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Alternate auto trans for FCs...

Old 01-15-19, 09:38 PM
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Alternate auto trans for FCs...

Threw this out in a different thread and didn't get too many thoughts, so I thought I'd try it in a different area of the forum.

I need to stay with an auto trans for a variety of reasons that kinda stink but here we are. I've got to think there's better auto trans options out there for a 13B than the 4 speed auto my 91 'vert came with back in the day. I'd like to find something fairly light and that can handle a higher rev limit. Here's a few I was considering...
  • 6AT from a more recent Miata
  • 10-speed from the current Ford/GM tie up (Kinda large?)
  • ZF 8-speed (not exactly light)
  • Nissan 7-speed auto
  • Subaru 6-speed auto (from the BRZ/FT-86/FR-S)
I guess there's some sequential manual trans options out there, but I don't know too much about them. Most are setup for drag racing (Jerico?).

Any other ideas? Anyone tried any of these?

All thoughts welcome!
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Old 01-16-19, 06:41 AM
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The 4N71 is actally a pretty nice unit. There used to be shift kits and such available.

You will not find a very high rev capable automatic, the issue is the torque converter. They tend to balloon at high RPM, so they are the limiting factor. You're not going to get away from that with any transmission, from any manufacturer.

Strength-wise, the 4N71 is a 3N71 with an overdrive stuck in the middle, and people have run 500-600hp drag engines in front of largely stock 3N71s. Besides everything rotary, this series of transmission was used in 280ZX Turbos and Starion turbos, basically anywhere a high torque capable automatic was needed.
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Old 01-16-19, 02:13 PM
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Thanks peejay! I thought about the Miata unit in particular for the RPM capability. What I didn't know was that the T/C was the real limiting factor. Now they you mention it, it makes sense...that's a lot of spinning around potentially ballooning the T/C.

Maybe a shift kit is what I'm looking for...I'd really like to tighten up the shifts and spin the input faster if I could. Maybe add a higher stall t/c?

Last edited by WobblyBobbly; 01-16-19 at 02:18 PM.
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Old 01-16-19, 06:20 PM
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Originally Posted by WobblyBobbly View Post
Thanks peejay! I thought about the Miata unit in particular for the RPM capability. What I didn't know was that the T/C was the real limiting factor. Now they you mention it, it makes sense...that's a lot of spinning around potentially ballooning the T/C.

Maybe a shift kit is what I'm looking for...I'd really like to tighten up the shifts and spin the input faster if I could. Maybe add a higher stall t/c?
This has actually been something I've been looking into. (I have a pair of FB 3N71s on the shelf now, and my left knee isn't getting any better as I get older, and constantly shifting in and out of gear with the bridge port engines for the past 9-10 years has not been kind to it) I have yet to get down to the point of actually buying parts to investigate, but it APPEARS that the electronic-controlled automatic used in the S5 FC and NA Miatas is just the good ol' 4N71 with a different valve body. So it may be possible that a 1.6 Miata converter could be an upgrade for an RX-7. Stall speed is a funny thing and it is dependent on engine torque. More torque means the stall speed is higher.

Which makes things kinda funny when you send a converter in to be restalled, and they make some wrong assumptions about your "300hp, 300ft-lb turbo 2.5l", and you get back something that ends up with a stall speed about 700rpm higher than you wanted. They weren't expecting something that made 300ft-lb at 1900rpm, you see. Oh well, it has a lockup clutch so it's not too bad. (I should probably change my signature... I think I drove this car more in the past two weeks than I drove the RX-7 all last year)
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Old 01-16-19, 07:31 PM
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Agree 100% and t/c's are weird. The ones I worked on are kinda different. The biggest one is about 7 feet across, holds 80 gallons of fluid in the sump, and backs up a 4000 HP engine on a mining truck. All the same principles though!

So for an NA 13B in an FC, we're looking at 140 lb-ft at 3500RPM or so right? So if we set the stall up that high, it will probably slip like a greased goose down low. Not so good.

Reading up on the 4N71B's today, it seems like the shift kit isn't too hard to install. Might get farther with that.

Sequential manual is an interesting option, and apparently these guys have hooked up to a rotary before. Not sure how many $$$ it would take though.
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Old 01-16-19, 07:31 PM
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I see the late model RX-8 6 speed auto that enabled the 6 port 9,000rpm redline to be retained going for under $500 on ebay.

Wouldn't that be a slam dunk for an NA 13B auto application?

For a turbo application that has to handle torque, you should be able to pick up a Cosmo 20B transmission for next to nothing since no one uses them on 20B swaps. I think this is the same beast that is in the turbo Z31 300Z and Pathfinder- they are good for serious torque with some easy mods.
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Old 01-16-19, 08:07 PM
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Originally Posted by BLUE TII View Post
I see the late model RX-8 6 speed auto that enabled the 6 port 9,000rpm redline to be retained going for under $500 on ebay..
I thought the RX-8 auto downrated the RPM to like 7000? IIRC they tried to have it take the full 9000 but hen gave up on it later. Been a while since I read that somewhere, so I could be off on that.

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Old 01-16-19, 08:53 PM
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Originally Posted by BLUE TII View Post
I see the late model RX-8 6 speed auto that enabled the 6 port 9,000rpm redline to be retained going for under $500 on ebay.

Wouldn't that be a slam dunk for an NA 13B auto application?
....Interesting. Very interesting. I wonder if it is a clutch-to-clutch style transmission, or if it is something that MegaShift could control. I know it appeats that MS can't control the Ford 5L45 for hardware reasons. Off to Google!

... No easily found info on what trans it is, other than it was used starting in 2006, and '06-up autos had 6-port engines. Changing tack, JATCO did not make a 6 speed auto so that is out. Aisin-Warner made several 6 speed autos for rear drive* but I can't find info on which one, if any, the RX-8 used. Mazda's own PDF brochure says that the automatics were redlined at 7500, even with the 6 speed, so I'm going to not search too hard anymore. Maybe I can find more info tomorrow at work.

* - My car has a transverse 6 speed Aisin trans. Same unit is used in the Mazda6, I think. It's a clutch-to-clutch trans if I read the docs right. Even so, it would not be conducive to aftermarket controls because the TCM is kind of part of the trans, not a separate unit. You COULD remove it, I guess, to control the trans with something else, but you'd have to either make a connector, or hack up a TCM to turn it into one.
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Old 01-16-19, 11:49 PM
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D:

I just learned that even though the 2006 automatic RX-8 went to 6 speed auto and 6 port induction the redline is still 7,500rpm to protect the torque converter.

Sorry for my mistake of thinking they had raised the automatic redline when they changed the engine to 6 port on the newer 6 speed auto.
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Old 01-17-19, 12:10 AM
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One member on rx8club claims he has remapped his ECU(s) to 8,500rpm redline on his auto RX-8 and RE Amemiya can remap it to 8,500rpm or 9,000rpm- but recommends changing ATF and putting in a transmission fluid cooler.

Post# 13

https://www.rx8club.com/series-i-spe...it-auto-97890/

After that others in the thread chime in on their higher rpm shifting the autos.
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Old 01-17-19, 10:34 AM
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our database provider says the 1990-1997 Miata uses a L4N71B transmission, the 99+ cars use an Aisin Warner 03-72LE

the 6 speed Rx8, and NC/ND miatae use an SJ6A-EL, it says its a Mazda unit, and it says its a transaxle.

our database provider is not, well they have a lot of room for improvement.
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