T50 to AZ6... a "new" option for gearbox issues

[oldeskewltoy]

A brief overview... the AE86 GTS is going on 20 years old. It is rear wheel drive. It was originally equipped with the 4AGE twin cam 4 cylinder, a 5 speed transmission, and a solid rear axle. In recent years the AE86 has garnered "collectors status" as well as becoming a cult "drifter". The AE86's popularity is VERY strong!

The 5 speed transmission is known as the T50. The 5 speed T50 evolved from the earlier T40 4 speed. The T40/T50 was originally designed for engines producing 40 to 100 hp. The stock power of the AE86 4AGE is 112 horsepower. The T50 in stock form tends to have 2nd gear difficulties as it has aged. Cost to rebuild a STOCK T50 (complete rebuild) are quite expensive, parts can cost $900+ and the labor would add another $450. There are TRD performance gearsets for the T50, they would add somewhere between $1200 and $1900 to the rebuild cost. The TRD gear set is more substantial but overall the T50 remains fragile.

Besides its fragility, the stock gearing is VERY poor. So poor that when it was originally tested by Road & Track in February 1985, they commented,

Quote:The GT-S was a fraction slower... ...there were times and places when the engines power couldn't be used... ...the jump from 2nd to 3rd is too wide and the jump from 4th to 5th isn't wide enough

adding a TRD gear set improves the gear spacing, but because the TRD gear set's 1st is so much tighter (2.6 or 2.3 to 1 as opposed to 3.5 to 1) it makes use on the street MUCH tougher.

Move ahead in time 15 years... The Miata is introduced with the AZ6 6 speed transmission. In essences the AZ6 is a close ratio 5 speed(gears 2 through 6), with a taller 1st gear. This in essence gives you the advantage of a close ratio transmission WITHOUT sacrificing the ease of use that a tall 1st gear offers. The AZ6 was originally designed for the Altezza, the Miata and the RX8, its power capability is well over 250 horsepower. The AZ6 is capable of running just about ANY type 4AG - n/a, s/c, turbo. The gearing is just about perfect for the 4AGE AE86 - placing redline shifts at or above 4500 rpm(TVIS engagement) for each successive gear


The problems to overcome:
1) Toyota gearbox is a self contained unit, the bell housing isn't intregal to sealing the gearbox itself, the Miata bell housing is also the front of the gear box case. This is the biggest problem to overcome. Doing research I've found that this isn't an unknown swap (AZ6 into AE86) but it is VERY RARE, but it has been done a handfull of times, but in all previous cases the Toyota bell was welded to the AZ6 bell and a clutch mixture has been used. The problem with this is the engineering involved with the clutch working range was never taken into consideration. The reliability of this is unknown. "We"* decided that the most reliable way to do this was to use the entire Toyota clutch... complete bellhousing, hydralics, disc and pressure plate, all Toyotas engineering. To do this we needed to cast an adapter. This adapter would closee up the AZ6 gear box, support the output shaft bearing, and have a flange to bolt on the Toyota bell housing.
2) Since we decided to use an adapter, the input shaft becomes too short. Also, to use the Toyota clutch, the splines are wrong anyway. The Toyota input shaft will be grafted on to the Miata input shaft. this will extend the length needed, and fit the Toyota clutch
3) The AZ6 is physically larger than the T50, a bit wider(no so wide to be a concern) and quite a bit longer. The length on the AZ6 will be compensated by shortening the stock 2 piece drive shaft. The front section will be shortened and the AZ6 output shaft will be added. This will keep the driveshaft support bearing in the stock place, thus keeping the driveshaft to axle angles all intact from the stock driveline.
4) As stated above the AZ6 is larger, one of the issues in this swap is the shifter would come through the floor in the AE86 about 9" behind where it does originally. Now if this was a racing example, this would not be an issue, just move the parking brake and cut a hole, BUT I don't want to do that, so the shifter section is going to be shortened so the shifter will come through the floor in the correct location. This will require shortening the top of the AZ6 case and shortening all the shift linkage.
5) The AZ6 as fitted to the Miata does NOT have a rear transmount as is conventionally known... the AZ6 in the Miata has a link to the rear this link holds up the back of the AZ6. This link section needs to be modified to hold a conventional cross type transmission mount.
6) The last problem to overcome is speedometer drive. The Miata uses a digital signal, the AE86 uses a mechanical cable. For now... we are looking at fitting a mechanical drive to the AZ6.

Now.... there are a lot of VERY intricate details that seem to be "over engineered" for a one of a kind swap. Now some of this is because I want a fully useable car when I'm done, but there is a second larger over riding purpose... go back... look above....

T50 remains fragile AND The AZ6 is capable of running just about ANY type 4AG - n/a, s/c, turbo. soooo.....

Rob and I decided that most of the T50s are now junk, and for nearly the same cost as a rebuilt T50 with a TRD gear set you can have an AZ6! The final costs will be set once the engineering is done, but as of now the total cost for a complete AE86 AZ6 swap would come in UNDER $3000 That might seem like a lot, but once you compare it to rebuilding an inferior trans...(or purchasing a Quaife for $6g!) it makes PERFECT sense!

* - "We" are Rob Golden owner of Pineapple Racing http://www.pineappleracing.com/ - renown Mazda builder....
and me

Why.... bet that is running through some brains.... simple really... T50 sucks. W box is fine... but could used better ratios... AND one more gear(for use with a 4AGE). In essence(for 4AGE use) look at the AZ6 as a 6 speed W box.....
But why?.... well, the one part of the puzzle that isn't there... is the addition (in my case) of a 4.10 Richmond Racing rear diff. The slightly taller 1st gear of the AZ6, matched to the 4.10 offers me nearly identical starts.... as the T50 and the 4.30... and between the 4.10 and the .843 6th gear I should lower high rpm by over 700... which I'm expecting a 10% boost in mileage...


Ratios for those that want them.......
Original Transmission T50

gear ratio
1st 3.587
2nd 2.022
3rd 1.384
4th 1.000
5th 0.861

Final Gear Ratio
4.300

TRD set#1 For T50

1st 2.630
2nd 1.891
3rd 1.384
4th 1.000
5th 0.861

TRD set#2 For T50

1st 2.341
2nd 1.607
3rd 1.195
4th 1.000
5th 0.886


AZ6 from 1999-05 MX5

1st 3.760
2nd 2.269
3rd 1.645
4th 1.257
5th 1.000
6th 0.843

The pattern - what the casting will look like once poured