Hi Bill, Try this explanation:  1. Torque is a direct result of the
  combustion
  chamber's volumetric efficiency (rotary or piston engine). When this
  efficiency peaks; torque peaks.

  2. The V.E. is mainly governed by the intake system. In the intake pipe
  the rush of air resonates, that is each time the combustion chamber
  intake closes (whether rotary, 2cycle piston or 4cycle piston) the
  air rush stops (sometimes even reverses).

  3. This very brief stop sends an energy pulse back up the pipe toward the
  open inlet.

  4. # 2,and 3 change with RPM.

  5. A perfect length intake pipe for a GIVEN RPM will contain that energy
 and
  revert it to push the next charge TOWARD the combustion chamber.
  This is "resonating."

  6. This is important: Look at #5 again. The pipe length that works
  perfectly at XXX RPM is not the correct length for YYY rpm. So, what to
 do?

  Adjustable length intake pipes have been used on racing engines but
  are too cumbersome for aircraft use. So the next best choice is to
  pick an RPM that the engine likes and tune the intake to operate
  there and accept less V.E. at RPM's above and below that engine
  speed.

  The aircraft piston engine (big bore/big stroke = lots of
  mass moving back and forth large distances) is happiest at about
  2500 RPM and the intake is designed around that.

  NASCAR engines
  (big bore/ short stroke = lightweight mass moving back and forth
   short distances) are happiest around 8000 RPM and their intake is
  designed around that (put a NASCAR intake manifold on your family
   ride and you will be terribly disappointed)

  The rotary engine
  (lots of mass but NO DIRECTIONAL CHANGE) is happiest around
  6000 RPM (rotor speed is around 2000 RPM, bearing loads are
  lightest, V.E. is pretty good, vibration is quite low, power
  output vs. engine wear is very good).

  So with an engine that
  wants to run forever at 6000 RPM that's where aircraft conversions
  are run and the intakes are designed for that speed. To get back
  to a proper propeller speed requires a +/- 40% reduction box.
  Seems like a hell of a penalty until you look at the whole picture.
  Even with the added weight, the power to weight ratio for a P-Ported
  rotary easily surpasses today's standard aircraft engine.

  If you've
   ever shock cooled your Lycoming, your repair bill exceeded the cost
  of TWO COMPLETE rotaries. Can't happen with liquid cooling. Have you
  priced a Continental exhaust valve lately? Rotaries have no exhaust
  valves. This is only some of why we're here. Stay with the newsletter,
  there are nuggets of valueable info here. Hope this affords some more
   understanding.
  Dave M



 I agree with most everything you said Dave - except for the bit about
 aircraft engine intakes being tuned for 2500rpm.  I don't think
 they're tuned at all except for easy packaging.  Regardless of the
 engine type, tuning for 2500rpm will require _long_ runners.  This
 site is interesting:

 http://www.wallaceracing.com/runnertorquecalc.php

 This page has a model for calculating runner specifics.  2700 rpm and
 360 cu-in in four cylinders (a Lycoming) gives runners 31 inches long.
  A similar length pipe would be required for the exhaust.  Most
 aircraft don't have room for such long runners.  The F1 racers are
 probably the closest - with headers-collectors on some of the O-200's.
  They're quite long (24" - a guess), but the engines turn ~4000 rpm.
 Headers optimized to work at the stock 2750 rpm red line would be
 significantly longer.  The F1 rules don't allow diddling with the
 intake configuration - otherwise you'd see long intake spiders too.

 The model doesn't really appear to be set up for a Wankel rotary.  A
 160 cu-in two rotor (13b) at 6000 rpm calculates to a 14" runner,
 which experience has shown is too short.  There's probably something
 about the multiple rotor faces sharing the same runner, and that the
 rotor turns at 1/3 of the e-shaft speed that comes into play - I don't
 have that all figured out...  It seems like the calculated runner
 length is about 1/2 what it should be.

 I would add to #2 above that the reason the intake is so important to
 VE is that on a naturally aspirated engine, only atmospheric pressure
 is available for cylinder filling.  I think things look different if
 you have forced induction.


 Regards,

 Matt-


Matt,
A tuned intake is a function of velocity which in turn is a function of
intake diameter and length and RPM.
If you have a set diameter and RPM you can work out the rest.
George (down under)

Or you can just copy this.

Paul Lamar