For certain cowlings this 180 degree tuned manifold might fit
better. It is more compact. It is based on a 3 inch center line radius 180
degree
2" tubing bend. the center line distance is 6 X 3.1416 or 18.8 inches.
We need 24 so this comes out about right..

It is easier to make as there are fewer
tig welds than the front mounted configuration.

It is also really trick looking :)

Paul Lamar



Looks nice.. What about having a very short "block hugger" style
intake manifold to the carb for better packaging and then a
plenum + long tube combo to give the needed length?

Vlad

It is possible that might work but the origin of the pulse that
energizes the intake manifold takes place right at the port.
I suspect that moving the carb near the port may seriously
affect the tuned system. I have not tried it however. For the
same reason you don't want to put the venturi and the  butterfly
throttle somewhere in the middle of the runners. When the slide
throttle on the other hand is wide open for take off and for high altitude
cruise so it has no effect..

Paul Lamar


I see your point, however, I'm going to play devil's advocate. Looking at
the relatively short runner length on domestic v8 intake manifolds, one
could easily craft a tubular intake manifold for the rotary that is the
same
length or more likely longer and still have it be very "block
hugger" -ish.
Given that many producers like Edelbrock, Dart, World Products, Weiand are
producing intake manifolds of such a short length, tuning the carburetor
must not be a problem. Now take a short runner style manifold and attach a
plenum with long tubes that lead to a filter such as those produced by
Spectre and you 'could' potentially offset the power band short manifold
back to a more desired rpm. While comparing a domestic v8 to a rotary is
an
apples to oranges exercise, you get the idea. Just adding fuel to the
fire.

Vlad

Those V8 spider manifolds are old tech. New V8's use long runners.
A piston engine cylinder fires half as often as as rotor and the intake
time is only about 90 degrees compared to about 270 degrees for the
rotary so the optimum lenght of the intake pipe is different. Probably
close
to twice as long at the same RPM Like the Dodge ram charge. I see a few
properly tuned V8's these days.. The new LSx Chevy engines have the
runners
wrapped up in the lifter valley.

Paul Lamar

Old tech they may be, but that's the name of the game when it comes to
carbs. The point is that no matter how old the design and technology, they
work.. They worked back when they were state of the art and still do
today,
otherwise, no one would produce them. I would agree that mounting the
carbs
as far away from exhaust is good for both efficiency and safety, however,
you also have to consider how you're going to route carb heat.

Vlad

Silly me, I forgot where I had seen this whole "shorty/block hugger"
intake
manifold.. Racing Beat made them for older SA22 RX7's. Here is a link of
what
I mean: http://www.mazdatrix.com/r-holley.htm or
http://www.racingbeat.com/FRmazda1.htm under "Holley Intake Kit." Seems
like
it worked for them. Just use a kit like that with an added plenum to lower
the power band.
Vlad

Those are side port manifolds. They work but are totally unsuitable
for our A/C needs IMHO. P-ports have 20% more power across the board. I
think
Steve Beckham, Carry, Richard Sohn, several other people on here that have
built and dynoed p-port engines and Power Sport may agree with me.. Since
the
pumping losses are higher they will probably burn more fuel in the process
all other things being equal.

I am done doing R&D on intake systems so good luck. I basically copied
the Mazda Lemans p-port engine as they have achieved more HP per rotor and
spent more
money in the process than anyone else. Particularly at 7500 RPM where we
need
it.

Please put it on a dyno and tell us what you find out.

Paul Lamar

       Yeah, I know it's for a side port, but the idea is there. Point is
that if
the set-up as a whole worked on the rotary, be it side port or peripheral,
it works. While this set up does allow for uber easy carb heat plumbing,
you
do have the possible fire problem if anything ever leaked, and it would
eventually. Racing Beat made heat shields for the set-up which would help
with vapor lock, power output and prevent fires from leaks, but how much
trust can you put into a sheet of metal?
Vlad

Wings are made out of sheet metal :)

Paul Lamar

Very true ;)

Vlad

Paul,

OK now we have conventional carbs on a RX8 P Port engine.  Why can't it be
done on a 2nd or 3rd gen P Port?  Is it because of the exhaust impinging on
the intake via overlap?  I guess the side exhaust ports on the RX8 motor
slow the gasses down or mitigate the effect.  In any event we still have to
build the P Port housings.  I talked with several people involved with the
Powersport design which flew on several aircraft and found that they sealed
the tube with an interference fit.  They made the opening smaller and soaked
the tube in LN2.   So is there a carb set up for the old engine?

Joe

That is precisely the problem Joe. If you have a 2.8:1 gear box
it might not be all that bad as you can idle the engine at 2500 RPM
or so. One would have to fly it  and see if one was happy with
the idle.

Paul Lamar