Currently I am tearing down a 12a from an 85 with the top mount oil cooler from a crashed 7. I've also purchased a 1982 project car. Being a family run machine shop we are currently contemplating an 18A. The shaft seems easy enough along with the oil pan, intake and exhaust. But what else other than bolts pieces do I need?

Will the extra heavier rotor from the 82 throw the balance off? Is there anything from one engien that should be used over the other? I've often heard that the lighter flywheel from the 82 should pair with the 85 internals?

On another note, what port job on the 12a goes best with forced induction?

Any help would be much appreciated..

 

force induction will work with any port

 

I apologize if this comes across as being a know-it-all, but I find your proposed build a bit troubling. If this isn't your first rodeo, take what follows in stride.

Generally speaking, you can't mix and match rotors from different series in the same rotating assembly. In terms of tuning, such a difference could wreak havoc on making the engine run properly due to differences in the compression ratios between said rotors. The 20B uses three S5T2 rotors and its own unique counterweights to yield a balanced assembly. If one substituted a S5 NA rotor in place of one in a 20B build, it would exert greater force on the E-shaft where it is placed, compared to the other two rotor due to the higher compression ratio. I believe it would result in twisting the E-shaft at its weakest point progressively more with each revolution untill it snaps in half. This could be measured in minutes or months, I'm not really sure as I'm not a metallurgist, mechanical engineer or physics professor but I personally wouldn't risk it on the principle alone. Unbalanced engine = Bad JuJu

As for the rotors actual weight, the rule of thumb is that they need to be within one weight code of each other without ill effects on balance when using stock counterweights. A "C" and "D" weight rotor can be used together in an engine, same as a "B" and "C" or two "C" weight rotors.

The rotating assembly and counterweights will be a bit tricky to get right. I believe that you may find some clues by taking a close look at the 20B's rotating assembly vs a S5T2 assembly. By close, I mean machinist/machine shop close. Another option would be to send to assembly to Mazdatrix for dynamic balancing.

What did you have in mind for intake ports? Planning to keep it a stock/modified side port or peripheral port? This will determine how you approach the E-shaft (length and necessity of removable front lobe) and second middle iron (AKA the Thick middle iron on a 20B). Side intake ports will require a middle iron akin to the 20B's proprietary thick iron, while peripheral ports can use a normal 12A middle iron.

 

yes.

you probably won't be able to use a stock flywheel on a 3 rotor, because the balance weight will need to be in a different place. finding a 20B weight and taking weight off when you balance it is probably the easiest.

 

Just get three rotors that match. And do what j9fd3s said. Then for intake ports, use the intermediate plates as primaries/secondaries. Don't bother peripheral porting unless that's what you want.

 

So how exactly does one bi-pass having the wider housing/plate without PP?

And what would be used for a distributer?

 

The primary runners from a Y or R5 engine are tall and the ports are the same size as secondaries. This is not an option on 86+ irons. However since you're looking to do this with 12A stuff, you have this option to use them as side port primaries/secondaries like Hurley did.

A distributor is easy. Make a plate that mounts the three pickups 120 degrees apart. Then you get direct fire on all three leading plugs. Don't need trailing, so don't make a second plate, unles you want to.

By the way, leave the leading reluctor alone with all four tips intact. If you go through the unnecessary process of adding trailing ignition, you will have to chop off two reluctor tips fromt he trailing reluctor located at the top of the shaft. But don't bother as trailing adds like zero HP. It's just there for emissions. Plus you'd need to canbalize three distributors for a total of six pickups, not just two for three for pickups used for leading only. And then there are three coils vs six and making room for them all...

Or avoid all that and get the 3 rotor dizzy mod stuff from RB. Notice its 6 point reluctor? Yep.

RX7 1986-1992: Ignition: 3-Rotor Distributor Cap - 20B 3-Rotor Engine -

And one more thing (man I need to stop) have you ever driven a peripheral port? Better start with a 2 rotor then. Also research j9fd3s' project peepers. I can tell you I didn't want to go with a peripheral port on mine.

 

It is necessary to install a 3rd bearing and stationary gear to add a rotor. There is no way to do it without a 3 piece eccentric shaft (except for a 2 piece stationary gear, which is a terrible idea, as proven by RPI).

It is not reasonable to use two different series rotors, as they have different geometries, and more importantly, different weights. Also, all manual transmission rotary flywheels have integral counterweights, which means they would be useless for a 3 rotor.

That said, if you have the capability to make an eccentric shaft and properly heat treat and straighten it, then you can get some more appropriate parts and do a little more research, and build a proper 3 rotor.

If you are going to build it from scratch, I recommend that you in fact go in the other direction, and make a 3 piece eccentric shaft with 4 main bearings, as this will remove the major weakness of the 20B style 3 rotor.

 

Patman knows his ****, I think my days with the 12A are done..

 

So how exactly does one bi-pass having the wider housing/plate without PP?

And what would be used for a distributer?

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