Savington

iTrader: (31)

You are the anecdote I was referring to. I'm just nervous about grenading a $3k gearbox. I'd much rather sell it before blowing it up and step into a $4-5k option that I know is bulletproof.

Just FYI, the S15 Silvia guys have the same reliability issues we have at similar power levels. The gearboxes are also expensive.

Savington

iTrader: (31)

TII ratios:

3.483:1
2.015:1
1.391:1
1:1
.762:1

Those ratios are worse than the T5 ratios. Totally unacceptable for a track car IMO.

muoto

Another broken 5-R shift fork checking in.

I’m surprised that no one here had taken apart these gearboxes and swap broken parts? It can’t be that hard, can it?

theshdwconspracy

iTrader: (1)

For some one that isn't buying a $4k transmission and is frequently breaking 6 speeds I don't think that is an unacceptable solution.

bbundy

Not sure I like the gearing plus I don't like all the funky adapters and special parts required, movement of shifter location, and I think it all weighs at least 10 lbs more. My big complaint with the 6 speed gearing is I couldn't go fast enough in second for Autocross use. I never use second on any track however and I cured the second gear issue by turning up the rev limit to 8500 rpm and suddenly the gearing was pretty awesome almost always in the power band and can just rev it out on the long autocross straits past peak power but it still pulls.

theshdwconspracy

iTrader: (1)

Funky adapters? 1 plate that bolts to the motor and you buy an Nc clutch disc for your current clutch setup, a new driveshaft and a trans bracket. It's not that much work for a transmission you won't break.

You're going to do what you like and I'm not arguing that this is the best thing on the planet but I'm just explaining to you and the other people reading this thread that it isn't that hard or a bad option. Obviously you're very particular about your transmission suiting your needs. Best of luck in your quest.

bbundy

NISMO Reinforced 6 speed Claims gear ratio selection and design allowed bigger more robust gears to be fit into the box. Again I’m still betting 6 speed Miata front belhousing casting and tail section casting and shift linkage can bolt up to any of the AZ6 AISIN family boxes. The design allowable engine torque for the NISMO box is listed at 289.3 ft-lbs. pretty close to what I am putting to the ground but I’m sure it is significantly Higher than The Miata Variant. gear ratios are kind of a tight ratio 6 speed and it doesn't have the stupid low first and second. probably would like a 3.909 instead of a 3.636 maybe.
NISMO | Silvia•180SX Reinforced Cross 6-Speed Transmission

Not a cheap solution.
http://www.frsport.com/Nismo-32010-R...5_p_26092.html

bbundy

6 speed with 3.636 and 5 speed with 3.909: Both of these work for my purposes in terms of gearing. 5speed has better second gear for autocross. 6 speed might work better on the track with 3.909 but produces an un-usable second gear for autocross. 4th gear is a weak link in the 6 speed Miata box.


RX7 TII 4:10: Sucks for gearing worse than all others. Huge gaps in 80 mph and up range. A 4:30 would help a tiny bit for track use but would make second unusable for autocross. I would be unhappy with this.


RX8 3.636 : interesting they changed only 4th gear from the Miata Box and the change makes no sense from a gearing stand point. Id be willing to bet this change was made entirely to address weak fatigue strength in 4th gear!



S15 box with Miata 6th: With this I would swap 6th with miata gears which is easy. The stock 6th is way tall and probably for fuel economy not good for using it in anger. The result is a slightly tighter 6 speed slightly more useful second. One could only hope that the 4th gear is stronger than the Miata version. seems eqivelent Pricing to Miata 6speed.

bbundy

Quafe wide 4:10: Decent track gearing second is actually too tall for
autocross use.



NISMO 3.909: Pehaps the ultimate of what I have looked at. Would take a few more revs in 6th on pacific but everything is better. NISMO claims Design torque for gears is 30%higher. In terms of fatigue life that is huge. Wish I had ~$6500 to blow on it.

bbundy

Interesting find FRS/BRZ has the same gear ratios as the S15 and Cusco sells this to make first and second gear taller.
2013-2014 Scion FRS 6spd MT Close Gear Ratio Transmission Gear Set -

mr_hyde

iTrader: (4)

Late to the party here but I'd be interested to see one of the blown 4th gear boxes taken apart to see if it gives any hints about what is happening. My theory is you have an unusual car used in an unusual combination of events so it will take an unusual solution like those being discussed.

How many other cars are at ~350/350whp/wtq (you didn't turn yours down until recently), do autocross launches on 275s with a co-driver, and run >10 seconds below Spec records at ~2,500 pounds with driver on an unsprung clutch? That clutch puts a huge amount of stress directly on the gearset.

I picture the all the on/off power transitions and launching of the autocross season fatiguing parts of the transmission that don't get let go in 2nd gear but can't take it months later when you are blasting out of a corner or down the straight in 4th gear at peak torque. Other high power guys are consuming 6 speeds also but none at the same intervals as an oil change. I'm stating the obvious, but that transmission was fatally wounded already to blow up under 300wtq like it did.

Do you think you are going to keep the ~14 psi lower power level next season? I wonder how much of the extra 60whp is useable for autocross and if going to a sprung clutch disk would help the problem. I know they get slaughtered launching with 275 A6s but disks are cheaper than transmissions and basically the same time to replace.

Just throwing ideas out. I have a 4.10 R&P if you come to a solution that needs a new final gear and I'm sure I can help you dispose of that 3.63 set while we're doing the swap!

bbundy

I dynoed the car and tuned it with the intention to run 17-18 lbs of boost. Data logs from the track when I broke the one at Laguna showed I wasn't going over 15 psi All weekend. The gage on my dash is 3 to 4 psi optimistic. Re- dynode the car to make sure I was under 331hp to run TT2 and with the boost between 14-15 it was making ~325 hp ~285 ft-lbs Dynojet SAE corrected. The last two transmissions broke 4th gear at that power level on the track. I like running 14-15 lbs of boost on the track, the car still goes like stink and it is more reliable now than it has been in the last 12 years of my tracking a turbo Miata at that boost level.

Autocross rarely sees 3rd gear almost always in second. 8400 rpm rev limit required. I did quite a lot of it this year though. Won national tour events Pro Solo and even won the Canadian national championship. Had a co-driver for all of it as well. So yea a shitload of abuse in first and second gear was inflicted. nothing done to 4th however.

From what I have been reading though I think the big problem is running around 35% more torque than a S15 AZ6 was designed for and I suspect the Miata version is even weaker.

mr_hyde

iTrader: (4)

The car certainly goes like stink. Since your car is built and tuned for ludicrous power, it will probably be pretty reliable at your TT2 power levels... except the transmission of course! Could you run a sprung clutch at ~325/285?

DeerHunter

Bob, I've found the Quaife gearing quite salubrious for autocrossing. Now, our autocross courses might have been a little more wide open than usual, but once out of first, I never have to shift again (even coming out of a tight Chicago box). My redline is 7,700 and I have a 4.10 R&P.

TurboTim

iTrader: (9)

If the miata 5 speed would work, then so should the T5z with a .8x 5th.

sixshooter

iTrader: (12)

I have thought of this as well. Since piston engines don't generate power smoothly, but rather in a series of violent pulses like an air-impact ratchet, it would follow that removing the compliance and impulse smoothing would lead to higher momentary forces. An unsprung clutch, hard motor mounts, hard diff mounts, and, to a lesser degree, hard rear suspension bushings could contribute to the intensity of each impact.

Example: Lay a wheel bearing race on an anvil and strike it with a hammer and it leaves a pretty good dent in it. Put the race on a wood block and strike it and it doesn't hurt it nearly as bad. Put the race on a rubber block (like an old diff bushing) and strike it with the hammer and you will not hurt it much at all. The same amount of force was absorbed all three times with significantly different results to the metal race.

Driver comfort isn't the only reason a factory engineer builds compliance into any drivetrain. Component stress and fatigue life is a consideration.

muoto

I wonder if this is the reason why Norwegians here (hf-mx5t & Inglar) are keeping their gearboxes together. IIRC, They both use FM lvl2 sprung clutch with their 400whp+ cars

soviet

iTrader: (10)

Close ratio T56 Magnum with 4.3 rear



The T56 is heavy and expensive and no bellhousing but food for thought. The ratios seem ok.

pdexta

iTrader: (17)

I really like the S15 6 speed for the street. There's one on ebay for $449, if the guts swap out that should end up cheaper and give you a taller 6th than a 3.63 swap. It looks even more attractive when you consider you could sell the S15 transmission with miata guts for little or no loss when you were done with it.

glade

iTrader: (2)

So why not just buy the 6th gear and swap it into your trans? Seems like a cheaper, easier option?