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Runner length and theory?

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  • Runner length and theory?

    How long are our runners?

    I was toying with some runner math (basically whats shown here: http://auto.howstuffworks.com/question517.htm) and I keep getting the results that our runners should be APROX 28ish inches long to get pressure pulses to line up correctly for a stock setup with a peak at 5,500 RPM...

    BTW, the math Im using...

    @5,500RPM it takes 0.01091 seconds for approx 1 crank revolution, and our engines intake is closed for 478* or 1.3277 revs, so thats 0.01448 seconds its closed.

    ASSUMING sound is traveling 1,300fps, which Im sure is off, but Im told its based on average intake tempuratures and intake air speeds

    1300 * 0.01448 = 18.824 feet up and down the runner, so 9.412 feet one way. Then we assume we are going to let the wave bounce 4 times so 2.353 feet...or 28.236 inches...

    I think to find the true speed of sound in our case, we'd have to weld up a set like 30+ inches then dyno and cut, dyno and cut, then when we fine one that peaks at 5,500, run the final length backwards through the equation with the speed of sound being X to find the true speed.

    Also if that is longer than stock (which Ive never measured but stock it seems alot shorter than 2 1/3 feet) how much fatter would we have to make the runners in order to maintain the origional stats (velocity and such) of the old ones?

    any input in the error of my ways (as you can tell Im very bored)

  • #2
    I think they are about 18in long from the factory, for the 91-95s.

    Mine will be about 10in long, but I am also not using a plenum style, and I'm looking for max power at about 7800rpm.

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    • #3
      Originally posted by Aaron
      I think they are about 18in long from the factory, for the 91-95s.

      Mine will be about 10in long, but I am also not using a plenum style, and I'm looking for max power at about 7800rpm.
      Yea, I guess you wouldnt have to be bothered by pulse tuning if you are just putting filters on the ends of the runners

      But as far as velocity tuning, does anyone have any equations I can play with

      Im curious to see how much wider the runners would have to be at 28inches inches in order to maintain the same velocity (or more) as our stock ones @5,500RPM

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