Quote:
Originally Posted by whatthe
Why do you say that acoustic tuning will not be effected at part load? As I see it, the air is still pulsing even past the throttle valve. If you go home and stick an 8 foot section of pipe on your intake, go for a drive, and tell me that it didn't make a difference at part throttle.... I will be incredibly surprised. I have an exhaust/intake theory book that has a few graphs showing the effects of cylinder pressure/filling against valve opening for a few different lengths of intake. This length of pipe was placed before the "carburetor/intake manifold" (older book).
From an incredibly limited experience stand-point, and a premature update for Vanner, I put a 6-7 foot length on my GT yesterday, put the wideband on, and went for a drive today. The pipe is longer than the theoretical values I was thinking of, but I used some old intercooler pipes and common sense says it's easier to make it shorter than longer later. There have been some interesting changes that I can better get into after a tank of gas.
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Tuning upstream of the throttle valve will change the acoustic signature (sound levels) apparent to the driver and outside environment, yes. Car makers add Helmholtz resonators to the intake boxes to quiet certain frequencies that would sound objectionable to the driver.
But those effects are lost on the other side of the throttle valve, inside the manifold. Acoustics there, which would cause local pressure wave optimizations right at the intake valve (for improved volumetric efficiency) are almost entirely (>90%) caused by reflections between the throttle plate itself and the various parts inside the manifold and intake ports. The throttle valve is still more than 50% closed at those operating points where we run for best FE, so that plate is a HUGE chamber end reflector for acoustic waves inside and outside the manifold.
Now, open the throttle valve and yes, the entire intake system becomes important. A plate at 90 degrees to the flow will have a minor acoustic impact. Racers and those looking for wide open or no, (i.e. diesel) throttle operation should look at the impact of the whole system for tuning effects.
A bit off topic, but for part throttle FE I think we'd be better off looking at a tuned exhaust system. That's always "wide open" and adjusting its length for constructive interference acoustics at the exhaust valve (where constructive means improved breathing) will work at ALL throttle openings. With a better exhaust flow you ought to reduce the total engine friction, but overdoing it could cause some of the incoming intake charge to be pulled right out of the exhaust valve. Reducing cam overlap could help optimize that.