iluvtruenos

First off, I'm not the stereotypical needy whiny bitch. (I hope)

I did my homework, and I think I'm right, but not sure, so I need someone to help me.

First off, I have a 1992 Miata with 43k miles on the clock, all original, really fun car as is.

However, I know a person that is selling a Cosmo front clip, the one with the 20B 3 rotor engine.

Now, I've read and read, and I think I'm right, maybe not.

The 87-90 RX-7NA and Miata used the same trans. Now, is the 13BNA and 13BTT the same engine, but with a turbo, or does it have different mounting points, bellhousing, etc?

Now, the Cosmo was offered with a 13B or a 20B. So assuming the transitive property, this means that a 20B will mount to a Miata trans without any modification.

Am I even close to anything remotely correct? This is all theory, so I'm running it by people to see if this would hypothetically work. I just want the trans to bolt to the engine, the rest I can just transfer from the Cosmo.

Also, the Miata trans is pretty stiff, their diffs are weak-ish, however. Anyone know where I can crossreference a diff? The Kia Sorento uses a 2.2l engine derived from the Miata, so do you think the diff would be a relative as well? I would assume that the diff itself is not the problem, rather it's the pinion/spur, so could those be cryogenically hardened, or am I just talking out of my ***?


Thanks a lot guys.

EDIT: Wait, would a 13B be more economical to do? I'm leaning towards the 20B because it has three rotors, so it can divide the stress between three parts, instead of only two in the 13B, so the 13B is more 'fragile'? Which one is easier to get more power out of?

psychotic7

Well, there have been VERY FEW conversions where the 13b-t is put into the Miata. I heard it was very difficult, a lot of fab work. now the 86-91 n/a is only different from the 87-91 turbo by compression, turbo, intake manifold is less restrictive, the turbo is 4 port where as the n/a is 6 port, and WAY stronger transmission in the turbo, ihope i got everything. the right way would be to use the turbo tranny to fit the 20b, it bolts right up! beware, one of the engineers from Mazda did this swap on a 91 miata and he even said it was a bitch and that was with a 2 rotor! but hey if ya know how to fab really good, then go for it! it will be a BEAST!

iluvtruenos

I know how to fab somewhat well.

Diesel blower mated to a 1996 2.2l Toyota Camry engine. Is that good enough? Hahah.

Anyway, the turbo trans is stronger. Ok, what exactly is stronger? The gears, housing? Would a third generation turbo trans fit in a Miata, or would it be easier for me to strip the turbo trans of its gears and put it in the Miata housing? I think I'm going to go for it either this winter or next winter as I have enough money, tools, time, but not enough knowledge about the engines. If I knew that this would definately go together, then I would be all for it, but....eh....

psychotic7

well, if you use a N/A tranny on a 20B, I would give it 3 months tops before it breaks.....the TII tranny can probably handle 450whp pretty easily. The TII tranny also bolts directly to the 20B and is the cheapest 5 speed option for that engine because the 20B only came in automatic. A TII tranny can be bought for $200 easily so why not use it? also if you are planning on going beyond stock 20B power levels I would strongly recommend a chevy or ford rear end, they are cheap and can be cut to fit......beware, this swap is expensive!!!

20B front clip around $3000+
custom mounts cheap if done by yourself, if not $100+
custom crossmember $75+
custom driveshaft $200+
custom rear end god knows???
aluminum radiator $200
electric fan $50
stand alone management unit at least $300 if ya go with Megasquirt
255lph fuel pump $100

theres probably more, but thats the basic stuff

why dont you just buy a TII with a blown engine for about $1000 i see tham all the time? Then all you have to do is make the mounts and go with a stand alone management unit for now.....the rear end can hadle about 500 hp and you'll already have the tranny you need!

psychotic7

oh yeah, also the second gen RX-7 will also give you more engine bay room.....ive seen Miata's with Ford 302's stuffed in the bays, but ive seen big blocks stuffed into RX-7s

C. Ludwig

iTrader: (27)

The Miata and NA RX-7 trannys are internally identical for all intents and purposes. The difference that you need to concern yourself with is that the Miata input shaft is slightly longer than the NA RX-7 tranny and the bolt pattern of the bellhousing is different. It doesn't bolt up. So why not swap the NA RX-7 tranny into the Miata with the rotary of your choice? Because the trailshaft housing and mounting arrangement is different. The Miata uses a PPF like the 3rd Gen RX-7. The tranny and differential are joined by a structural member. The NA RX-7 tranny does not have the mount for the PPF. You could make it work and here's how.

1) Remove the inout shaft and have it cut down to match the RX-7 shaft. It's been done and there are threads on this subject. Search.
2) Swap the RX-7 bellhousing onto the Miata trans.
3) Bolt it all up. Slight fabrication required to fit the 20B. ------ can you hear the sarcasm there?


Now the problem with all of that is that the Miata and NA RX-7 tranny are weak. They break regularly in 250 whp Miatas that are not treated kindly and I've broken my share in our roadrace NA RX-7 making much less power. Their weak. To make the 20B swap worth your while, outside of the cool factor, you'll need to make 350+whp IMO to justify the work over simply turboing the 1.8 in your Miata. At that power level, easily achieved with either 2 or 3 rotor engines, you'll break Miata trannys like you change your undies. In the end don't bother.

So that leaves you with the turbo tranny. It's a solid piece and as others have said will hold a decent amount of power with good reliability. What's better about it? Everything. Bigger cogs, stiffer case. It'll bolt to your engine but it won't bolt to your chassis. The 2nd Gen TII mounts like the NA and doesn't use the PPF like the Miata. You can't swap the Miata tailshaft housing on to the TII trans so you are stuck fabbing a mounting system. Not rocket science but not something for many people.

But wait, the 3rd Gen uses the PPF! But it's on the wrong side. The Miata PPF runs down the passenger side and the 3rd Gen runs down the driver's side. Or do I have that backward? No matter, the trans mount is on the wrong side.

In the end it is a HUGE undertaking to swap a 2-rotor into a Miata. Going 3-rotor is even worse. With mild turbocharging you can make an easy 250whp out of the stock Miata engine and have good reliability. If you haven't driven a 250whp Miata don't immediately say you need more. With some work you can make 300whp and have something that eats 98% of the cars on the road in a straight line, out brakes everything, and out corners everything save for the occasional pissed of Lotus. And the best part? It's soooooooooo much easier than trying to drop a rotary in there.

Sell the front clip and finance a turbo setup for the 1.8 FWIW, the 1.8 diff is fairly stout. You'll break trannys and half shafts long before you break the diff.

iluvtruenos

Great, C Ludwig, thank you for everything.

That is pretty much the answer I was looking for.

And I have the 1.6, not the 1.8, the PPF is on the passenger side, it's more straight whereas the Third gen is drivers, and more of a bow in it, so you're absolutely correct.

Now as for 250whp. That is not a tall order at all, I think, after all, the 323 GT-X is the same block, turbocharged from the factory with 170hp.....in 1989, so I think the 1.6 can easily make 200HP.

Now I Have heard that people have used the Ford Exploder 8.77inch rear end, the one from the 2003, IRS, so all I would have to get made is halfshafts, and I can save a 1.6 diff, which I've been told can go with the stock 116 horses.

Now for the rotary swap: Basically it's pretty much impossible, right? Right, but you never said anything about making a custom PPF, or maybe even two PPF's, one on each side, one bolting to the stock holes on the trans, custom on diff, stock on diff, custom holes on trans.

To top it off, a GReddy kit good for about 200hp can be had for maybe 2k, whereas a turbocharged 13B is good for what, 250 at a price of 4k? What is the limit of HP in the 13b?

B6T

iTrader: (2)

Actually the 1.6 turbo made 132hp 136 ft.-lbs.

The BG GTX (1990-1995 style) had a 170 or 180hp 1.8L Turbo BP (same as the newer Miatae). Maybe thats the one you're thinking about.

ronbros3

transmissions should be looked at by there torque rating, my take, a miata trans cannot take the torque of a turbo 13B, for very long. you might be better off with an automatic, and you cant miss a gear. Ron

psychotic7

Well I will tell you this....a 13B can make tons more power than a Miata engine. There are people on here that have 500+ whp on the 13B and thats on the street. Drag RX-7s make WAY more than that....well, put it this way, in Australia there is a 2 rotor that is running a 7 sec 1/4 mile. Why not just buy a RX-7 and build it up while you drive your Miata?

iluvtruenos

Right, for some reason I was thinking of the 1.8. My bad. Over the last week or so I've been massively researching, and I think this is just too difficult of an idea. I think that I'll just try to build up the Miata 4 banger.


offtopic: A 1.6 front clip of a GT-X is about $1k shipped. Worth the hassle of swapping over some stuff for a turbo Miata?

C. Ludwig

iTrader: (27)

No. If you're going to do that just go with a 1.8. They're plenty boost friendly in stock form and the small amount of extra displacement makes a world of difference in low end torque.