Here's some interesting read on calculating the runner length for
rx7. Gets indept with lots of intake and exhaust math.

http://www.rx7club.com/general-rotary-tech-support-11/heres-how-figure-out-intake-runner-length-199788/Back to planes, for the 2.85 psru gearing it looks like around 24" gives you the best HP from what I read on Pauls site. What about for Neil's upcoming psru and his 3.xx gearing, do we have a length to aim for? (I forget what the gear size is)

Right now I have 23 inches to play with. I'm just spending the day
making a plenum box, not sure if we have a volume we are striving
for.

- Matt Boiteau

Side ports. For the P-port I just copied the p-port le mans engine
as closely as possible. If you err on the long side the fuel burn
will be slightly lower at cruise.

We need to do more research on Helmholtz resonators. I suspect they
would save some space under the cowl.

Paul Lamar



So I should try to shoot closer to 18" then, not sure it's possible
with the 90degree bend and injector blocks. When you add the slide
throttle, it's almost 8 inches right there from the engine to the
end of the throttle. Wish we had an engine dyno and free time to
play around with intakes and HP.

The link I sent, the guy talks about Helmholtz resonator and how the
RX8 intake is very well done compared to older generations.

Matt Boiteau

   You are spacing your slide throttle further out than most.

   Most people have kept that as short as possible for idle reasons
   starting with NSU from the very beginning. Power Sport copied
   NSU and put butterflies in the rotor housings.

   However the time to climb p-port with the throttle on the other ends of the runners
   is idling fine at 3000 RPM. 1000 prop RPM.

   Runner length depends on the prop as well. What prop are you planning on?

   It is not that critical. Hopefully you will have a ground adjustable prop.


   Paul Lamar


Sorry I meant to say from the start of the P-Port on the engine to last blue rubber coupler (near the start of the 90degree bend) is about 8 inches. Between engine and slide throttle is 2.5". So anywhere between 24" - 18" is good, and try to lean closer to 18".

I don't have a prop picked out, any recommendations out there? I'm surprised nobody has put on a hydraulic pump where the A/C went for a constant speed prop.

Matt Boiteau

A constant speed is very very heavy. Not including the governor. My C182 prop is 50 pounds.
These rotary powered experimental s are very high power to weight ratio so they get off
the runway in seconds. It is not like you are lifting 3,000 pounds. Yes, in theory, you can have your
low pitch for take off and high pitch for economical cruise.  I suspect it is not worth it
for most people. Set the ground adjustable prop for best cruise RPM and accept the
take off performance. IMHO don't buy a fixed pitch. 90% of the time it is not right.
We have been there and done that many times in the past 18 years.

Sensenich has some nice ground adjustable blades and 3 blades would be about right.
Other companies are making ground adjustable blades and Warp Drive started the trend.

I would try the 24" runners you have know and take a bunch of fuel burn data. You can always cut
it down later.

Paul Lamar

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