None of this matters if you are COMPARING the WORK of the typical 4-stroke with the WORK of the rotary.  This means by definition that you MUST find a common means of measurement of that WORK.  The only place that this takes place is at 720 degress of crankshaft/eshaft rotation.  At 720 degrees of eshaft rotation the rotary displaces 2.6L.  If you aren't trying to compare the 2 engine types then 3.9L is correct for the rotary since it rotates 1080 degrees in 3 rotations of the eshaft to WORK all 6 faces of the 2 rotors (but this isn't a valid comparison to a Lyc or Chevy!).  Ed A. had it right earlier even if he did change his mind.

Ken Powell
Bryant, Arkansas
501-847-4721
C150 / RV-4 under construction
 

    -------------- Original message --------------
    Well guys here you all go getting tangled up in you analytical undies.
         I keep trying to tell you to stop getting confused about the
    hardware and just pretend it does not even exist.
         A fixed amount of air and fuel goes in.
         It is compressed.
         It is ignited.
         It is expanded
         It is exhausted
              Now the question is for this one packet what is the maximum amount
    that can be ingested at atmospheric pressure.
         .65 liters per chamber.
         what constitutes a chamber?
         A rotor face.
         How many are there?
         How long does it take to run all the little packets through one
    complete cycle?
         The true *_thermodynamic_* eqivalent is a 3.9L 6 cyl motor spining
    at 1/3 eshaft speed.
         You may choose a 1.3 L 6 cylinder spining 3 times as fast if you
    like and get the same numbers.
         You may also choose a 2.6 L spining 2/3 as fast
         Or you may play any game of two cycle four cycle six cycle etc.
         Or you could just make up some arbitrary way to make the numbers
    work out.
         You could also say that 302 cubic inch engine is actually a 604
    cubic inch engine if spun twice as fast and choose to calculate
    displacement via the cam drive.
         The point is the only thing that matters when comparing oranges
    and oranges is that one little chamber or single cylinder. Not
    half a cylinder or 2/3 of cylinder or chamber.
         The working fluid is the key-not the hardware.....I promise.
              I still maintain that it doesn't make a hill of beans of
    difference. The mental exercise beats watching Oprah, however.
              Monty