Howdy folks! I'm going to keep this one short and sweet: I'm looking for the absolute strongest automatic transaxle that will:

A: fit in an 88 Formula
B: Bolt up to an SBC and still fit in the Fiero.

AFR (the head company) is telling me to expect 600HP at the crank out of this engine. I need something that will hold to that kind of power at a max of 6500RPM. Not just looking for budget options, need something that's going to handle the abuse.

Thanks in advance for any advice you have!

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- Darryl

If you mean bolting to an SBC without adapters, there are none to my knowledge.

So, assuming you are open to using an adapter plate: The TH425 would be the strongest. That will mean custom motor and trans mounts, and custom axles. It is a longitudinal FWD transaxle based on the TH400, they have been used in several big block Fieros. You will lose some or all of your trunk.

If you want to stick with a transverse mount engine, the 4T80E would be the strongest. That will need an adapter, custom mounts, custom axles (I believe, I haven't really looked into a 4T80 swap. But stock axles certainly won't work.), and a trans controller (either stand alone, or integrated with your ECM). I know that on the N*/4T80E swaps the cradle needs to be modified, but I don't recall if that is for the trans or the engine.

I haven't paid much attention to the newer 6+ speed transaxles from GM, so I'm not sure exactly where they stack up against the 80E as far as strength.

Thanks for the response Jimmy! Definitely want to stay transverse. 4T80 was one of the most logical conclusions to me, however the concern of whether or not it'll hold is definitely a strong one!

I wonder if the 9T65 would work? Those are mated to the 3.6L v6 which is rated for 310HP and 268 lb*ft. Barely larger and hardly heavier than the 6TXX transmissions.

The 4T80 is what is most commonly used with the turbo LS4s pushing 800+hp. You will need an adapter plate to run it as well as a transmission controller.

That's what I like to hear FieroGuru! Stock 4t80s or are there quality rebuild kits available?

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- Darryl

A lot of the high HP 3800 supercharged builds use a built 4T65HD which is lighter than the 4T80E.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

Hey Sanderson! I did a ton of reading on the 4t65s last night. I also came to the same conclusion but Triple Edge Performance informed me this morning that the parts are no longer available to build them that strong. Any idea what parts are needed? I read that a 1" chain is needed and found a company reproducing them. Pricey, but with the money I'm putting into this engine, I'm not going to cheap out on the transaxle.

Lighter is better for me. The entire engine build is designed from the block up to be as lightweight as possible. Hoping for an engine that comes in at about the same weight as the all cast OEM 2.8L with over 4X the crank HP

quote Originally posted by Reborn756: Hey Sanderson! I did a ton of reading on the 4t65s last night. I also came to the same conclusion but Triple Edge Performance informed me this morning that the parts are no longer available to build them that strong. Any idea what parts are needed? I read that a 1" chain is needed and found a company reproducing them. Pricey, but with the money I'm putting into this engine, I'm not going to cheap out on the transaxle. Lighter is better for me. The entire engine build is designed from the block up to be as lightweight as possible. Hoping for an engine that comes in at about the same weight as the all cast OEM 2.8L with over 4X the crank HP

Let me see if I can find the parts list from when Triple Edge Performance built my trans.
4T80would be a good choice if you can't find the parts for the 4T65E

Thanks Tampa! Would prefer 4t65 to meet my weight goals, even if it costs more than a 4T80 setup. 4t65e-HD is only 40 lbs more than the TH125C when both have their converters from some rough numbers I saw. I can easily make up that 40LB weight increase by moving the battery to the front and making some modifications to the exhaust system, along with the weight savings I should see with the new engine.

4T65E-HPS4
High Performance Street/Strip Transmission Rebuild
Upgraded Input Shaft
OS 3rd Piston
Stage 2 Valve Body
Custom Precision Industries Torque Converter
2,800 rpm stall speed
Milzy Motorsports 1" chain set
3.29 Gear Ratio

[This message has been edited by tampalinc (edited 03-19-2023).]

Thanks for that info Tampa! How much power are you putting through that transaxle?

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- Darryl

If you decide to explore the 4T80 option, I got mine through Triple Edge Performance. It took much longer to get than originally planned, but he also did the footwork to get the custom torque converter for me that used an M300 input shaft from Precision industries for me. Using an adapter plate it would bolt up to my SBC, though required cutting about an inch long piece of metal off the SBC block where the stock starter location would be.

While Sonnax lists parts of the 4T80E that they categorize as "Heavy Duty", I reached out to them and asked if they are stronger than stock parts. They told me no, but the 4T80 has some parts that can be used in other transmissions and that's where it's an HD upgrade.

LS4King (Brandon Furches) has not had any issues with his second 4T80. He found out that 1-2 shift was too "long" and it burned up his first transmission. He modded the trans valve body so the 1-2 shift is pretty firm/fast now, and hasn't had any more issues (not a line pressure mod, something about flipping a piston in the transmission valve body, I don't recall the details right now). That guy competitively does 1/4 mile racing with his Monte Carlo, and is pushing 700+ horsepower and torque.

It is a heavy transmission, around 255 pounds (I think that's without fluid). The 0411PCM does have stock flashes out there that will operate the 4T80 with an SBC (it'll think it's a 4L80. They have the same solenoid 'pattern' for all the gears), as long as your SBC is setup to think it's an LS (digital distributor, coil packs, that kind of stuff. FieroGuru did that conversion on my SBC).

The guy currently doing all the involved work on my car did not get to figuring out a starter mounting solution for the 4T80/SBC combo before I switched up my engine to the LS4.

So there are downsides to the 4T80, a little more complexity (and less documentation - most people are putting them on LS4), a little heavier, and probably(?) a little more costly, but also seems to have a much better track record at taking abuse than the 4T65. I think one of the big deciding factors will be the costs between the 4T80, and getting the heavy duty build-up 4T65. 4T65s have more documentation out there on a variety of platforms, so most of the technical issues are solved for, depending on your fabricating skills or wallet.

Or, if you really want to get experimental, as FieroGuru told me years ago "The most interesting would be the 6T75". However, I replied the same way you did "If the 4T80 is stronger, that's what I'm going with!" lol