The Factory PP is large enough with an opening of 50mm x 43mm,  about 3sq"  ( 50mm dia), which is fed by a 43mm inlet tube, about a 16% reduction in diameter.

 

I can't for the life of me figure out why the port opening has to be so much larger than the inlet tube. It may be the duration of the open inlet they are looking for.

 

Do you know what sort of power they found with the 13B stock PP?

 

Any how for those interested, I've settled on a 44mm inlet straight tube with a late opening and late closing - same a stock PP IO 86 BTDC and 75 BBDC.

 

I can buy 41mm and 44mm tube and have opted for the 44mm ID inlet tube, only 2.35sq" area.

 

George ( down under)

I have heard that the 13B Pport has over 330 HP around 10,500 RPM. The port size at the rotor is just a result of late closing for operation at the high RPM. It reduces velocity and hurts lower RPM power. Same with the side port. Increasing bowl volume slows velocity and move peak power up the RPM band. Where rules allow, side port people fill in part of the bowl with epoxy to keep velocity high after extending the closing line.  We are not allowed to add material at all.

 

Lynn E. Hanover