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  1. #1

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    Question on supercharger air intake/boost versus RPM

    I installed a new custom intake today that is basically a 4” pipe coming off the SC, with a “Y” connector going into dual 5” hoses that end at a 4” flow-rite on each side of the boat (with catch-cans, etc.).

    I started the boat on the trailer (connected to a flush hose), and at idle, I could faintly feel air being drawn into the flow-rite. Then I rev’d the engine and was surprised that air was blowing OUT the flow-rite, instead of being sucked in. I didn’t rev it super high, because that’s not good to do with the prop out of the water, probably up to just 4k - 5k briefly.

    I remember someone here mentioning that the supercharger sucks above 5000rpm (produces boost), but blows under 5000rpm [no laughing, please], so the supercharger produces zero boost below that mark.

    I know someone will have a good answer here, so I ask: How is the engine getting the air it needs below 5000rpm, if air is blowing out of the intake? How does that work? I assume upon rev’ing it over a certain point, the blow would shift to a suck. I just curious of the mechanics going on there. It was a huge volume of air blowing out of the flow-rites.

    Thanks-


  2. #2
    HOSS's Avatar
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    What you felt was not air actually blowing out. This is gonna sound weird but with that huge of tubing you felt actual sound waves. I do it with exhaust all the time.

  3. #3

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    Quote Originally Posted by VX1 View Post
    I installed a new custom intake today that is basically a 4” pipe coming off the SC, with a “Y” connector going into dual 5” hoses that end at a 4” flow-rite on each side of the boat (with catch-cans, etc.).

    I started the boat on the trailer (connected to a flush hose), and at idle, I could faintly feel air being drawn into the flow-rite. Then I rev’d the engine and was surprised that air was blowing OUT the flow-rite, instead of being sucked in. I didn’t rev it super high, because that’s not good to do with the prop out of the water, probably up to just 4k - 5k briefly.

    I remember someone here mentioning that the supercharger sucks above 5000rpm (produces boost), but blows under 5000rpm [no laughing, please], so the supercharger produces zero boost below that mark.

    I know someone will have a good answer here, so I ask: How is the engine getting the air it needs below 5000rpm, if air is blowing out of the intake? How does that work? I assume upon rev’ing it over a certain point, the blow would shift to a suck. I just curious of the mechanics going on there. It was a huge volume of air blowing out of the flow-rites.

    Thanks-
    It still pulls in air... above 6500 the supercharger produces more than the motor can suck...N/A

  4. #4

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    Hoss, you are very astute when you’re off the bottle! I didn’t consider sound waves, but it sure makes sense, since the engine’s not going to run if it’s blowing air out the intake. The sound waves were so powerful, makes me wonder if they are impeding the flow of air into the engine (not that much could be done about it, if they are). Sounded very good, though! My engine is now in stereo.

  5. #5
    HOSS's Avatar
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    Quote Originally Posted by VX1 View Post
    Hoss, you are very astute when you’re off the bottle! I didn’t consider sound waves, but it sure makes sense, since the engine’s not going to run if it’s blowing air out the intake. The sound waves were so powerful, makes me wonder if they are impeding the flow of air into the engine (not that much could be done about it, if they are). Sounded very good, though! My engine is now in stereo.
    Yes this will impede cfm numbers. The tubing is too large for any possible gains in performance. Velocity is your friend. I`m actually going back with a 3" intake on my ski. I am still running a stock ECU. The Riva 3" pipe works well, then add a 4" to go under the steering. I ride in rough waters so the rear intake is hurting my capabilities (fun).

  6. #6

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    Wouldn't the same sound waves exist in the 3" pipe, just more concentrated? I'm not seeing how a smaller diameter pipe helps. With so many 4" intakes out there, I can't believe this hasn't been discussed before.

  7. #7
    HOSS's Avatar
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    4" intake tubes don`t seem to pose a prob as the air keeps moving, or enough velocity. Remember: velocity is your friend. 3" would have much more velocity but less cfm CAPABILITY.

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