Dave

Thanks. You mentioned perhaps the biggest reason a housing with leading
dual plugs should be designed from the get go. That is the force of the
combustion event against the threads. This explains why the current
housing plug boss design is so robust. It also explains why there is a
little half moon ridge near the plug holes on the outer housing.
Beefing up the housing to account for loss of strength near the large
breach no doubt. I suspect but haven't confirmed yet that there is a
stainless boss welded to the outside of the liner near the plug hole.
That would make sense because the flame propagation starts there. Nut,
screws and bolts !%#`!

Next idea. Common ground Y shape Siamese twin plugs that fit into the
present 14mm hole.

One less hurtle or one more concern

Doug in Japan

Sorry Doug but wrong! The robustness of the plug boss isn't to protect
the threads. It is because you need a castable boss there and to
reinforce the side of the ROTOR HOUSING against the HOLE being put
in it. The threads are fine, in fact engines seeing higher combustion
pressures than we will EVER see, AA fuel dragsters running 90% nitro
use standard plugs, just very cold ones.
Bill Jepson

Hi Guys;

Are you really sure that the boss is there for structural reasons
Then, why not complete the boss circle

BTW, some people run the RX7 just fine on leading [lower] plugs only.
I may give that a try this summer.

Cheers
Cary

There is a possible cooling problem with side by side plugs.
The lower plug region is the hottest part of the rotor
housing. You could over temp the material between the two
plug bosses due to inadequate water flow. The NSU spider engine
cracked excatly where the spark plug boss was.

Paul Lamar


Cary,
The rotary WILL run fine on just the lowers. It will just be down on
power and
use more fuel. Mazda wouldn't have gone to the trouble of a THIRD plug
for
LeMans if there wasn't a measureable difference, albiet a minimal one.
Why do
all the backyard tuners think they can easily make a better motor than
Mazda
who has the most rotary experience of anyone For your plane you want
all the
power and efficiency you can get.
Bill Jepson

Hi Bill;
I am quite certain Mazda can produce a better motor and the 3 plug race
housings are probably near optimal for power, but they are not available.
The reality is we are adapting a mass produced engine designed for daily
driving with production costs, installation issues, warranties, fuel
economy, noise and emissions regs.
One question is:   what were the Mazda design goal(s)?

Backyard tuners can and do improve on many stock parts, particularly the
older 13Bs.
Mazda has also made incremental changes and improvements through the
generations of 13B.
IIRC, the plug timing changed slightly 89+.
For racing or aircraft use, even the best mix of stock FD/RX8 parts can
be improved by many mods -
PPort intake and exhaust, tuned intake/exhaust, eshaft, water and many
oil mods... a very long list.
With custom EFI, people routinely improve the fuel economy (and possibly
power) of the overly-rich stock systems. (Is the goal to keep the
exhaust/cats alive?)
Read some of Lynn's amusing commentary at nopistons.com

I would also love to see "the rest of charts" related to the ones in the
KY Rotary Engine books and SAE papers.
For airplane use, I would gladly exchange some HP for dual plugs that
can both produce near 100% power.
It is interesting that the dual plugs on the Rotomax appear to be near
the center-line?

If people are going to the effort of doing a clean housing design,
designing in provisions to test some of this may be worthwhile.
IIRC, the stock S4 plug positions were ~19(L)/28(T)mm?, very close to
the 18(L)/30(T) on the charts 4.54-4.56. 81edition.
These low load charts hint at other possibilities - like moving the
leading plugs towards the center-line or having a far trailing plug.

To confirm, we would need Mazda's design notes or lots of dyno time and
production facilities.

Do you have dyno sheets showing:
- power on leading or trailing only?
- negative split (very early trailing)?
- with/without wasted spark?

Cheers
Cary