Plenum sizing (a little off topic)

[Bernd Felsche]

Scott Knight writes:

>Bernd,

>> The plenum size isn't as important as the resonance characteristics.
><snip>

>Here is the part where I turn into a complete putz.  My understanding
>was that the runners set up the supercharge frequency by themselves with
>their shape, area and length.  Then the plenum was the Big Damper that
>allowed you to run a single throttlebody and have all the intake port
>entrances see the same pressure (within reason).  The bigger the plenum,
>the more damping and consequently possibly soggy throttle response if
>too big.  Seems the pressure waves that come up from the closed valve
>reflect back into the port at the opening in the plenum.  I know this is
>basically what you said, but that 'resonance characteristics' part has
>me a little confused.  I didn't realize there were plenum shape (?)
>considerations.  This could easily turn into a whole study all it's
>own...

The transition from runner/tract to plenum is what causes the
reflection. An infinite plenum (free air) has no internal resonances
to take into account. However, one where there are walls in the
vicinity of the end of the runner, i.e. all practical sizes, will 
have it's own characteristic resonances detrmined by the dimensions
of the plenum. 

I'm not surprised at all that this makes you feel like a putz trying
to get to grips with it; The acoustic design is complicated because
you're not dealing with steady-state resonances; pressures,
temperatures and pulse strengths are usually widely varied.

>I know there have been engines with variable plenum designs.  The VW you
>mention, and the V6 SHO Taurus is the first that comes to my mind.  My
>understanding was that the SHO just opened the valve between the two
>halves to make the plenum volume(s) twice as big somehow enhancing high
>rpm performance.  Thought it was strictly due to the volume.  Is there
>more?

Yes; the pressure pulses pass from one plenum to the other.

The Porsche system (forgot it's fancy name) is also a three-stage
switching system where connection of the "plenums" results in
improved induction at the highest engine speeds.

The Helmholtz equations can be used to approximate the (ideal)
resonant frequency plenum/runner combination - assuming a single
runner:
	f0 = c/(2*pi) * sqrt(Am/(l*V))

Where "Am" is the mean cross-sectional area of the runner, "l" is
the runner length and V the volume of the plenum. I think you can
figure out what "c" and "pi" are. :-) "f0" is the resonant frequency
near which the system will act as an amplifier. At approximately
half and double f0, the system will start to act as a damper.

-- 
Real Name: Bernd Felsche
    Email: nospam.bernie at perth.DIALix.com.au
     http://www.perth.dialix.com.au/~bernie - Private HP
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