Plenums

[Bruce Plecan]

> Why is it desirable to have the plenum volume the same as engine
> displacement?

I started measuring all kinds of volumes, some years ago.   First started
with some ram air experiments I was doing.  Over time, it just worked out
that for what I liked the closer to CID things were nicer, was a gray matter
until I built a incar timer and then it started to show actual performance
gains.   A 1/4" spacer under the lid of a Crossfire just changed it's
personality entirely.
It allows the air to lose ineritia and bend around to fill the runners
without having to lose energy.

> If the plenum volume is less, what are the bad things that happen?

At lower rpm, keeps the column moving as more of a column, and helps with
low rpm cylinder filling.

> If the plenum volume is more, what are the bad things that happen?

I've seen some stuffup at 1.5xCID, and no down sides, at speed.
You might have some **laziness** off of idle but I don't *drive* there.
I personally think it's used as a way to better optimise the *tune* for
emissions (keeping it small).

> I know I'm going to regret asking this but what functions does the plenum
> perform?

It just is where runners meet.
Basically, you have
IR   Individual Runners
Single Plenums
Dual Plenums
Variable Plenums

> Does the Bosch book discuss such things for plena and runners?

My turn what's a plena?.
Bruce



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