If you are a fan of Japanese performance cars from the late 90’s and onwards, chances are you would have rowed some gears on a variant of these gearboxes which are variants of a transmission platform made by Aisin. Manufacturers like Toyota, Nissan and Mazda took the basic platform and tweaked them for their specific cars. Some parts are interchangeable between them, and some are not. 

Variants include:

Nissan FS6R31A - VQ35

Nissan FS6R92A - VQ35

Nissan FS6R92A - SR20

Mazda Y16M-D (F&B) - NB8C 

Mazda Renesis - 13B-REW 

Toyota J160 - SXE10 

Toyota TL70 - FA20 

Toyota TL70 - FA24

I have grouped the gearboxes into the groups that are most related, and I have created tables that show the gear ratios (apologies, the Nissan gearboxes are mislabeled Toyota):

The Mazda Y16M-D gearbox in the NB MX-5 is a true close-ratio gearbox, likely developed extensively by Mazda specifically for the NB roadster due to its lack of parts interchangeability. It shares its design with the RX-8 gearbox, though the 3rd and 4th gear ratios are slightly different for unknown reasons.

The gearsets of the Miata transmission and the S15/FRS/Altezza gearboxes are not interchangeable due to a thickness difference, indicating Mazda’s unique development path. Unverified forum posts claim that the RX-8 bell-housing can fit onto the S15 gearbox. Notably, the 6th gears of the S15 and SXE10 are interchangeable. The Miataturbo community has attempted to develop thicker and stronger gearsets, but the current setup cannot accommodate significantly larger gears due to spatial constraints.

One of the drawbacks of the Y16M-D gearbox is that its shift forks are made of aluminum. When the gearbox heats up, the expansion of the aluminum causes gear shifting to become erratic. However, once the oil cools down, shifting returns to normal. To fix this issue, you can install a transmission cooler by using the fill plug and drain plug as the inlet and outlet. Additionally, incorporating an oil cooler and a temperature sensor will help regulate the transmission temperature more effectively.

The TL70 in the ZX6 is a newly developed version of the AZ6, sharing most of its architecture with older versions. Although the gear and synchro setup between the TL70, J160, and FS6R92A are largely the same, the external cases vary significantly. The TL70 has a different shift pattern, placing reverse beside 1st instead of beside 6th, necessitating considerable revisions to the internal shift linkage.

Toyota, Subaru, and Aisin AI engineers collaborated with Takeshi Kaino, a researcher specializing in shift feel, to develop the ideal 6-speed manual transmission. Tomohiro Ishikawa, the 6-speed manual transmission development director, shared, “Perhaps 80% of the design had changed. More than that, if you look at blueprints for individual parts. Most of the parts were changed, I bet.”

The TL70 shares a similar style case to the J160, both utilizing remote-mount shifters, while the FS6R92A has the shifter mounted directly on the tailshaft housing. The TL70 shares gear ratios with the FS6R92A, whereas the J160 has a taller 1st gear and a shorter 6th gear. The TL70 and FS6R92A have a shorter 1st gear and a taller 6th gear, likely selected for better fuel mileage.

The Toyota 86 was initially launched with model code ZN6 and it's mid-cycle refresh emerged as the ZN8. In this new version, the gearbox was significantly revised. The model code is TL701V8ACA. The Toyota process of constant improvement is evident as beyond the obvious strengthening of the transmission to handle the extra torque of the large displacement engine, there are several other changes made.

1. The 3/4/6-speed fork has been upgraded 

2. The 4th gear now features a carbon synchro. 

3. There has been many small changes to improve shift feel and durability. 

4. The 6th gear has a new spline pattern 

5. Shift bushing clearences are tighter.

Here is some information on the evolution of the gearbox up till 2024:

On the transmissions older than the Toyota TL-70, there is a well known modification that significantly improves torque capacity (from approx. 220nm to 500nm) called the Circlip modification. This is a two part thrust washer that stock is only cut part of the way into the gear teeth and under high load they chip the edge of the teeth leading to inevitable transmission failure. Cutting the slot deeper and installing a new custom thrust washer resolves these issues. The shafts must be sent to a specality shop in order to be machined correctly to suit larger and stronger circlips. 

A special note is to be made that contrary to some sources, the Honda S2000 transmission attached to the F20C and the F20C is not an AZ6 variant. It is a fundementally different gearbox altogether. 

Here is the source directly from Honda's press release: "With the engine already moved as far back in the chassis as possible, space for the transmission was at a premium, so Honda engineers designed the transmission to be as narrow as possible. All six speeds and reverse are on two parallel shafts. Both transmission shafts are coupled at the output end, a design feature borrowed from Honda's front-wheel-drive transmissions. This reduces the load on the gear synchronisers by as much as 40 per cent, which, in turn, allowed the gears to be smaller."

"Shift feel has always been a focus at Honda," Civic Type R chief engineer Hideki Kakinuma told Road & Track magazine over email. That leads us to the technical answer to the question—Honda puts a lot of work into all the components that go into a transmission.

"Everything that they've done has just been top notch, from the shift handle all the way to the gearbox, and that makes everything," says Chris Weismann of Traction Products, a longtime racing-gearbox design firm. "You can have a great shifting gearbox, but if the linkage to the gearbox doesn't work, you're not going to get a good feel. That's part of the equation too."He adds that synchronizers are especially important for achieving great shift feel, too, and Honda's are particularly great, benefitting from great attention-to-detail, high manufacturing standards, and years of continuous refinement. "All the rough edges are not there, all the fits between mating parts are correct and well within their high standards of tolerance," Weismann says. "Basically, the quality control of the finished product is excellent, and so whatever [Honda makes] functions better when all working together."

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References:

https://www.sq-engineering.com/tech-articles/j160-conversion-and-modification-guide/

https://www.club4ag.com/forums/viewtopic.php?t=21510

https://www.sq-engineering.com/product/j160-gearbox-shifter-relocation-kit/

https://hondanews.eu/eu/lt/cars/media/pressreleases/34329/honda-s2000-99

https://www.roadandtrack.com/car-culture/a42029867/why-are-honda-shifters-so-good/