sure.
    An idea to share that I havent tried yet, fill that low at the center with some type shaped solid material, could serve to decrease crank case volume & subsequently increase velocity. So... what shape?, my 1st guess is obviously a cone Next would be some sort of mushroom shape that fills all but the highest pressure areas.

  3. #23
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    I would say a cone similar to the incoming side of the big three cylinder merc. It would fill the void and direct the positive pressure signal outward toward the reeds from heel to toe. The redirected signal "might" allow for a better reed response as it rebounds away from the "toe".

  4. #24
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    You know. Come to think of it. With that pressure signal rebounding off the outter circumference and across the reeds, that compliments my earlier suggestion and could bring a thin reed open sooner as the primary side transitioned from positive to negative pressure. Hmm

  5. #25
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    Does anyone know if there is a rep with CCMS on this site?

  6. #26
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    Hey everyone thanks for all your help so far.

    So we took the boat out this weekend and raised the motor (almost 4 inches) and added a little more tilt. our best was 37.2 mph GPS with me (130lb) and a 110lb of stuff add another person 200Lb and dropped to 36.3 mph. At 37.2 mph our RPM was 6060. I have a couple of pictures now just for test proposes we just used a peace of wood and some clams, hoping to buy a hydraulic jack plate soon. does anyone Know what would be the best timing to run? do you think we should go with a 16, 17, 18 pitch cleaver or chopper from Ron hill ? and we are looking at a over hub.

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  7. #27
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    37.2 mph @ 6060rpm? If your info is correct, I'd say you just topped out in terms of a prop. Those figures indicate a near perfect prop.

  8. #28
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    That Emustard is a good step forward, when your near potential an mph is hard to find, 3 is grounds for a party. Whatever #'s the prop you got that on are your safest bet on the next, a performance clever of the same hopefully will be a bit better, going up would risk lost gain.
    Since as soon as the h20 leaves the transoms edge it rises to seek level at your motors position it would likely be about an inch above your straight edge. If it will cool & the prop will hold 40 might be an inch away. You can run a small hose from the indicater spray up front so you can see it full time looking forward.
    All this is proof that props & set up are the things to do & afterwards motor work can be icing on cake. Good Luck.

  9. #29
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    That's what our tac and the GPSs said. I was very impressed with our results this weekend now just looking for a new prop and a stock tuner to test.

  10. #30
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    I would hold off on the prop. Get a trim plate and fine tune the set-up. If it remains the same, I would look at more mods for the engine. By my calculations, you will not find as more efficient prop the way it is. Hold off on the prop till another improvement is found elsewhere. That's MHO.

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