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Well,
I have nothing better to do on a Saturday night so
I will subject you all collectively to my random thoughts (assuming you read
this)
I have been trying figure out how to cram a
muffled, tuned, road-race, exhaust system under my (tight) cowl. No easy
task.
I am going to be running a Pport NA high
compression motor.
The prevailing wisdom is to use 10-26 inch primary
pipe into a collector to a 24 in megaphone that goes from around 2 in to 4 in
dia. over it's length.
After looking at this thing for so long my head
hurts I have concluded that the rotary shares aspects of a two cycle and four
cycle but is nothing like either in terms of exhaust system tuning.
Two cycle recips can use a divergent/convergent
cone type expansion chamber. The timing of the pulses alternately draws gases
out of the combustion chamber to assist scavenging to the point of
overscavenging at the operating point, and then "stuffs" the overscavenged
mixture back in at the last moment as the port closes to effectively
supercharge the engine.
The rotary is similar except for the fact that the
port is never "closed" so the convergent reflective wave will never arrive at
the right time. It will either impede flow from the exhausting chamber or it
will "stuff" exhaust back into the intake side. The combination of overlap and
the fact that the port is open to two different chambers when you could use a
positive pressure wave, makes it impossible to use typical two stroke tuning
techniques.
Likewise it is impossible to use "interference
working" as is typical on a four cycle recip engine. This uses one branch
of the header as an organ pipe resonator when the other exhaust valve is closed.
You can only design a system for a rotary that operates on the
"independence working" principle.
The literature (I have a graph from the pope's
newsletter that he bogarted from a Mazda technical paper and
"copyrighted") and racing practice shows a primary length of around
31in to give the best results in the 5-6Krpm range. That is a long pipe to
fit under a cowl. These results were obtained with a single rotor engine. From
the previous discussion it should be evident that positive reflections cannot be
used with the rotary. Only negative or suction waves will help. With a two rotor
feeding into a collector and then into a divergent cone, the effective
tuned primary length is half of the single rotor. This is somewhere
around 15.5 inches. The cone should then be another 15.5 inches long. Each
rotor now sees the 31 inch tuned length plus an extra suction wave from the
other rotor. This of course assumes no positive pressure waves go back up the
collector (use as shallow an angle as possible). Alternately the cone could be
24 inches long (available from racing beat) and the header primaries could be 7
inches long. I don't suppose anybody has ever tried anti reversion
headers?
The muffler is a another issue, better quit while
I'm ahead (I think)
Monty
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