Sender: <marv@lancaironline.net> (Marvin Kaye)
To: flyrotary
Date: Tue, 05 Nov 2002 19:13:55 -0500
Message-ID: <redirect-1853718@logan.com>
From: peon@pacific.net.au
MIME-Version: 1.0
Content-type: text/plain; charset=US-ASCII
Content-transfer-encoding: 7BIT
Subject: Re: [FlyRotary] 12A vs. 13B
Reply-to: leon@promotorsport.com.au
Priority: normal
In-reply-to: <list-1852753@logan.com>

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/