George Lendich wrote:

Paul,
Can you ( or anyone else) point me to where I might find and evaluate the argument about a higher Intake velocity (smaller diameter intake pipe) having a distinct advantage over the larger diameter
intake pipe.

I know there is a difference of opinion on this, but I have a feeling that intake velocity might have a bearing in regard to postponing the backpressure in a P-Port configuration.

In reading remarks from builders on the internet, there are claims that Tracy has had some success with smaller diameter tubes, improving the performance of his installation.

I looked at Bruce Turrentine's site but found nothing on the subject - but I have seen it briefly mentioned before on other web sites.

George ( down under)

I'll take a stab at explaining it. It has to do with the rate of negative pressure
rise at the runner opening in the intake chamber. What you want is a rapid increase
in the velocity of the air molecules. Think of them as a bunch of light weight tiny 
marbles. At low RPM the negative pressure rises rather slowly as the face of the
rotor moves away from the inner surface of the rotor housing. Therefore you want
a small diameter intake runner. This results in a rapid acceleration of the air 
molecules (AM) and high intake air velocity as the chamber reaches max volume. 

Newton "F = M x A"

As the chamber starts to get smaller the high velocity of the AM and the 
consequent inertia of the AM results in more of them entering the now rapidly getting 
smaller intake chamber. As the port closes the inertia of the AM causes them to
compress with an increase in pressure at the rotor end of the intake runner.

Newton "Objects in motion tend to remain in motion."

Due to the adhesion and elastic properties of AM that results in a low 
pressure on the intake end of the runner and high pressure on the port 
end so consequently the pressure wave changes direction or bounces and 
now heads towards the intake end of the runner. At constant RPM this 
phenomena happens over and over again.

Therefore there is one optimum diameter and a corresponding length of the
intake runner for each RPM number that results in the max number of air 
molecules in the intake chamber. BTW this also results in the
lowest pumping HP losses so the BSFC is optimized for that RPM.

Working against this process is the friction of the AM on the walls of the 
intake runners. As the runner gets smaller the ratio of inside  wall surfaces area 
to volume of the runner gets worse. This results in diminishing returns as the 
runner diameter gets less and less. At high RPM's three runners of the same
cross sectional area as one large one results in less AM's getting into 
the intake chamber. We are mainly interested in max HP at 6000 to 7500
RPM for aircraft use so one large runner is called for optimized in both
diameter and length.

The rotaries biggest problem for automotive use is low speed torque. Mazda 
must get as many AM's in there at low RPM's as possible. After all, the engine 
is only equivalent to a 2.6 litter 4 cylinder piston engine in displacement and it 
is asked to compete in HP with 3.5 litter to 4.5 liter piston engines. When 
it comes to low speed torque there really is no replacement 
for displacement. This is why the RX8 engine uses three small ports
and runners rather than one large peripheral port and runner.

Paul Lamar
 
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