We are using two inch tubes with .065 wall or 1 7/8 ID.
About the same as the Lemans engine. The idea here is to copy
the Lemans P-port set up as closely as possible as Mazda
spent millions developing that engine. HP is flattening out
as the current tubes are too short and the engine is going
lean on the top end. Here is a graph from Mazda showing the
torque peak RPM position as a function of runner length
increments. You need to know the minimum runner length which you
can scale from the included side view drawing.

That should be:

HP is flattening out
as the current tubes are too**** long **** and the engine is going
lean on the top end.

Paul Lamar