Motorcycle Myth # 5 - You need big ports to make big horsepower

Discussion in 'Dirt Bike Mods & Maintenance' started by Rich Rohrich, Jun 23, 2014.

  1. motometal

    motometal Lifetime Sponsor

    Sep 3, 2001
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    Thanks for the detail. Regarding the sonic boundries portion, I would guess some engineers and tuners have software that will help predict when/if this will be an issue, but otherwise just knowing this is a concern and what to look for is the key. Are you saying it's possible to exceed the speed of sound? On a related note, this can be an issue with .22 rfles, better accuracy staying subsonic.

  2. Rich Rohrich

    Rich Rohrich BioHazard
    Staff Member

    Jul 27, 1999
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    Localized sonic flow can happen in places where the port changes direction or sizing. Some call this "sonic choke" because it can cause a small shock wave that can essentially stop flow. Like most things with airflow I don't know that it is intuitively obvious when you are reaching that point in a port. It's difficult to model all portions of the port accurately even with really serious software like Riccardo WAVE CFD software.

    For those of us with more modest shop capabilities we have the trusty flow bench and pitot tube to measure pressure in various areas of the port to determine the relative velocity of the airflow. It's far from perfect but it has served us well for a fairly long time.

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