Hey Guys,

For years now I've been hoping to collect enough parts to do the "Ultimate Streetable 12a". I never thought about the fact that things like Rotor Housings would become NLA for the 12a, and so I did just about everything else to my car first.

So I started thinking, if I miss the bandwagon for building a kickass 12a, what would I do? So I figured I'd start taking ideas for the Ultimate Streetable Carb'd N/A 13b.

What have I got so far?

- TII 4-port Irons, street ported or half-
- 13b Rotor housings (does it matter which?) with ported exhaust
- RENESIS rotating assembly (rotors, e-shaft, counterweights)
- 3rd gen corner seal springs
- RENESIS stat gears (they've gotta be hardened to take 8kRPM from the factory right?)
- Race rotor bearings
- 3 window main bearing
- High Flow oil pump
- High pressure (80psi) oil pressure regulator
- Race clearencing of rotors and balancing of the rotating assembly
- 2-stroke adapter for the OMP
- Oil pan baffle plate

And a bunch of stuff I'm sure I'm missing like a lightweight steel flywheel and a good clutch.

I've already got a big aluminum rad and an e-fan, and the S4/S5 Water pumps are already larger than the one the FBs come with. I plan to mount a FMOC for good measure of course.

The idea is that I would get a GSL-SE front cover and oil pan, and the RacingBeat brace that allows me to drop it in and bolt it in place. Hopefully a GSL-SE Road Race header would allow it to bolt up to my existing 2-pipe RB "Street Port" 12a exhaust system, and if I could pick up an intake manifold from a mid-70s 4-port 13b, I'm hoping that I can just start with my Sterling for now. Basically build the engine and leave the rest intact.


So what am I missing guys? Let's make the wishlist for this beast.

Jon

 

I don't think that intake manifold from the 70's will bolt up to a TII block..???

From what I remember when comapring... the TII ports are HUGE compared to a n/a 13B.

 

The renesis apex seals are too short to be used in norman 13b housings, so you have to use older style apex seals and to use them you need to have the rotors machined. for the cost it's not worth using them. .3 increase in compression would be good for about 3whp against s5 rotors if the engine was tuned perfectly.

 

What do you mean that they're "too short for the old housings"

I knew that they were half as high, so you couldn't use old style seals in Renesis rotors, or Renesis seals in older rotors, but I thought if you matched the rotors and apex seals you were still ok? I mean the rotors are still 13b, so they should still be as "wide"...

So how does that work?

Jon

 

Yeah it probably won't as-is now that I think about it.... I may have to make an adapter plate or mod the manifold itself... We'll have to see I guess. I can't think of another way to get it on there except that I once saw someone mod an RB Holley intake mani to take a Nikki... Though again that was for an -SE, not for a TII block..

Jon

 

Alot of awesome info in this thread. https://www.rx7club.com/naturally-aspirated-performance-forum-220/highest-power-challenge-2-rotor-13b-non-bridge-non-peri-712131/
Something about them not working with the perifial port exhaust. i for get why but it's listed in a build thread I read last week. In the NA performance forum.

 

I would of said u could use a lower TII intake manifold w/a upper RotaryShack to a Weber... That's what the shack does for his blowthru's why not for a n/a...???

But then I noticed you want to keep your Sterling...

Maybe an adapter plate...??? I have one that mates a 12A intake to a 4port13B... You'd just have to find a good machine shop.

But then you'd have to consider the size of the TII ports..??? Is the Sterling carb going to flow enough for that motor configuration..??? Just poppin out some questions to which I don't know the answers..?

 

I must have missed this thread. Ok, what parts of the above mentioned, do you have right now?

 

The manifold runners are small though, compared to a REW engine.

 

For the engine I have nothing... But I have a Sterling and a Racing Beat "Street Port" exhaust that I would like to re-use, at least short term...

So the issue is how to mate a Sterling Nikki to a Franken-13b, and what kind of frankenmotor to make.

Jon

 

I have a franken motor I built about 4 years back and I couldn't get it to start. It's been sitting tucked away in my basement doing nothing but looking real pretty on a motor stand.

I used a pair of GSL-SE rotors...

S4 TII rotor housings, which looked real good to me... no flaking or grooving anywhere... problem I learned after the fact was the guy who I got them from went tooo big on the GIANT exhaust port job and that was why he got rid of them so easily. But then again this info I received was from a backyard mechanic like myself and he could be wrong...

S4 TII plates, with port job done by myself... and again I have NO CLUE about porting except at the time my mentality was to go BIG.

Anyways my point is I have ALOT of motor parts just sitting around... such as another set of TII rotor housings and plates, both sets in real good condition w/no molestation done to the ports... various sets of rotors... etc.

I could get this project underway also and use my YAW carb which is in the same catagory as the Sterling...

I be willing to start right away being the cold is creepin up and I'm looking for some inside work to keep me busy. I don't mean to hijack your thread but it's basicly exactly what your looking to do and I'm always looking to learn.

 

Well the combination that I've found that people have actually done that seems to work is

S5 Rotors
S5 4-Port TII Irons
Pretty much any 2nd Gen rotor housings
Rx8 E-shaft
and S5 N/A Rotors (9.7:1 comp)

And with street porting the irons and rotor housings, it should be quite the beast. You'd also end up re-balancing the rotating assembly due to the weight difference of the rx8 e-shaft... though some say the difference is so minimal that it's not needed in a street engine, I don't know if I believe that.

I haven't found any other references to the fact that you can't use Rx8 rotors with other 13b housings... From what I've seen as long as your rotors, your e-shaft and all your rotor seals (apex seals, side seals, etc...) are RENESIS you can go ahead. I've seen one or two mentions not to use it for Peripheral Port engines if your PP is very big because the thin apex seals might be more apt to break in the middle as it goes over the intake port since there is no support... No first-hand accounts, but definitely some words of caution.

So if someone has a thread or a personal experience as to why you shouldn't use RENESIS rotors with all their associated seals and e-shaft and rotor bearings in an engine built with S5 TII Irons and Housings, I'd love to hear it/see it. Otherwise I'd be *very* tempted to try and build an engine like that.

Jon

 

Another question:

Does anyone know the differences between all the 2nd Gen rotor housings? Mazdatrix has them listed as "front" and "rear" and separated between S4 and S5 AND Turbo and Non-Turbo... For some reason, the S4 Turbo Rear rotor housing is $65 cheaper than any of the others. Is there any reason *not* to use a pair of these, especially if you're porting them anyway and would be running a header?

Jon

 

Jon, the differences are T2 has normal sleeves, NA has diffusers. S5 has knock sensor threaded holes above the trailing spark plugs, and leading spark plug location was moved slightly. Also, the difference between front and rear has something to do with an emissions hole under the exhaust port. I beleive soem were drilled and others were just blank castings there. j9fd3s should know the exact differences. It's not a big deal for you, I would think.

If I could make a suggestion, you could have a 1/2" aluminum plate cut for the T2 port shape and bolt pattern, and have it welded to a Sterling manifold, or a stock '79-'80 one (since they flow a bit better than an FB manifold, appareantly). I have a '79 manifold like this, with a 6 port plate welded to it. The aux ports won't flow that well, but a contraption like this would work much better on a T2 engine since the stock 12A runners are a lot closer in size and shape to T2 runners. And the most important part is you could use your Sterling carb.

It's more difficult to use a Nikki on an old Hitachi manifold because they locate the carb higher up and would cause the top of the carb to possibly touch the hood leaving no room for an air cleaner assembly. Also the Hitachi manifolds fit a 4 port 13B, not the T2 bolt pattern.

You might be interested in this; I recently picked up an S4 T2 engine dissassembled. Someone streetported it and milled the rotors for 3mm apex seals. You wouldn't want the rotors because they are low compression 8.6 or whatever. You'd be better off with a set of NA 9.4 rotors, which I also have.

I sent you a PM with more details.

 

Complete S5 NA rotating assembly. They came balanced for the 8k rpm limit and hardened stationary gears. Don't recall if they came with the 3 window bearings or not. 3rd gen corner seals, e-shaft oil mods, upgraded ropr, (85 psi), TII irons, S4 or S5, any housings with TII exhaust sleeves, lightweight flywheel with TII clutch and tranny.

Carburation will be porting dependent. My preference would be a mild primary sp, large sp or 1/2 bp secondary. I have doubts that the Nikki would support an sp effectively, definitely not a 1/2 bp. For the 1/2 bp, my favorite and the only streetable option, Rotary Engineering's dual DCDs. That will need a custom made mounting flange to mate the differing port openings and bolt patterns. Plate would need to be 1 to 1-1/2" thick aluminum.

The above is the basics of the engine I've been building in my head for the REPU. It should be very streetable, reasonable fuel mileage and haul *** when opened up. One benefit is I can run true dual exhaust on the REPU chassis. HP should be just under stock S4 TII's 180 hp and the REPU weighs the same as the FC's, about 2800 lbs. The engine could be easily turboed, keeping boost under 10 psi with high comp rotors, hp should jump to 250 with a full 3" exhaust using stock TII parts, MegaSquirt and upgrading the Hitachi turbo to a BNR stage 2.

 

I'll be watching this thread lots of good info here, one small contribution I could make is about the engine mount. If you are planning on running the re-speed rack and pinion kit (which I seem to remember you wanting) you wont need the RB engine mount brace, the Re-speed kit has provisions for a 12a or 13b already...

 

*smacks forehead* OH YEAH! I *have* the ReSpeed rack and pinion steering kit on the car I'll be dropping this engine into and you're right, the front mount can be loosened, moved the 20mm forward, and re-tightened in the new location! Props to Billy for thinking of that, it takes one step out of dropping this engine in. I had just been working on the parts car so much that I forgot about the different mounts for the ReSpeed kit.

Jon

 

Yeah that's what I was thinking. Start with a 1/2" piece of aluminum, drill bolt holes in the appropriate places and use a gasket to scribe the 4-port 13b Ports on one side, and the 12a ports on the other side, lining them up in the middle. Then I'd cut through from one to the other and try to shape them so that the transition is as smooth as possible with the given space.

If I'm lucky, the bolt holes won't interfere with each other and I can just drill and tap spots for studs on the aluminum adapter. Then I'd be able to bolt the adapter to the engine, slide the manifold onto it, and use nuts to fasten it in place. That would allow me to use my ported 12a manifold and Sterling carb, without removing the ability to swap them to a 12a engine if I needed to at some point (which I sometimes do for testing on other cars).

We'll see once I have all the parts in front of me (or at least the housings, the aluminum and a pair of gaskets) how this will all work. Worst case yes I could do some grinding and welding but I'd much rather keep them separate if I can. I think this is actually going to be the trickiest part of the swap, assuming the GSL-SE "Road Race" header bolts to the S4 engine and to my existing RB exhaust system. It should, since they didn't move the exhaust studs from the -SE to the FC engines methinks, they're still centred and the same distance apart... but I'll have to check to be sure.

Jon

 

After spending some time in the 2nd gen section I'm thinking S5 N/A rotors with front and rear counterweights, but an rx8 e-shaft, stat gears and main bearings. Race rotor bearings. Yes the rotating assembly should be rebalanced, and most likely will be though from what I've heard it's not a huge deal since the weight change is so close to the centre. Since I'm starting with no parts at all, the rx8 e-shaft is cheaper, lighter, and the stat gears come with 3-window bearings stock, and are hardened to take the rx8's 9k redline.

3rd gen corner seal springs are a no brainer. I'd do that in even a 12a build.

Whatever rotor housings I get will have clear exhaust ports. I'm getting them ported anyway so if they have the diffusers they will be removed.

Lightweight steel flywheel and appropriate clutch, but for the time being I'm going to keep my 12a tranny.

Basically I am building this to be a "plug and play" swap of just the engine, despite the huge differences in the engines themselves.

I don't know... As much as people say that a modded Nikki "can't" run a street port 13b, I'm going to try it anyway. When you run the numbers there's no reason that a 465cfm carb can't flow enough, even at 8krpm, it's just a matter of tuning. We'll make the adapter plate, drop in the engine, hook it to a dynajet and see what happens. I think people will be pleasantly surprised. The Sterling isn't just *any* modded Nikki, and I've spent enough time with it that I'm confidant it can do fairly well on it. Unless someone's willing to donate another carb (or carbs) for comparison dyno testing, I'm going to stick with the Sterling unless I see something on the dyno that suggests I change.

For one thing, my supporting systems are fairly well set up as it is, and if I let myself get carried away doing more than just the engine I could end up spending thousands on things that don't need to be done.

Jon

 

Wasn't sure on the RX8 specs, I've paid little to no attention to them. My point using the S5 NA rotating assembly is it's plug and play, balanced for 8k rpms. Using the FE e-shaft, bearings, etc., along with the S5 rotors would require lightening, rebalance and, iirc, new rotors. To be able to use the 9k redline, bearings and rotors will need clearanced too.

I too have faith in Sterling's carbs and his mods. It will most likely work for a 13B-sp. But adding a 1/2 bp and 8-9k redline, I have doubts. Since your thread is titled 'Streetable', that probably eliminates the 8k+ redline and 1/2 bp. Given that premise, your parts list works. It will be interesting to see not only the net hp/tq, but also powerband range.

 

It will run with a Nikki and a 1/2 sp/bp. It will drive fine as well. Did that before, but when it got a 600 holley, it seriously woke up.

 

You had a Sterling on a similar engine?

Jon

 

I found an old thread talking about the Sterling, carb flow, Volumetric Efficiency and other wonderful stuff:

https://www.rx7club.com/1st-generation-specific-1979-1985-18/why-big-carb-536072/

So I ran the numbers for an 80CI engine

--------------------------------------------------------
At 10,000RPM, at 100%VE (both numbers that aren't realistic, for your "best case" scenario on the engine) you get:

80CI x 10,000RPM = 800,000CI/m (cubic inches of flow per minute)

800,000CI/m / 1728 (cubic inches in a cubic foot) = 462cfm
--------------------------------------------------------


Which would make the Sterling *just* enough. Of course you don't want to run that close to the max flow of your carb, but for the heck of it, let's run some more realistic numbers for the flow of the engine. You will never get 100% VE throughout the entire powerband, and especially not at the upper end of the RPM range since that little port isn't open long enough for enough air to rush in and fill that space (assuming standard atmospheric temperature and pressure). PLUS, I won't be going 10,000rpm for fear that my clutch assembly will explode and take out the bellhousing, the floor, and my legs.


--------------------------------------------------------
So let's say a max of 8,000RPM and a very generous VE of 90%. Even with porting you're not going to get that, but it's a little closer to being realistic.

80CI x 8,000RPM = 640,000CI/m

640,000CI/m / 1728 = 370.4cfm * 90%VE = 333cfm
--------------------------------------------------------


That's a number that puts us well in the "tunable" range of the Sterling without completely maxing it out. It's 71% of the max flow of the Sterling (465cfm) in fact.

So I'm confidant that the carb will perform well on that engine, assuming I take my time to put the car on the dyno and jet it.

Jon

 

This is an excellent point, Jon, that is not often made, and that I don't make enough myself.
As you begin to max out the carburetors flow, efficiency through the carb falls. What was once a beautiful slipstream of air current that caused a textbook depression right below the apex of the venturi, where the bottom of the booster venturi and fuel inlet tube sit, becomes impeded by a combination of friction, and air compression at a very relatively low measurement, but certainly enough to effect the delicate depression measurement that is needed to draw out the fuel in a precisely metered, delicately atomized mixture...

-In other words, at some point approaching the maximum flow of a carburetor, everything begins to go to hell within the carburetor. What exactly that point is, and just how quickly it happens as the flow increases, I don't know. I believe it happens rather abruptly, and very close to the maximum flow rating.
In a discussion I was having with a physics major regarding when carburetor flow falls to pieces, he suggested it might have something to do with the terminal velocity of air. It wasn't the first time that had been proposed to me. Though I believe that would be a different issue with different effect, neither of us were sober enough at the time to look up the numbers and start crunching them to see if that's even achievable.
I suspect that the "maxing out" of a carburetors performance potential has a lot more to do with engine porting than people realize. The same engine ported for an average VE of 94% between 4K & 8.5K can use the same carb as a stocker that only gets 87% in the same window. That 8% increase in volumetric efficiency requires a complete rejetting. The question is not whether the carburetor will breathe enough; obviously it will. The question becomes, is it tunable enough to accommodate the change in the ported engine.

We can mathematically reverse this perspective;
Assume the carburetor can flow a high enough amount for both applications, as described above, a 13b (80 cid @ 87% VE) engine, and a ported 13b (80 cid @ 94% VE).
The needs of the stock 13b (theoretically) are that of about a 69.6 cid engine @ 100% VE, where the needs of the ported 13b (theoretically) are that of about a 75.2 cid engine @ 100% efficiency. -Again, an 8% increase, which is a significant maximum flow increase, but should be well within the working window of most any carburetor choice for the application.
But, is the carburetor TUNABLE enough to accommodate that wide of a variance. Most are not. Holley carburetors were designed for the narrow rpm range of V8s, and lack the dynamic range of tunability for the extremely wide (by comparison) power band of the rotary. Consequently, they deliver a rather narrow usable power band, often at the high end, leaving the rotary lacking it full power potential at low flow.
Two barrel carbs are of the same nature, but for different reasons. However, the tunability of a two barrel set up on a race track where low RPMs are rarely used makes any production four barrel carburetor hard pressed to compare with them, save maybe an appropriate sized Demon carb.

However...
Just about all of this is moot if your manifold sucks. For the most part, the stock Nikki manifold sucks. That's why I'm working (off & on) on trying to find a comfortable porting configuration and plenum combination for "general" application. Pulling out some hair in the process, I have not ruled out a Racing Beat Holley manifold adapter as the way to go. Hood clearance is an issue. But, with the appropriate runner sleeves to reshape the inside of the manifold, and of course an adapter plate, I have no doubt that the Sterling would be competitive against anything on the track.

 

Here's your answer for the rx8 rotors. The side seals are not in the same posisition as the older rotors, so standard porting does not work with the seals. You have to be very carefull when porting, or you'll drop the side seals into the port and end up with an epensive paperweight. You can open the ports earlier but you have to be very carefull on how late you can close them. As I said before the apex seals are too short to run over the perrifial exhaust ports. They are too weak to support themselves with the big hole there, so you have to get them machined to fit the older style seals.