If you end up getting an S15 box and want to sell the 6th gear let me know. It sounds great to me and I haven't been able to find the gear online.

 

LOL same thing I said above in post 137.

 

And the same thing I discussed with Bob on the phone after I made post #111.

 

Have you put a lot of research into the s15 gearbox?

As I said earlier in this thread its not uncommon for s15 owners to change it out for the 5 speed s13/s14 sr20 gearbox once they break a few 6 speeds. They've been broken at stock power, though that probably has more to do with abuse than power. I don't know **** about gearboxes, but could you rebuild the miata and use the 4th gear from a broken s15 gearbox?

 

Took apart two transmissions tonight. Looks like I can make one that works using a combination of parts from both fairly cheap. One had a thrust failure on the secondary shaft. Front bearing is toast including the shaft. Forth gear on looks fine but it tried to thrust and failed a snap ring and thrust ring broke the ends of the reverse gear teeth on the secondary shaft. Second transmission sheared all the teeth off 4th gear but the secondary shaft looks fine. All the syncros and everything look perfect.

It is worth noting 4th gear ha smaller teeth than the other gears.

 

Did you see any unusual wear or damage that may have caused the 4th gear grenade besides the smaller teeth? The Frankenstein transmission would be fine to drive to work over the winter but it would die at a miserable time next season - like Saturday morning at Laguna.

I guess you may as well blow it up too. Nobody on this forum would give $50 for a used transmission that has been in your car!

 

Half of 4th gear is machined directly on the main shaft. The only gear that can be swapped independently of the rest of the gear set is sixth because both gears of sixth are pressed on the end of the shaft. So you pretty much need all the gears out of the S15. But I plan on running a Miata 5th.

I am still holding to the theory that the Miata has the weakest 4th gear in the AZ6 family of transmissions. It is the weakest because of the ratio selected results in small gear teeth. Miata has 36 and 35 teeth on the fourth gear combo and the S15 has 35 and 34 with the same shaft spacing so the teeth have to have a bigger base to their profile. Small difference about 2.8% but if you know anything about metal fatigue small differences can make a significant difference in fatigue life. The next step up that meets my criteria for gear ratios and weight with a higher strength cost 10 times as much. So Im still taking baby steps. S15 box doesn’t really cost more than another Miata six speed to try out.

S15 uses a 28/33 on the primary and Miata uses a 27/33 so slightly less engine torque is transferred through the secondary shaft. And the input shaft won’t interchange either. Input splines I believe are 1”X24 rather than 15/16 X 22 like a Miata or 1” X 23 like Rx7/8. So custom clutch disk will be required as well.

 

I figure I need a spare on hand anyway.

 

I hate you guys.

If someone can dig up a Nissan part number for the gearset, and it's reasonably priced, I wouldn't mind being the guinnea pig. My motor is being assembled so the transmission is out anyway, it'd be a good time to swap 6th out.

 

This site has some good info. They have the part number for the Nismo 6th gears. Some more information I have gleaned is the S15 box uses a a different primary gear ratio so slightly less torque gets transmitted through the secondary shaft while it spins faster and there are fewer teeth on 3rd and 4th gear sets so the teeth are beefier. Engineering theory says the box will handle a tad more torque than the Miata version. Its still not enough but it is better. If I can get one or two seasons out of it instead of less than one season it would be worth it.

S15 6 Speed Transmission for Dummies - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

 

If the secondary shaft turns faster the pair for 6th will not have the same ratio when moved to a Miata box (they may even be identical).
Drive 28-33 teeth
6th 28-43 teeth
The 1.179*0.651 gives 0.767. What's the Primary gears in the Miata AZ6 (i.e. what would the final ratio be)?

 

My plan was to try the .767 on the street car and swap the 4.1 gears from the track car with the 3.63. By my math, the 3.63/.843 would be <2mph taller than the 4.1/.767 combo at 70mph. It would be awful on a track car but looks like it would provide a reasonable overdrive for the street car at mostly legal freeway speeds. /threadjack...

 

The Miata 6th gears in the S15 box will yield a .813 sixth. Tad faster than in a Miata. I might shop for the NISMO 6th which yields a .862. Stock Miata transmission works out to .843. For my purpose I don’t need a faster gear than the stock Miata transmission. Not looking for top speed or a highway cruising gear. 160mph is fast enough. Tight even ratios that progressively get slightly tighter as the speed increases is what I’m looking for. low gears can use more rev range high gears stay tight on peak power point of the rev range between shifts.

S15 6th gear in a Miata would yield a .795 sixth instead of the .767 of the S15.

 

Counting teeth.

By tooth count and gear design for those tooth counts The S15 should be stronger in all gears. especially second gear. probably 10 % or more stronger in 4th. Interesting that most published data posts truncated numbers instead of round off numbers. probably done by marketing instead of engineers.

 

FRS/BRZ
3.626
2.188
1.541
1.213
1
0.767

Interesting the FRS/BRZ has the same gear ratios as the S15 but a much taller second. Gee I wonder if engineers were thinking of building a dominant stock class autocross car with that selection, very good choice I think if they were. It’s AN AZ6 box but pictures look like the center casting may have been updated. I don’t think it is the exact same casting but might bolt up to a Miata front and back sections. They are not cheap yet either.

 

Interesting, there are a few 400+whp FRSs, but I doubt they have been abused on track like you do yet.

 

Freekin awesome looking frankenbox idea.

CUSCO first and second in an FRS box with Miata Sixth gear and Miata front and back portions of the housing. run it with a 3.909. autocross and track gearing awesomeness in one. In theory it should be a bit stronger than the Miata box.

FRS Mod &mdash; Cusco Close Gear Ratio Transmission Set for FRS BRZ

 

Looking at that chart 2nd appears about 10mph too slow for Autocross use.

 

Would an 8500 rpm redline help?

 

Not with an 8500 rpm rev limit that gets you to 74 mph. And it would be right in the powerband out of slow and mid speed corners. This is how you shift it for autocross.