IMSAaspirations

Hello all! I'm new to rotaries and I'd love the advice of those of you who know better than me. I know it's possible to build a 13B out of 12A irons and 13B internals. If my goal is to (one day) build an NA 13B racing motor for my 1983 IMSA GTO build would it be worth it to convert one of my 12As or should I start with an NA 13b from an FB or FC? I have a 2 engines in the garage, a complete and totally stock 12A from an 83 GS auto and an old Pro-7 motor from my racer and I'm tempted to use the irons from one for another engine. What do y'all think? Am onto something or am I barking up the wrong tree? Should I convert of just find an NA 13B?

j9fd3s

iTrader: (3)

to do a 13B from a 12A, you need rotors, eccentric shaft, stationary gears, dowel pins, tension bolts, oil pan, and the intake/exhaust manifolds. most of that are not wear items, so they are pretty easy to find. you reuse the irons, oil pump (depending on the year of 12A), front cover and water pump

if you can find an NA FC engine, it gives you a lot of this stuff but with better rotors. the housings aren't compatible, but they are also not P ported..

so whichever is cheapest/easiest

KansasCityREPU

iTrader: (9)

If the 12A engines are currently good runners, I would not take them apart. The value in a 12A is if it runs.

It really depends if you want to stay carb, FI, turbo, etc. Determine the intake and let that guide you. The other thing is if there are any restrictions based on race class.

peejay

You're going to need a 13B for 90% of the engine anyway, no reason to pull apart a good running 12A.
If you were closer I'd GIVE you a set of 12A side housings that need refinished (Chips Motorsport does a good job at a good price) and then you get a FC core engine for its rotating assemble and 13B length related parts, and a pair of new GSL-SE rotor housings, and presto, 13B.

IMSAaspirations

Hello all! I'm sorry I've been AWOL for an entire month; life got very hectic and taking the time to reply to my numerous threads simply couldn't make it onto the to-do list. I'm really thankful for all the input! Hopefully y'all don't get too upset at my resurrecting a month-old thread but I've news and I need some more advice:

I understand that one needs the entire rotating assembly from some NA 13B, and Friday I acquired an NA 13b from an 1987 FC! I pulled it from a pick-your-part for $275 sans alternator. The bad news is its completely locked up, and I can see the rear rotor through the spark plug holes and its super rusty. The engine is TOAST! As j9fd3s pointed out a lot of what I need to convert the 12A to a 13B aren't wear items so hopefully I can salvage all those parts and have a good base to start. Rotors and e-shafts can be procured, and I think Mazdatrix may have some hardened stationary gears still in stock. If they don't maybe I can get some from Mazda Motorsports? I'm worried about the irons as the 86-88 13Bs can only lose .002" before the O-ring seats are compromised so I hope I don't need new irons...

KansasCityREPU mentioned intake, and my goal is to have a weber carbureted NA motor like some of the IMSA and Group C racers form back in the late 70s-early 80s. Allan Moffat's rotary powered exploits, particularly the 1985 Dayton 24 HRS car, are my inspiration. My goal is to build an SVRA Group 10 car so anything period correct will do.

The reason why I asked about building a 13B out of 12A is that I have one 12A motor that needs a rebuild anyway. It was a Pro-7 motor that has failing apex seals or apex seal springs. It's the motor I've mentioned in my other threads that will only start if bump started at a decent speed, but seems to run really strong once it gets going, so we assume the irons and the like are all in good shape. That motor hasn't been torn down yet, but if its parts a healthy I'd love to put it back together as a 13B. Peripheral ports can always come later. It'll be a race motor so it won't let forever; it'll need a rebuild again at some point!

peejay may have answered my next question, but can I use a 87 rotating assembly and housings with 12A plates and make a Frankensteins's motor? Or if I want to go that route I can use all the non-wearing assembly parts and a GSL-SE rotating assembly? I've searched the forums and read all the tips and tricks on Racing Beat's page and I haven't found a simple answer to that question.

​​​​​​​I'm sorry, y'all but I'm impressively technologically inept so if my searching skills have failed me and you din't want to answer here please point me in the right direction! I have yet to tear the motor down so IU'm open to all the advice y'all have on what to do next. In the next couple weeks I'll drive up to see Daniel at GarageLife and he'll disassemble and asses the parts I have

diabolical1

iTrader: (3)

the rotating assembly? yes
the rotor housings? the simple answer is no. the other answer is maybe .... you would need to get them machined for the water seals.

rxtasy3

iTrader: (2)

rotor housings will need to be pre '86.

peejay

Rotors are wear items, gotta watch the apex seal grooves. '86-up rotors are much lighter wearing than the 3mm seal stuff, at least, so finding good rotors is not as much of a challenge.

Likewise, you will not need hardened stationary gears with '86-up rotors. They have a much better way of retaining the rotor gear, it prevents as much shock loading to the gear interface. I'd been running for years on '88 rotors with GSL-SE stat gears and I spin my engine right on up there.

IMSAaspirations

So what I'm understanding is that the assembly components, rotating assembly and and the irons are pretty much interchangeable, but the rotors and housings are specific per generation. I'd assume that the 84-85 parts would be more difficult to find and I'd need rotors, counterweights and housings at a minimum to make a 13B out of my spent 12A. I have no idea what I'll need for the 87 motor so I guess the next step is to tear it down and assess the parts. That'll tell me if I have anything worth using. I hope the irons aren't trashed!

Y'all are awesome! Thanks for all the info and advice! I'll be back when I have the next step figured out

j9fd3s

iTrader: (3)

so Mazda made a bunch of revisions for the FC. the important one is that the water seals moved from the rotor housings to the irons, which makes them incompatible with the older parts.

the rotors got lighter, and got better seals, so these are a nice upgrade. (the rotors, flywheel and counter weight are a set, so keep all of that) the stationary gears and tension bolts are the same as any 13B.

oh and the water pump housing is a nice upgrade too, aluminum vs iron

diabolical1

iTrader: (3)

i think you should pull the engine apart, clean and inventory everything. rust inside it is not a good sign, but it's also not necessarily the mark of futility as well. you won't know until you check it out. i think this would be the easiest path toward 13B power. of the 4 "types" of Gen 2 engines we got here, the S4 6-port seems to be the one you're likely to find parts for at any given moment, so i would say try to make it work. if it's usable, then you will have complete, factory engine, and besides, as stated a few times above, the S4 rotor assembly is better than the S3 (GSL-SE) set, anyway.

if the rotating assembly is okay, but side housings do turn out to be junk, then you can look at trying to find the SE rotor housings and build your "old school" engine. you'll pretty much have all the bolt-on parts except for oil pan (maybe oil pump, too) and front cover.

peejay

Oil pump is different 85-earlier vs. 86-up, bolt pattern wise.
I actually prefer the 12a pumps. Yes they flow less at low RPM, but why do you need that? They cavitate less at high RPM, so you don't get oil pressure dropoff.

IMSAaspirations

Yea I'd hope Mazda made some improvements when they introduced a new series of engines! A cooling passageway change is definitely a big deal so it makes sense why the S3 and S4 irons and rotors won't mix. I also read somewhere that the shape of the housings changed slightly in 86 and that prevents and rotor interchangeability too.

I am of the same mindset in regards to having the later motor if possible. Though I'm wanting to build a period correct IMSA racer, I'd much rather have the better motor with the better parts availability so I can race the car with much less worry! I don't need the old school, S3 type 13B when an S4 runs pretty much the exact same, save maybe a few HP and RPM. Its good that it seems S4 6-port parts seem to still be available seeing as that's what I've found. I'm defiantly going to have Garagelife inspect the motor and help me take inventory on what's useable, but until I have that sorted I'll keep planning for the worst while hoping for the best. Hell if I only have to source a rotor and an iron or two that may not be too difficult. I plan on getting a 13B in my racer at some point so I bought an NOS 13B pan from Mazdatrix (ow, my wallet!) and when the time comes to drop it in I'll buy Racing Beat's 13B mustache bar. I'm going to just use the front cover off of my tired 12A. My research has led me to believe they're compatible and I plan on tearing down that tired motor anyway to better store it.

Its funny you mention preferring the 12A pump as I swapped out my 12a pump for the 13b pump in my racer's current engine before I dropped it in! The power curve for that engine has me shifting at 7200 anyway so I'm not too worried about caveating and losing pressure at stupid high RPMs. One day I'd like a 10,000 RPM NA 13B, but my budget won't allow that so I'm no too worried about high RPM issues yet.

peejay

Suuuuper low budget. 12a pump, ported oil passages, teardropped oil passages in the eccentric shaft. Atkins 2mm seals. Holley manifold with a junk carb with a Ford something TPS, Megasquirt and two injectors.

I shift my stock 12A at 8....

IMSAaspirations

Well damn, if Peejay didn't break it, its probably not going to break...

Well that video makes me wonder: one of the big things everyone involved in rotary engines says to avoid is over-revving the motor, but if the stock redline is 7k, and you shift your stock 12a at 8k, and this 12a with slightly modified oil ports can go to 10k what actually is over-revving for a rotary engine? I guess i should be concerned with mods that'll help eh engine live longer and be stronger before I try porting and going after power... But oh how I want a P-port NA 13B... we've all got have goals, right?!

peejay

A stock 12A will blow over 8500. Maybe not when you actually rev there, but very shortly after.

There's really only a few things you can hurt... you can get insufficient oiling, and the rotors can smack the side or rotor housings when the eccentric shaft starts turning into a noodle. Or the apex seals can bounce so much that they just sort of come apart.

Oiling... Lynn Hanover instrumented an eccentric shaft so he could measure the oil pressure inside it with the engine running, and found that at some RPM (8000?) there was only 7psi of pressure, with 100psi measured in the normal place. Oil is fighting centrifugal force to get pushed from the main journals in to the shaft so it could get to the rotor bearings. This is not helped if the oil is aerated. The Mazda fix is a dry sump oil pump, which gets you a huge de-aeration tank so the oil is "solid" before the pump gets to it. The Pete way, which is cheaper, is leading teardrops on the main bearing journals to scoop the oil in, and lead-out teardrops on the rotor bearing journals to assist in flowing it back out. And porting all the weird bumps and ***** out of the oil pump inlet and outlet passages and rounding as many right angles as possible in the oil passages in the front side housing.

Rotor smacking... I clearanced about .002" from the sides/tips of the rotors with an angle grinder with the rotor chucked in a brake lathe. Very much meatball machining, but it's okay. I did smack the rear rotor into the rotor housing, but only lightly. The FC rotors in GSL-SE rotor housings MAY have helped me out here as the rotor housing dimensions are slightly different.

Apex seals.... Carbon, ceramic, or 2mm. 2mm is the cheap, long-lasting way, and you get better rotor gears that don't require expensive hardened stationary gears.

j9fd3s

iTrader: (3)

we were too cheap to buy gears when we raced an FC, so we just revved the thing to 9400rpm. engine was from a junkyard too. the engine is tired, but its actually still around.

we then moved to a honda, if you over rev by like 1rpm the engine just shatters. oh and the Honda was much more expensive, everything wore out. if we had the Honda budget with the FC we would have had a museum piece

IMSAaspirations

I'm definitely going to have to make those mods and clean up the oil passages; gotta keep the thing slippery! Thanks for that explanation! Those modes of failure make sense and they're definitely something I want to avoid!

peejay I've read many places that the FC rotors don't work in the GSL-SE housings. Was that info less than accurate or did the .002 clearances you machined into the rotors make them usable with those housings?

j9f3ds that's awesome! That junkyard motor seems pretty stout! I wish mine was that ready to go! Not that you are, but if you're ever gonna sell that motor send me a PM please!! If its tired and haggard it seem to be right up my alley haha! You've mentioned that Honda in another thread of mine! That thing sounds like an absolute nightmare but its tales of woe do always make me laugh. It seems that these rotaries aren't nearly as fragile as everyone claims. I don't want to do anything to damage my engines, but its good to se they seem at least a little forgiving, unlike that glass Honda.

My minimum-wage-earning-debt-ridden-college-student budget loves all the tried and true low-budget tricks. Hopefully I can make something work with all my miscellaneous parts

Beryy2

I want to buy a second Honda car
tracking software

IMSAaspirations

Actually, y'all before this thread goes cold: although the rotors for different generations of the 13b are all slightly different from each other, and the 12a rotors differ from the 13b's, do they all use the same stationary gears? Or are the stationary gears generation and engine type specific as well? Basically I'm asking if either my 12a or 13b set will work with any rotors I can find, of if I need to get the gears for the right year of motor? I'd like to use my tired 12a and make an old style 13B with either 88-92 or RX8 rotating assembly. Can I use either set of stationary gears? Or do I need to match them to the rotating assembly? I know I could prolly search that but since I have you all gathered here what's the verdict?

diabolical1

iTrader: (3)

12A stationary gears are different from 13B gears. the 13B gears have wider teeth.

the 13B gears you have will work with any 13B rotor.

peejay

The only weirdness with stat gears is that RX-8 rear gears do not have a groove for the O-ring. I think FD front gears might be odd in some way as well due to the thrust bearing, but I've never played with those engines.


As far as rotors in different rotor housings goes, the rotor housings are very slightly different in shape, but the difference is on the order of a thousandth or two in some areas. Effectively, it makes no difference, and people have been running FC/FD and even RX-8 rotors in early rotor housings no problem. I probably have 80k miles of FC rotor in GSL-SE rotor housing use.

IMSAaspirations

Thank you for a crystal clear, succinct answer! I'll be sure to keep this response handy lest I forget!

Awesome! A man who's done it and can attest! Thats really helpful to know as I was conferring heavily an RX8 rotating assembly. The fact that I can still get those parts new is great and I like the idea of lighter, better constructed rotors with higher compression for my NA racer build. I understand they have to be machined to work with the older, peripheral port style housings but that does seem well worth it in the long run.

Thanks so much for all the help, y'all!

peejay

I have no direct experience on how the shallower RX-8 apex seals tolerate sliding over a peripheral exhaust port.
Gut instinct says they would be fine, because Mazda had 3 piece apex seals for about a decade and a half, the long piece was split height-wise, so the business half of it was much shorter in height than an RX-8 seal.

Bridge port engine is back to life.

IMSAaspirations

I believe most people who put the RX8 rotors in earlier engines machine the rotors down to accept the larger seals. I read that in a thread on the subject a while ago it seems. They also mentioned using racing or high performance RX8 seals as an alternative to machining but idk anything about that myself. My engine builder said that the RX8 rotors work and just mentioned balancing the assembly. He didn't say anything about the seals. Weren't the early 12A and 13B seals 2mm as well?

It's awesome you got the motor back working! I need to pick up another set of 12A irons so I can make an old school ported 13B. That way I can have a set I can fill in to make a p port motor and one I can bridge