Stacks

08/24/2003

If you have access to Vizards "How to build horsepower Vol II Carburetors and
Intake Manifolds"  flip to Chapter 9 - other performance carbs  - page 97 top.

He has a group of pipe openings and their flows.  Figure 11 - The elliptical
flare has the very highest flow of +5.8%.   A stamped hard edged plenum
opening could be very easily made and these could even be brazed on.

Heck, they could even be modeled with clay and finally formed with plastic's.

If this fits your plans it could save you a grunch of weight and machining.

If not enough surface area, he has a number of flared bell mouths that would
stand inwards from a hard flat surface and nicely affect flow.

Don't get caught in the trap that all the air has to flow gently to one point.
Plenums work on the trapped mass of air creating a flow sufficient to filling
a cylinder.  Dead area's such as behind the port bell mouth provide the air
mass needed for them to work - but no air ever has to move about in them.

Garies description of trumpets in a plenum can simply be bell mouths leading
to the port in a completely surrounding stamped steel box.  A straight
unchampered opening is -6.7 on total air flow to the most optimum of +5.8 - a
spread of 12.5%.  

The mental trick is to forget ports and runners yada yada.  Think of the
opening between the plenum and the valve seat as a extremely short ram pipe.

This pipe will not be long enough to resonate even at 20,000 rpm so we
eliminate the concepts of acoustic and kinetic tuning.  But the form needed to
maximize flow thru the pipe still is valid.  Thus all you need facing into the
plenum are the appropriate bell mouths blended back into the port.

One effective trick is to bend the air entering the plenum such that it goes
straight down and splays and splatters off the plenum floor.  This prevents
any port inlet from directly seeing air from the turbo and instead is filled
from the well.  

Don't worry about the nooks and crannies - air moves to equalize pressure at
the local speed of sound because by definition, the local speed of sound is
the speed air moves to equalize pressure.  As long as the air never directly
aims into a port bell mouth, it makes no difference where the air enters at.  

Macginnes covers the situation where air is conveyed directly to a cylinder
and how to make certain the manifolds do no cause issues - but this is a
plenum. 

On the rover, you can drop the bottom of the plenum way down into the valley
to gain more space.  As long as you are using unleaded gasoline, you do not
need to be overly careful about where you spray supplemental fuel.