Even with a variable pitch prop, where is the need to tune over a broad
 > rpm range?  If you tune for cruise rpm & can make, say, 160 hp at
 > cruise, then you will probably make  190 to 200 on takeoff. Is it really
 > worth the complexity (in the airplane) of variable length intake ports
 > to get another 5-10 hp which will be used only on takeoff?
 >
 > I see a lot of value in using it on a test stand, along with variable
 > length exhausts to determine the most efficient combination for cruise
 > in a plane.
 >
 > Perhaps Lynn can weigh in on this;  I suspect that all this intake
 > tuning stuff will change completely when the exhaust is changed.
 > Hmmm...Maybe that's the best argument for making it adjustable.....

Well, yes and no. Everything affects everything, so a nice rule of thumb
might be:
Engines that have little or no overlap (intake and exhaust ports connected
for a small number of crankshaft degrees) are relatively insensitive to exhaust
system tuning. Engines with large overlap periods (large number of crankshaft
degrees) are relatively
very sensitive to exhaust system tuning. So, Note the cast iron reactors on
early engines. Exhaust system disasters, but they ran fine. Not much overlap in
a stock engine.

If you have a periphery port or a bridge port, the exhaust tuning becomes
more critical. A big street port extends the closing point a bunch but does not
add much to the opening point, so it can be left with the stock ported engines.

Leave in at least the outer part of the port liners. Make the primaries that
size or just a hair bigger, like 1 7/8" or 2" ID. The first concern would be
to make both primaries the same length. Shove an old sweeper hose through the
primary from the flange end. Keep adjusting and cutting until both pipes get to
the collector as the same length. The second concern would be to make the
first turn as far from the engine as is possible. A long collector makes a
broader power band. A short collector, a bit more peaky.  Primary lengths in
multiples of about 11" work fine. The same rules for piston engines. Stand around the
local hot rod shop for a few hours with a tape measure in your hand looking
at V8 headers.

One item that the rotary does not like at all is back pressure from a poorly
designed muffler.

As for the intake, the four tube numbers most people are building looks like
it will flow way better than the stock manifold. As for the correct length, I
have no idea.

Lynn E. Hanover

 
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