Paul,

The publication, Street Rotary - How to Build Maximum Horsepower &
Reliability into Mazda's 12A, 13B & Renesis Engines , states that  optimum
header length for a P Port is 10 to 18".  Have you, with experience with
throtary run into this theory?

Joe Berki
Limo EZ

It all depends on what RPM you want the HP to peek.

22 inches gives us peek HP at 7500 RPM.
That implies a 2.85:1 PSRU and a 6' dia. prop.

If you are using a 2.17:1 gear box you want
the HP to peek at about 6000 RPM however  you
will get less HP. HP is torque times RPM.

We verified this on the dyno and Mazda confirmed
this in http://www.rotaryeng.net/Mazda_R26B_US.pdf

You can download the paper and read it as a pdf.

Paul Lamar