intake plenum size & reversion

[James Montebello]

Reversion is a when, during the overlap phase, the intake will be blown
all the way back through the plenum past the throttle.  In extreme cases
(small plenum, lots of overlap, short headers), the exhaust pulse may
even come back through.  The plenum is NOT the primary cause of reversion,
the cam timing is, followed by the exhaust design.  Triples may normally
have an exhaust setup that allows lots of reversion in a high overlap
design, I don't know.  However, in a low overlap engine, you're not
going to see a lot of reversion no matter how many cylinders you have.

A big plenum is not universally good, since the bigger it is, the
worse your throttle response will be, since the throttled volume will
be larger.  Think about it, the only thing you have direct control over
here is the throttle angle, and the delay between the time you change
the throttle setting and the time the engine "sees" the change is going
to be directly correlated to the volume of air between the throttle and
the intake valves.  You want to make the plenum as small as possible
for good throttle response.  Many full race engines use a throttle per
cylinder with a tiny throttled volume and no plenum at all.  Tuning these
engines is difficult, since MAP values are unreliable.  You can use
a plenum upstream of these throttles with a MAF, but then you're back
to the response problem again (delay before changes in demand by the
engine are "seen" by the MAF) as the plenum grows.  You can also use a
small plenum just for the MAP sensor, to smooth out the intake pulses,
again at the expense of responsiveness.

About the only disadvantage I can think of to a small plenum is intake
pulses will mess up MAP measurements, and how big it needs to be to keep
things sufficiently smooth is going to depend on a lot of variables,
cylinder size, engine speed, and cam overlap likely to be three of them.

FWIW, my 2.5L V6 has a plenum (OEM) that's a good deal smaller than 2.5L.
1L, maybe.  My 1.8L four, however, has a much larger plenum that's
probably close to the 2L.

james montebello

On Tue, 3 Jul 2001, Roger Heflin wrote:

> Chris Capowski wrote:
> > 
> > It is more complicated than that.  The number of cylinders you have makes a
> > huge difference as well.  Three cylinder engines tend to have (or they
> > should at least) the largest plenum/engine size ratio.  Reversion is bad...
> > 
> > Chris "Mighty Mouse" Capowski
> 
> What exactly is reversion?
> 
> The situation I have is the motor is a chevy V8 small block 6.6l engine, with
> what someone else measured to be a 3L intake, with 3-4" runners.   This motor
> is a late model chevy LT1 motor, and the intake has IAC passages built
> into it (basically a separate plenum/runner setup going to each cylinders
> runner, and this air is separately controlled with a idle air hole-much like
> you would drill in a carbed motor, and a iac motor setup).
> 
> At idle (without using the special IAC passages in the intake, or without
> using them enough) the motor runs consistantly different side to side 
> (one side runs with say a blm of 110 or so and the other side run 160, 
> basically -15% fuel on one side, and +25% on the other side), as the 
> throttle increases things even out. The above was determined by averaging 
> a large number of samples (10-15,000).
> 
> Now increasing air routing into the IAC passages evens out the above numbers
> at low throttle and has minimal affect at higher throttles (it has some,
> but in areas the engine does not often run, and the differences are much
> less significant).  When this is done the TB blades are adjusted down
> such that the total air in the motor should be about the same, and the
> idle is about the same speed.
> 
> Several others have noticed exactly this same behavior and that upping
> the air into the IAC passages fixes the inconsistancy.
> 
> Would reversion exhibit this sort of behavior, and would the IAC passage
> setup be a way of fixing the part throttle issues?
> 
> 				Roger
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