Plenum sizing (a little off topic)

[Bernd Felsche]

Scott Knight writes:

>> Course there is the split manifold like the Viper has using two throttle
>> bodies.
>> Lot smaller packaging, and more along the lines of IR, but I'd imagine
>> different cross sectional area (larger) then a common plenum one, but that
>> is guessing on my part.
>> How about 8 butterflies, and a true IR, with a common filter area?.

>Well, there certainly is plenty of room to do about anything like that
>if I wanted to.  The problem is that for right now, the engine bay needs
>to at least look 'normal' because I autocross and street race this car. 

>At least I could stick a SuperRam on there if I wanted a big plenum in a
>bulky package, and that requires only minor fabrication to work with the
>LT1.  Just about anything else is going to be built from the ground up. 

>I might go there in the future if I have time for that, but for now, the
>important thing is to get it running, and I thought it would be easy to
>add on to the OE intake.  In fact, there is nothing on the top or rear
>of the plenum except space, so I could really add all I needed if I
>didn't care about ugliness.  Would be fun on the dyno to see what it
>really wants.

The plenum size isn't as important as the resonance characteristics.
Intake pulses will reflect off the large cross-section change at the
end of the intake tract. The frequency can be calculated to provide
the desired dynamic supercharge.

Depending on how you do your airflow measurement, the larger plenum
will dampen any pressure/flow changes which you sense in either the
plenum or in the main inlet leading into it at the throttle valve
end. That makes measurement of transients more difficult.

A smaller plenum will not mask those transients as severely and will
allow pressure pulses from appropriately-phased cylinders to be used
for dynamic supercharge as well; so you get (at least) two boost
peaks throughout the rev range, dependent upon tract lengths.

An example of the use of multiple plenums is on the VR6 engine with
variable intake manifold. There are three; one smallish one for two
grouops of 3 cylinders each, and a large collector plenum which
feeds into them. The short lengths to the small plenums act as ram
tubes, and also provide a high-pressure pulse to a neighbour at
higher engine speeds. The long length to the main collector comes
into play at low speed.

-- 
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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