Intake Plenum - Tech -

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Message: 1         
   Date: Sun, 13 Feb 2005 19:23:09 -0500
   From: <dh@xxxxxxxx>
Subject: Plenum size

There is an optimum plenum size for every engine.  There are 1st order
effects, that will make big differences and 2nd order effects that will
make small changes.  Some factors that contribute are runner size (between
plenum and intake valve), valve duration, valve overlap, effective exhaust
diameter and others.  

The first order effects are number of cylinders, cylinder displacement and
induction type.  In reverse order induction type is important because with
a carburetor (or a throttle body injection) there is fuel suspended in the
plenum.  

The amount of fuel in the plenum that is gaseous and that which is
suspended liquid is constantly changing with velocity, pressure and
location.  As the throttle is cracked open from idle in a large plenum the
pressure goes up from a (relatively) strong vacuum to near ambient
pressure.  The fuel that was in suspension will 'fall out' of suspension
and the engine will be very lean.  That condition will exist until the
plenum has been replinished (from the carb or FI.)   That is the reason
for the accelerator pump on the carburetor - to provide additional fuel as
the throttle is opened.  FI does that electronically.

Cylinder displacement is important to plenum size because that much air
must be provided from the plenum to assure steady air flow through the
metering device.  If the metering device is a carburetor it is rated at
the amount of air that will flow with a continuous 1.5 psi pressure drop
(I believe that this is the right value - someone correct me if I'm wrong,
and tell me if I'm right) for a 4 barrel carburetor or 3 psi pressure drop
for a 2 barrell carburetor (who knows why).  This is not the limit of the
carburetor but the limit of the carburetor AT THAT PRESSURE.  If the
pressure drop increases the air amount will increase, but not linearly. 
If the pressure doubles, the flow will increase by about 40%.  So what you
want is a steady flow through the carburetor large pulses of air wil not
provide as much flow as a steady flow at a lower pressure.  That is the
primary purpose of the plenum.  The plenum provides an area that 'smooths'
the air flow and provides an flow through the carburetor that is even and

There is a trade off of the two factors above for a given number of
cylinders.  The trade off is basically, maximum power vs. throttle
response.  However, the effects of the two are minimized as the number of
cylinders are increased.  A one cylinder four stroke would be ingesting
air 1/8 th of the time (actually this is an extreme oversimplification). 
Without a plenum the flow through the carburetor would be eight times the
average flow.  With eight cylinders the flow is relatively smooth without
a plenum and the helpful effects of a plenum are minimized.

Note that this discussion has been restriced to carbureted and throttle
bodies.  To be certain, these technologies benefit most from optimal
plenum size.  But even tuned port injection benefit from improvements in
metering capability.  Having a mechanically smoothed indication of
pressure which tends to indidate flow is an advantage for nearly all otto
engines.

So, to sumarize.  As the number of cylinders goes up, plenum size goes
down.  As displacement goes up plenum size goes up. Plenum size is
important to carburetion, throttle body injection and port injection in
that order.

dh

             Brien.
         NEW YORK
   eagle registry #501
eagle kammback registry


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