So, I may have a line on a '74 RX-4. I've been interested in a rotary-powered vehicle for a while now, but very hesistant to get one that would have to be smogged. This one is not only pre-smog, but I can get it for a song.
I know a little about the proper care and feeding of rotaries, but a quick perusual shows the aftermarket to be pretty lax regarding these early 13B cars. I believe it's a 6-port, and information trickled down from Bob Maybell is that these have the largest stock ports put in a 13b, as well as good high-compression rotors, making the plates really desirable by the ITA guys.
What's the board's thoughts regarding these? So far the only thing that has me at all hesitant is the leaf-sprung rear (as I dont' really know any good way to lower/stiffen a leaf setup, other than using blocks and add-a-leafs), but considering I'm planning to use this as a daily that will see the occasional autocross, I'm not too worried about it.
gamby
SuperDork
9/16/08 10:38 p.m.
Rare, rare, rare bird these days. Really neat cars.
I can imagine a carbureted rotary must be a JOY to keep running properly, though.
I'd recommend reading up on them in the Mazda forum of www.japanesenostalgiccar.com
If the engine is original it is a 4 port 13B which is the best NA 13B, they make a great basis for a bridgeport, as for the rear, you can have the springs reset to any height you want and the rate increased as required by any decent spring company. You can also add control arms, panhard and bolt the front end of the leaves together so they don't slip making them into a form of control arm.
If you don;t want it I do....
Thanks for the link, gamby, I've been drooling over maxg765's blue low mileage 4.
Now let's hope this actually comes together!
(I still can't get ahold of the guy doing the 3S-GTE conversion to my Camry AllTrac!)
you want to do things like this right? of course you do. buy it.
Can someone explain "Bridgeport" with regard to rotaries? Thanks.
As there are no valves or cams in a rotary you open up the intake port to let more fuel and air in, in the case of a "bridge" port the opening extends beyond the line the seal makes when rotating around the housing, so to allow the seal to continue its path a bridge is left and the port is continued beyond this bridge, making two distinct openings and radically increasing the ability to get more fuel/air into the engine..
Does that help.
Stock rotary intake port:
Bridge port:
They are called 'bridge ports' because of the 'bridge' (duh) that keeps the seal from popping out into the port.
Woody wrote:
Can someone explain "Bridgeport" with regard to rotaries? Thanks.
If you've ever heard a rotary do this......brap brap brap brap brap brap brap brap brap brap
then that is a bridgeport
bridgeports are only second to a peripheral port in the world of cool.
thatsnowinnebago wrote:
bridgeports are only second to a peripheral port in the world of cool.
And what's that mean exactly? I see peripheral and one other one as port types... didn't the Renesis go to peripheral ports?
found this article on porting: porting article
ReverendDexter wrote:
thatsnowinnebago wrote:
bridgeports are only second to a peripheral port in the world of cool.
And what's that mean exactly? I see peripheral and one other one as port types... didn't the Renesis go to peripheral ports?
In this case the intake ports are on the periphery of the rotor housings like the exhaust ports. Generally only used in a uber high reving application such as the Grandam Speedsource Rx-8 and mega drag race cars.
The production Rx-8 aka Renesis engine went away from the peripheral exhaust ports so both exhaust and intake are in the sides. Reason was to eliminate port overlap/cleanup emmissions
My buddy put a carb'd, bridged, 13B race motor in his miata. Surprisingly, he accomplished it all for less than the price of a flying miatas prime turbo kits. Hes no pro mechanic either. 200whp, NA lawnmower sound. Best part? He shuts it off after taking me for a ride, one minute later it backfires like a shotgun. I love that car.
Here's a page with pictures of each of the different kinds of ports on a rotary. And this is where you can buy your very own peripheral ported motor for only $9,805!
Another reason Mazda went to side exit exhaust ports is they could finally fine tune the exhaust port timing. Given that you have to put the coolant passages, spark plugs etc in the aluminum rotor housing that severely limits where the exhaust port can go and thus the port timing. The hard part with the side housing exhaust ports was that carbon would build up in the exhaust ports and chunks breaking off could cause serious damage, the peripheral exhaust port didn't have that problem.
The 13BTT made ~255 HP with twin turbos and all the other crap on them but still just squeaked by on emissions testing, the Renesis makes 238 with no boost and passes much more stringent emissions testing. I'd say there was a real good reason to pursue that side housing exhaust port avenue. 
why is the miata not a rotary powered product again?
I can't be the first to ask that question. It just seems to obvious a match.
belteshazzar wrote:
why is the miata not a rotary powered product again?
I can't be the first to ask that question. It just seems to obvious a match.
Because its a girly car and therefore requires a girly engine
has anyone experimented with tuned pipes a la 2-smoke engines to widen the power band? seems like an obvious option since the rotaries are basically 2-stroke engines anyways.
Strizzo wrote:
has anyone experimented with tuned pipes a la 2-smoke engines to widen the power band? seems like an obvious option since the rotaries are basically 2-stroke engines anyways.
They're 4-stroke engines.
Bridgeports exist because some racing bodies don't allow PPorts. They're a stop-gap measure at best.
Strizzo wrote: ...has anyone experimented with tuned pipes a la 2-smoke engines to widen the power band?...
Rotary's have a power band? Isn't that anything above idle?
belteshazzar wrote:
why is the miata not a rotary powered product again?
I can't be the first to ask that question. It just seems to obvious a match.
The smart ass answer would be because:
~ They wanted a motor that wouldn't flood after running for a couple of minutes
~ Mazda thought the Miata should get better than 18 mpg in town.
~ Apex seals suck
~ Mazda wanted to avoid rotards deifying another car.
The actual answer, imo, has more to do with emulating british sport cars of old. 4 cyl for the nostalgia win.
I love jerking rotary guys chains! 
disclaimer - I'm the former owner of a 12A.
