BTW I am not sure if this was clear in my orginal message
but most Mazda people accept the displacement as twice
the swept volume of one chamber times the number of rotors as the
displacement of the engine. One chamber for a 13B would be
654 CC I think. One power pulse per rev just like a one
cylinder two cycle piston engine.

So the engine is 1.3 liters for two rotors.
Therefor a 2 rotor 13B is considered equal to a 2.6 liter four
cycle piston engine and I think the countries that tax by
displacement use that number as do all racing organizations.
I could be wrong.

Vance would have us believe it is really 3.9 liters or three times
the real displacement of two chambers.... I think.
Is that right Vance?

The very interesting chart below from Kenichi's second Rotary Engine
book (1981) shows how the chamber volume changes with output shaft
angle.

volchang.jpg

Paul Lamar

I can see how the internal workings of the rotary could cause confusion,
but there is a very simple way to look at this.  How much air can the
engine displace per revolution?  Nothing else really matters, unless you
want to nit pick and ask how much air it can displace per two
revolutions.  Either way, for the sake of comparison, the answer is the
same when comparing the rotary to a piston engine.  A 13B, which has a
swept volume of approximately 80 cubic inches, will displace 80 cubic
inches per revolution assuming 100% VE.  A four stroke piston engine
with a swept volume of 80" will displace 80" per two revolutions.  So,
at a given rpm, the 80" rotary will displace twice the amount of air
that the 80" four stroke will.  Since total mass flow is the major
determining factor in horsepower output, the rotary would need to be
classified as a 160" engine for the sake of comparison to a four stroke
engine of the same swept volume.

Paul Yaw

All that is true and I totally agree. Good way of looking 
at it but for tax purposes Vance objects.

The rotory is like a two stroke but it is a four stroke
so... do the tax people tax it on 80 cubic inches 1.3 liters
or 160 cubic inches 2.6? Do they tax two strokes differently
in Japan? 

The road racing orgs such as the FIA and IMSA consider the 13B  
a 2.6 liter engine. For same strange reason people rarely
race a two cycle engine in car racing these days. Are they outlawed
say in IMSA? Back in my younger days when SAAB was building
the three cylinder two stroke ding-ah-ling people used to
race them. I forgot how the SCCA treated them. 

There was a guy running four (count em) two cycle engines in 
a (one) Can Am car as I recall. Of course Can Am had no displacement 
limit at that time. Could have been Ray Caldwell current president 
of Sky Star (Kitfox & Pulsar) as I dimly recall.

Paul Lamar
 
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