Plenum sizing (a little off topic)

[nacelp]

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

Bzzt.
It all has to work in harmony.

> Here is the part where I turn into a complete putz.

Only if ya let it get away from you.
You can study things in such fine detail, that you lose sight of the over
all picture.

Simply put, on a v-8, 4 stroke engine, at any one time, you'll have a pair
of cylinders at each operating stroke, so there will be an intake valve
closing, and one opening (or close to that stage), the closing one will tend
to slightly "reflect/pressurize" the intake, and with the one valve opening
have a slight head start for cylinder filling.  All this other stuff is
kinda neat to talk about but loses it meaning kinda quickly in the real
world, when you start taking about engine accleration rates.  It gets to in
car testing.  With all the money gm/ford/fiat/VW/MB has if there was a good
mathmatical model for this stuff it would be a done deal, but it takes as
much computer modeling a predicting weather (long term).  The cross
sectional area, is more about tuning for accleration rate, the lenght for
rpm range, and plenum for HP.   You can dial these in for a steady state if
you want but that don't win at anything other then cruise conditions, its a
matter of give and take for what your application is.

  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).

Damper, well sorta.  But, most importantly is that it allows the cylinders
to draw on a large volume of air, which is necessary at higher rpm/VEs.
Like you mention thou you can go too large, but that isn't as bad as when
talking about carbs, when you had the problem of fuel falling out of air
stream.  Lots of opinions carry over from carbs to EFI, and simply don't
apply.

Like (not to start the same lame thread again), dyno tuning can optimize a
engine for running on a dyno, with the new programmable software, it's
getting alot better, but it still takes in car testing to get it perfect.

  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...

If that's all you want to do, yep, I'd rather leave things adjustable so I
can taylor the plenum, runners to what I want and tune in car.
Les Grumpy
>
> 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?
>
> thanks.
> --
> Scott Knight  mailto:scott at scottknight.com
> http://www.scottknight.com/ IRC:SS396man
> '95 Black Impala SS
> '94 Ducati 900SS CR
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