I couldn't really add to the bridgeport 12A thread in the 1st gen forum other than to say that the bridgeport is calling to me. I've already got a 17mm oil pump and a modded oil pressure regulator. The engine is a '73 12A and is apart and clean. I just need another rear 'side plate' and they would all need to be ported.

A couple weirdnesses about the '71-'73 engines other than the twin dizzies is they also had a top mounted starter. The car originally had one of these 12As in it when I got it. I have since fitted a 13B with a bottom mounted starter. I'm finishing up the 13B install this week and I'm going to drive it around after a brake job and other misc roadworthy issues are solved. After those problems are sorted out, I'll be preparing the 12A. It failed due to a rotor imbalance and a badly scored and worn 12mm oil pump (previous owner's fault, and/or maybe the oil overheated?) That's why I've got a known decent running 13B in it right now.

I have a matched set of FB rotors (most likely '85 since the front cover from this engine has a 12A turbo oil return line casting under the OMP). I'd like to use them in my '73 rotor housings and I hope the chrome will be strong enough to handle the iron seals. I'd also like to use a '74-'78 rear end housing (side plate) so I can retain the bottom mounted starter (in case I get a Weber). All I'd need to do is delete two tensions bolts. Mind you the old engines have two more tensions bolts than RX-7 engines, so it'll break even (hmm, bad pun). I just don't want to run the risk of deleting four bolts by using an RX-7 end housing. Thus I'm forced to use a '74-'78 'plate. I've got a known good one in my four port 13B which needs to be rebuilt thanks to an EGR intermediate housing with its tiny ports. Very Dumb since it cannot be ported very much. I think I could get away with tearing this engine down while building the 12A at the same time. At least the 13B could get away with an RX-7 end plate if need be.

Should I get a Mazdatrix bridgeport template? Should I just bridge the secondaries? I still want low RPM drivability and could block the channels that run between primaries and secondaries on a stock intake manifold. It's going into my MG Midget project so I can't fit a Racing Beat intake manifold, but I do have a Holley to Nikki bolt pattern adaptor. I know the flow won't be as high witha stock manifold, but it's a compromise in such a small car. I also have access to an RB modded Holley carb (550CFM for a 13B should work on a half bridged 12A with fully seperate runners).

Thoughts?

 

try the secondaries only.
also why not use a 13b? just use a 12a intermediate for a way to make larger ports. you will need to have a machine shop cut water seals in and will also need to fab bungs for injectors into whatever intake you are using.
sorry for the short incoherent reply. i am a little tipsy.

 

i got 12a intermed and front plates already j-bridgeported, but they have a groove in them and need to be lapped. interested?

brad

 

Incoherent considering it's an old style four port 13B (out of my REPU). I may not have made that clear.

So you think I should just do the secondaries only? Makes sense to me. If I stick with a 12A and its stock manifold, I'll be blocking the little channels between primary and secondary so I don't get much overlap at low RPM. It should save a little gas too.

I guess the reason why I want to go with a 12A is because I already have one apart and the 13B was going to go into my baja bug as soon as the 12A is finished. I want a torquey low reving engine for it, and the 13B would be perfect. Too much power would break something. Then again, I'll probably break stuff on a regular basis. I just don't want to spin the bus transaxle too fast, which may occur with a ported engine. Even a T-56 has some sort of rev limit or something, and it's a performance tranny! The bus trans is strong, but not unbreakable.

I've got to get the MG Midget running as cheaply as possible first, and go from there. Just throwing ideas out there so I don't get to the point of wondering what my next step should be. If it turns out to be an awesome little car that needs more high-end, then yes, I'll go bridgey. If it turns out to be a dog performance-wise and it's not due to the low reving REPU 13B, then I'll weigh my options.

DriveFast7, I'm not interested at this point, but who knows what the future will bring?

 

This I would not do. The primary ports in the intake manifold, by themselves, actually flow less than the primary barrels of the carb. This means that you won't be flowing your carb's full potential. It also means that you have to run with no spring in the secondary diaphragm (or just go mechanical, same effect) or the secondaries won't open as much.

because of the way they are shaped, you cannot open the primary runners up without actually hurting flow.

put it another way - i ran a manifold set up just like that. ran a best of 15.6mph. I made a different spacer between the carb and manifold, that had a channel between each primary and secondary. times immediately went to 15.4 and i could shift about 300rpm later. i tuned for that combo and i got down to 15.1. so having no communication between primary and secondary was costing me at least 1/2 second in the quarter mile. i say at least, because the manifold setup was still far from ideal.

that manifold has had the channels re-opened, and now basically it's an '83 manifold that works just like a '79-80 manifold. (great, except for the 40 or more hours or porting that it took me to get it to that point )

 

peejay, would it be ok to still half bridgeport even with a stock 12A manifold? I've got a '73 and '79 manifolds to choose from. I'll be using the '73 though because the '79 has no block off for the exhaust port under the intermediate ports, and a long weird casting to the rear which would need to be hacksawed off if used in the MG. They both have channels between primaries and secondaries. Better yet, here is my parts list:


Parts of a '73 12A (rotor housings and front and intermediate 'plates')
'74+ front cover for a single dizzy
'85+ 17mm oil pump
'71-'78 rear oil pressure reg shimmed for 80-90psi
'85 oil pan (because I'm moving the engine back by 20mm going from 13B to 12A and the oil drain bolt is further forward than the '73 pan and should clear the frame like the REPU 13B pan does)
'76+ stationary gears
'85 12A rotating assembly for the lightest rotors and a decent set of seals ('73 rotating assembly was damaged and needed new seals anyway). Can use a light flywheel from racing beat since they don't sell them for '71-'73 engines. 225mm clutch too although peejay may disagree
'74-'78 rear housing with bottom mounted '86NA starter for reduced weight and any '74+NA tranny will work. Two tension bolt holes will be plugged.
'74 REPU four speed tranny with the same gear sets as the '73 had.
RX-7 Nikki carb (not sure what year). Then I'll look for a Holley
'73 intake manifold with a Holley adaptor plate


peejay, could I get away with a Holley out of the box if I were to cut a channel between both primaries? Should I plan on bridgeporting the primaries as well?

The more I think about this project, the more it seems doable. I just need somebody to critique my parts list and let me know if the parts are incompatible with each other.

By the way, the reason why I want to use '73 rotor housings is because my header was originally designed for its closer stud spacing. I got away with installing a 13B with the header because I slotted all four holes at 45º angles. It doesn't leak with the 13B. I just can't bolt it to a '74-'85 12A because it would need to be slotted even more and there simply isn't enough material left. Was it a bad decision? Maybe, but a new header wouldn't be all that hard to make. I just don't want to do it though. I already have a nice set of '73 rotor housings with very little wear and I assume '85 apex seals should work without flaking the chrome. Infact, I have everything I need except for a good '74-'78 rear housing and a gasket kit. Well, I do have a '74 housing, but it may be too far worn to resurface. I also need a bridgport template.

 

I assume you mean, if you ran the fully independent manifold.

Yes, you could. You'd need to cut channels between both secondaries, too. I'd also run the smallest one you can find, mechanical secondary only (this means hunting down a 390cfm).

Better yet in my opinion would be an airdoor type carb like a Q-jet or a Carter AVS/Edelbrock carb, since they are spreadbore (secondaries much larger than primaries). I haven't done this yet, though, so if you try it you'll be doing the research on what works and what doesn't I do however have a stack of Q-jets as well as a couple Autolite 4300 carbs (Ford knockoffs of the Q-jet) and I am not afraid of experimenting with 'em Yet another future project that I haven't got the money for, and when I get the money I won't have the time...

 

I got my MG running yesterday with the 13B! I still need to fix the brake and clutch hydralics before I can drive it.

Ok so if I understand your post properly, I'd need a 390CFM Holley with mechanical secondaries, or an Edelbrock/Carter AVS? And this carb will work on my Holley adaptor plate? And I'd need to cut channels between both secondaries as well? Was this all going to work after blocking the channels in the stock manifold? Or do I leave them open? Should I bridgeport primaries and secondaries? Or just primaries?

Lots of questions, but I'm not sure if I understood your post. It's a good idea to clear this up now instead of later. Thanks!

 

The important thing when using an "american iron" carb is that you have communication between both rotors. This gives the carb the vacuum signals it was designed for and prevents it from going stupid-rich at the top end.

Racing Beat keeps all four ports separate, which requires a much larger carburetor (which is why they use 600's and 650's!!) and also requires a LOT of reworking to the carb's circuits. I say just use a smaller carb and cut a plenum...

It's up to you as to how you port it...

 

Oh ok, thanks. I was going to use an 'american iron' carb on an NA 20B with an open plenum anyway. Makes perfect sense to me! (I'm a little concerned about the idle though because it's going on a Jatco automatic). Er, that's a different project. Pay it no mind.

Back to the MG. I'm going to leave the little channels that join primaries and secondaries on the stock manifold alone and cut new channels in the adaptor to join both rotors together like you said. That is, when I get an aftermarket carb of course. I'll just use a Nikki untill then (for engine break-in and maybe for a little while untill I can afford the right carb).

So here's my porting plan: I'll bridgeport the secondaries only. Do you know if the channels in the stock manifold will screw up anything? Has anyone ever tried this on a stock 12A manifold before? Do you think the little bit of AF mixture/vacuum signal that actually flows through them will be enough to cause any idle or other problems down the road? Or should I just join the countless hundreds who've bridged all four ports and call it a day? I could go either way at this point. Please, any advice on this is greatly appreciated.

 

I've thought about it s'more and think it would be in my best interest to only bridgepoort the secondaries. So what if a little AF gets sucked into the secondary runners? Most of it will still be going through the primaries, I think. Hmm, the way the butterflies open allows lots of AF to flow into the secondaries. I think the phenolic spacer helps but not by much. Hmm?

 

Well, if you figure, the reason the idle is lumpy with a bridge/peripheral is because the high manifold vacuum draws exhaust gas up through the port and into the chamber again. So, theorectically (pulling an assumption out of my *** ) having the channels totally blocked might make idle WORSE with a half bridge.

It's something interesting to think about. GatorRX from the old Mazspeed forum (he still around? dunno if he's on here or even if he's back on Mazspeed) was running a half bridge with an Edelbrock carb. Incidentally he was having tuning problems with his carb (running way rich at top end) and I advised him of my theory to cut channels between each rotor's ports and he did that and that solved that problem entirely. (Which is how I KNOW that it'll work to use a boinger 4-barrel on a rotary) I wish the old Mazspeed forum was still around otherwise I'd cut and paste. Anyway, he reported that his idle was 900-1200rpm, and I'm very curious as to how his setup was myself!

 

Wow, me too. Was it on a 12A with a seperate runner manifold? I'm thinking I'll be the first to attempt this project with with a stock manifold. I'll definitly let you know how it goes! I've just got to get a few more questions answered first. I remember the name GatorRX.

It'll be a small bridge so it won't cut into the water jacket. It'll also need to be a daily driver, so reliability is one of my design goals.

Ok, the engine originally was streetported and had a Nikki on it. All I can do is assume it was basically tuned for a streetport. I know a Yaw carb would have more power, but I'll be going with this old Nikki at first. I bet the secondaries will go lean due to the higher port flow. I'm wondering if the primaries will have a problem with a half bridge since more of the AF will flow from the primaries barrels into the secondary runners. The carb will think the engine is ported more and may screw with the idle and low end power. That's a big concern to me.

 

I'm in the process of cleaning the parts for the engine. Some wear on the thrust plate and oil pump, but still quite reusable in my opinion. I also took a good long look at my '73 and '79 intake manifolds. Man, that '79 looks like a piece of crap. The runners are all interupted by stupid castings for all the little ports and stuff. I can see why Yaw likes to work on them so they'll flow better. Comparing the '79 to the '73, it looks like all four runners will flow way more AF mixture. The primaries look big and healthy. Even the casting looks to be higher quality. I guess they were still somewhat low volume back then, compared to scrambling to keep the showrooms filled with SAs back in the rotary frenzy of '79. I feel a lot better about using the '73 manifold on a bridgeport now.

 

I have no idea what kind of manifold he had. He also said it was a "small" eyebrow.

 

Oh, ok. I suppose it could have been anything then? I was going to do a small eyebrow too. I'd also like to go to an 'extend' port on the secondaries too. They're already streetported according to RB's old template which is similar in height to a stock '74 port. Well, I think I could get away with a taller port like what I've got on an old ported '74-'78 housing that I may use. I just hope the '73 front plate will take a tall port too.

 

I drove the MG today. Heh, this is the first time this car has moved under its own power since 1990! I burned out over a paint line near a fellow rotorhead's house, tee hee! My tires still have a little bit of white on 'em

 

I moved the MG into my garage yesterday so I can take out the old, slow reving REPU 13B to make room for my ported, multi-year 12A creation. The 12A isn't ready yet, but the 13B is going into another project so it's got to come out now. This means the MG will have to sit for a while. That's not a problem since we're getting close to the rainy season up here in the NW anyway. For a car that came from a rainy part of the world, it sure has lots of ways for water to get in. The wiring is prone to water damage too.

I think I'll get an '81-'82 engine gasket set from Mazdatrix because it is the cheapest. It doesn't include any of the junk needed for the oil to water coolers ('83-'85), and has an intake manifold gasket which covers the little exhaust port under the intermediate ports ('79-'80 wouldn't work). Yep, '81-'82 is the best year for a gasket set for my engine. Maybe yours too if you're doing an engine similar to mine.

I need your peoples' opinions on viton oil control 'O' rings vs stock Mazda ones. I take it the viton ones are more expensive? I've seen a set on eBay for like $54.00 or something.

Would it be ok to widen exhaust ports? They've been ported a bit, but I'd like to increase the size without increasing port timing. Will widening cause the apex seals to break more easily? I'd like to get up to 8400RPM with this engine.

 

What exaactly did you use on the 12A? I'm going to be putting my RX-2 together soon, and I plan on using the 73 engine because it has such low miles and runs so well. It already hs a single dizzy front cover and everything.

Should I find a 74-78 rear plate and port it to match the port job that the 73 already has? I'd love to be able to use a 5 speed/side mount starter setup I have.

Maybe I could bolt a T2 box to the 4 speed bellhousing and have a clutch disk adn stuff made? I don't even know if that is possible.

 

In the old days of peripheral porting, people would simply tap and plug the tension bolt hole in the upper corner of the rear plate to keep coolant from seeping out. From my test fitting of a '73 rotor housing to a '74-'78 rear plate, I think only two tension bolts need to be tapped and plugged. They're the ones right by the Leading plug and the exhaust port (these were relocated slightly in '74 for better spark plug location and a different exhaust port shape). However, there may be one more in the upper corner that'll need to be plugged as well. This is probably the same as the one that gets plugged with peripheral ports. This is only speculation, so you'll need to get a '74-'78 end plate and lay a '73 rotor housing on top of it. when I get some time to clean up my '74-'78 end plate, I'll check it out.

What kind of rotors were you going to use? There are no counterweights available for '73 engines as far as I know. I'll have the lightest rotating assembly using stock '83-'85 rotors with an auto counterweight.

Anyone out there have a 10A? Want to mate it to a 5 speed? It is possible, heheh!

 

I was going to use all the 73 rotating assembly. All I wanted was a rear plate with a suitable bellhousing bolt pattern.

 

Ok, then you should go with a '74-'78 since they'll have the maximum available tension bolts for when those two or three need to be plugged. If you were to use a '79-'85, you'd be loosing four bolts. Probably not a good idea.

Good luck with your flywheel then (the bottom mounted starter may not mesh with the teeth on your flywheel).

 

It might, its a 215mm flywheel, and the trans I want to use is off a 225mm car.

I'll figure something out. Maybe by then I'll have my FB plans finalized and I'll just use the 85 12A and 5 speed...

 

I started the process of removing the 13B from the MG yesterday. It's not much fun having to do this. I've just got to keep telling myself that the next engine that's going in it will have much higher performance.

 

I am putting a bp in my 79 and need an exhaust as quiet as possible. any recommendations