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G'day Larry,
"Ain't nuthin' " wrong with 12As, especially if they are given a bit of
un-natural aspiration, and the ports are given a bit of a tickle.
The same porting principles apply to 12A as 13B, and similar
extra power can be had, albeit at higher RPM, or higher boost.
As an aside, you may notice that as a general rule, a
turbocharged variant of any engine family is usually smaller in
capacity, but evenso usually produce considerably more power
than their bigger capacity naturally aspirated counteparts .
The extra RPM/boost are not as detrimental to a 12A as they are
to a `3B. Major 12A advantages are:
The apex seals tend to be more durable as they are shorter. They
also come stock with 3 mm apex seals, which saves the expense
of re-machining apex seal grooves. 12A parts are generally much
cheaper, and at this time, everything is still curently available from
Mazda dealers.
The overall package is somewhat lighter, as well as being 20 mm
shorter in overall length.
12A bearing loads are also less due to the lighter rotor weight, and
can be reduced even furher by lighteneing the rotors. I can't EVER
remember seeing a 12A run a main bearing in a race motor
(except of course for a very rare oil line failure), but I can sure
blubber in my "Foster's Frostie" (Oz equiv to a "Bud" in the US)
over 13B bearing failures, particularly on highly stressed PP race
engines. (12A & 13B use the same size bearings!!)
So the only real downside is that to get the same BHP, you have
to rev them a bit harder, or use more boost. Interestingly, a 12A,
as they came in the car, carburettored 12A engines were quoted
as 125 BHP with all the emissions control gear. The carburettored
13B was quoted as 130 BHP in the same configuration, albeit with
better mid-range torque characteristics (not an issue with aircraft
engines).
By removing the emissions gear, and fitting a set of tuned
extractors, typically, the same stock 12A will make 150 + BHP @
6,500 whereas you will get the same grunt at about 5,500 from a
13B. 6-8 PSI boost will bring the power output up to 200 BHP at
the same RPM. Tickle the ports with a die grinder and an extra 25-
30 neddies can be had for a couple of hours work.
I'm being conservative, and we are talking here engines without all
the pollution control equipment, no cat convertoers, and both with
9.4:1 compression rotors). So no "Nit Pickers" please, this is
illustrating a general principle, as there are several diffiernt types of
port sizes. The best end housings to use are the Series II / III ('81-
'85) type.(not 6 port - too much messing around)..
So the limiting factor really boils down to PSRU ratio availability
and and prop speed. The motors themselves are VERY durable..
To get the gunt without the need to rev the little sucker too hard,
they respond very well to forced induction. Turbocharger wise, the
12A can get away with a smaller, lighter unit than the 13B if you
don't want a lot of extra grunt..
There are some really sweet little ball beaing turbos with integral
wastegates made by Garrett that sell here for about $2,300 AUD
that are eminently suitable for 5-8 PSI boost on a 12A.
A good starting point for someone on a budget would be to grab a
turbo off a Nissan RB26 GTR. Lots of the Nissan tuning shops
take the stock ones off and fit larger ones to get more grunt. I can
pick them up here in OZ for the proverbial odd song or 2. In fact,
the reason I mention this is that is exactly what I have planned for
this summer's testing.
Contrrawise, for LOTS of grunt, I have several 12A powered cars
(mainly RX2s and RX3s) running around with the late model Series
V turbos that produce 350 + BHP at about 18-20 pounds and I
have yet to see one go "pop", even though the owners tend to
subject them to ritual serial abuse! . (Local registration rules
preclude 13B turbos in cars less than 1000Kg). I also still have
fond memories of my BAE kit equipped Series 1 RX7 12A back in
the early '80s.
What a weapon!! Devestatingly quick, and a real Porsche
muncher, ... and the local V8 worshiping Australopithicenes just
could never cope with being wasted by a "rice burner" or "chook
cooker" (as in rotisserie) as they disparragingly used to call the
Cork Company's (Toyo Kogyo) little marvel!. (I confess to having
been more than a little bit of a street racer back then!!) ... but I do
digress.
So I have been revisiting the 12A paradigm again and doing some
careful sums on the 12A lately myself. Just today, I was chatting
with one of the local SAAA guys who is a composites specialist,
and who is in the process of finishing his prototype "White
Pointer", a 2 place RV4 sized ship with LOTS of luggage space.
Current powerplant is a Jabirru flat 6, but he's looking for
something a LOT cheaper and with a bit more grrunt. (he is also
declared himself a closet rotaryphile). 12As are especially useful
where weight is a major consideration, and power output is simply
a function of RPM/Boost.
>From investigations so far, it would seem that for optimum
performance, a PSRU ratio of about 3.1:1 , or thereabouts would
be indicated. 200 BHP is easily obtained at about 8 psi boost and
7,500 RPM. This keeps the prop speed down around 2400 RPM.
More grunt is available simply by revving it a bit harder, or dialing
in a bit more boost. Bearing in mind that this would only be
needed on takeoff and climbout, the engine could quite happily
cruise @ 6,500 RPM and up to a couple of pounds boost and still
make 140 - 150 BHP. (All BHP quoted are at Sea Level!).
As for PSRUs, I know everybody raves about epicyclic PSRUs,
but the rubber band arrangement on Ken Welter's Coot seems
eminently reliable, and also give a wider range of available ratios.
It also solves the problems of manifold and water pump clearances.
(But then again, if you run "Plugs Up", these are no existant!!).
So do some homework, talk to Ken about his PSRU
arrangements, and if weight is important, then maybe a 12A turbo
is the way to go. The rubber bands also seem able to take
considerable abuse as Ken is know to give his engines a decent
shot of "laughing gas" to make the Coot unstick from water.
That also raises another viable alternative. Have a naturally
aspirated 12A "on the bottle" (i.e., fitted with "Giggle Gas"). Grunt
at the flick of a switch, and a lot lighter than a
turbocharger/Intercooler set-up.
So I hope this as given you (and others) food for thought. What I
have said above is NOT definitive, but I'm certainly already
investigating the feasibility of it, and in due course, after I have
done some testing over the Christmas period, I'll post my findings.
Cheers,
Leon Promet
leon@promotorsport.com.au.
On 4 Nov 2002, at 19:13, Schurr, Larry wrote:
> I have this opportunity to obtain an RX-7 with a 12A aboard.
> All the talk, research, and apparently flying aircraft seem to be
> centered around the 13B and 20B.
>
> What difference is there in the 12A and is it suitable for aircraft
> use? Too heavy? Too wimpy? Can I turbocharge it? Or should I just
> junque it?
>
> Larry
>
> >> Homepage: http://www.flyrotary.com/
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