brianpublic2(at)starband.
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 Posted: Tue Feb 27, 2007 1:27 pm Post subject: volume/pressure question |
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I know there are a lot of smart engineers out there, so I am hoping someone can help me with a calculation that is beyond my knowledge.
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BACKGROUND: My supercharged Eggenfellner STi engine starts and idles at a too-high value (1500 rpm) after I connected the supercharger plumbing to the intake manifold. It used to start at 1300, then drop back to 1000 after a couple minutes, then drop to 750 after it got warmed up. With the supercharger hooked up, and the wastegate open, it starts at 1500, then drops to 1300, then it kind of fluctuates slowly between 1100 and 1280 awhile, then eventually (the one time I ran it that long) back to 750. I’d really rather it didn’t start at such a high idle. Details & pics in my Feb engine page http://brian76.mystarband.net/engineFeb07.htm
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I’d like to try to find out in advance how much overpressure I will have at idle if I go to the not insignificant trouble of removing the supercharger plumbing, cutting off the ½” valve adapter, and welding on a larger valve adapter.
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THE PROBLEM: I am generating X amount of air volume in the supercharger ducting. That volume being vented through a ½” wastegate hole in the ducting is causing a 7 psi pressure in the ducting, in front of the throttle valve. If I change to a ¾” hole, what will the pressure be for that same volume of air being generated? What would the pressure be if I changed to a 1” hole?
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I suspect it’s more than just a simple matter of comparing the area of the holes and multiplying that comparative ratio times the 7 psi (which of course I can do myself). If that was true, a hole with 10 times the area would still be holding back enough air for there to be 0.7 psi in the duct, which I find hard to believe. I suspect it’s not a linear relationship, but maybe it is. I hope someone who knows about these things can help illuminate me.
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Thanks,
brian
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