From my understanding the 5th and 6th ports are so high on the iron face that they don't end up closing until the rotor is into the compression cycle. This pushes air out the port and creates some sort of resonance effect in the manifold which is supposed to increase flow while the 5th and 6th ports are open. Right?

When you replace the stock manifold with one designed for a carburetor and open the 5th and 6th ports wont it still push air backwards out the port with no resonance effect?

People say 4 and 6 ports make no real difference as far as street ports go and I was just wondering how this could be if the 6 port is actually pushing air out of the combustion chamber?

 

It is true that the engine is starting the compression phase while the aux ports are still open. However due to the fact that they open at 3800ish rpm, the inertia of the air in the intake runners does not allow that air to reverse and actually still adds air to the chamber. At lower rpms the intake air lacks this inertia and there is some reversion which is why the engine loses a bit of low end power if they remain open full time. There is always a resonance effect in any system. With a carb that leaves these runners open full time, you will lose low end power.

 

Right you are... but even the lower ports don't close until a fair bit into the compression cycle.

The resonance effect isn't from the air being pushed back in, it's more from the reflected wave when the port is shut. Think of playing with a Slinky in both hands... you can reflect the main mass of the Slinky between one hand and the other. Most manifold resonance tuning involves reflected waves between the closing port (or valve) and the re-reflection at the open end... Mazda simply capitalized on the fact that there are only two rotors so they reflected the ports at each other. The goal in either case is to have the airflow mass slam through the port as it is closing.

A huge street port closes at about the same time as a stock full-open 6-port, for what it's worth. The port flow quality is much higher though.

And the long-runner N/A manifold, I found, is absolute magic on a 4 port engine, if you can be bothered to adapt it. It's a lot of work.