| VG benefits low and midrange torque. The longer runners have more volume and the flow of air through them has more 'rebound' than runners will lesser volume. Since the air moving through the runners is moving in pulses, you are generating lateral waves of pressure within the air. When the timing of the pulses is just right, you create a resonance within the runner that takes advantage of constructive pressure rebound patterns which help move air into the cylinder. The lower the volume of the runner, the higher its resonant frequency is going to be and it will benefit high-rpm induction. Conversely, a larger volume runner has a lower resonant frequency and it will benefit lower-rpm induction. The volume dictates what the resonant frequency of the runner is and when the engine is operating at a specific RPM of which the frequency of the intake stroke of a cylinder matches this resonant frequency, you have what is called a 'sweet spot'. Since you aren't always running at WOT, the pressure in the manifold is constantly changing and this affects the resonance of the air within it as well. When boost pressure increases, so does the temperature. Both of these factors also play significant roles in the resonance of the runner. Harmonics of the natural resonant frequency also play a role.. i.e. If the port is tuned for resonance at 3KRPM, it will also play a role at 6KRPM and 9KRPM(if you were to go there without something breaking), but these harmonics at each interval higher have a lesser effect. Although the NA engine isn't worth 1000HP, it is worth a thousand words when it comes to reading its dyno-chart. This induction effect is clearly visible on clean non-turbo dyno chart. I love looking at the NA dynochart torque curve as it tells you a lot about the native induction dynamics involved in the TT engine as they are very very similar to each other. This is a bit of simplification of the dynamics I am aware of but it is the basis of the primary mechanics behind the induction system. Acoustical engineers would probably make great intake manifold designers.. Someone wanna call Bose? ;^)
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19:21:47 12/23/04
