Your exhaust port looks to be modified also as compared to
 > my N/A rotor housings. Is it a turbo housing or modified N/A ?
 >
 > Fred O.

The exhaust for that 12A periphery port race engine is what came on them
Mazda.  It is not similar to anything. It even has a different bolt pattern
so it takes special headers. There is no liner in it, and no attempt was made
to reduce the noise output.

Both intake and exhaust are optimized for best power at 9,600 RPM. Both are
taller (have more timing duration) than would be required for best power at
6,000 rpm. Having an intake with this much timing would have little affect on
tractability because that is diminished as displacement increases. (the
bigger the engine the more came you can run) however, opening the exhaust
earlier (moving the bottom of the port down) takes away some of the end of
the power stroke. Closing the exhaust later
(raising the top of the port) increases overlap. That is when the intake and
exhaust are connected across the face of the rotor. More intake charge gets
diluted with exhaust gasses. The best bet to start with would be to go a bit
wider on the port but
leave the timing stock and leave the insert in place. There is a definite
step in the full throttle curve where the Pport steps up on the "cam." The
12A has a very short intake runner, tuned for about 9,000 RPM. The long
runner of the aircraft engine would mask this very well. So you could build a
good sized intake port without the problems of the smaller 12A.
Lynn E. Hanover

 
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