George,
Where did you get the relative size numbers in percentages from?
I would assume the percentage numbers refer to area not linear

diameter.

Am I missing something here?

Rolf Pfeiffer

Rolf,
Considering some of the engine inlet sizes change from model to model,
including the 12A -I compared MY 12A inlet AREA to the 12A Carby

throttle,

trying to get a handle on what Mazda was looking to achieve in regard

to

manifold sizes and losses from the inlet i.e. drag and turbulence.

It turned out about 17.5% difference between the carby butterflies and

the

engine inlet - for the 12A I have. I initially calculated 20% for my

own

installation ( Paul's suggestion) and it's pretty close.

I also have late model 13B housings which I have milled to see the
difference in the water jackets and oil passages - not a hell of a lot

of

changes there but they do change for the RX8.

I was surprised to see that there wasn't much difference between the

primary

( AREA) sizes of the early 12A and the later 13B, which leads me to

believe

that too big a primary port may play havoc with idle - for a whole

range of

reasons.

QUESTION
Paul or anyone- At what RPM does the secondary ports start to work?

Anyhow I then looked at the 13B secondary port and it was massive

compared

to the 12A and massive compared to the difference in engine sizes

1146cc to

1308cc - only 14.13% difference.

So another conclusion is that the 12 A secondary port is undersized or

the

13B ports are oversized OR both. Which means I can't just assume their

the

ideal sizes and work my inlet manifold around the port sizing only. If

I

were to do that I would need 60mm ID(2.36"ID) manifold tubes for the

13B -

way too big!

Excuse my rambling on!!

What to do next, I looked Paul's suggested manifold tube size and

adjusted

the inlet sizing by 14.13% and it came out about to 1.5" ( 44mm ID -
14.13%). For the 12A that FEELS about right.

If I were to take Jerry's info of 1.5" - 14.13% =1.28" (33mm)- this is

where

I get confused and don't know which way to go.

Attached is a couple of port size drawings I did for comparison.

Paul do you have something better on the 13B port sizes?

George (down under)

Better? Nothing better so far.  Good work George. Here is what a third

gen 13B looks

like compared to a 13B second gen. The history of the rotary has been

ever larger port sizes.

The RX8 motor is larger yet.

Paul Lamar

Good project. Keep up the good work. Measure a 2nd gen 6 port and a third

gen 4 port.

You might be able to go to  Mazda dealer and measure or trace a low HP and

a High HP

RX8 engine intake manifold gasket.

Paul Lamar

Paul,
OK I will work on it, however I thought you had a drawing on the 13B for the
manifold you made - do you still have something on that, might save me a
little work.
I have the drawing of the 3 rotor which someone kindly made available on
ACRE - I think that was from the chap from (Perth) Western Australia.

Their different sizes again!

I figure if I keep on accumulating data it might give us some direction.

What RPM does the Secondary Port start to supply fuel ?

George ( down under)

I have a drawing but it is a bit undersize to accommodate casting irregularities.
I am not sure.

Paul Lamar

Then you have the huge ports for the late model Cosmo 13B engine like the one Bulent has.

Paul Lamar
 
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