how much torque have you guys put through your 6-speed transmissions without braking it? Is there some kind of official torque rating for it? a RX-8 6-speed tranny sort of fell into my hands and Im wondering if its worth converting it for my ford cosworth engine which tends to brake the original mt-75 boxes. Its a nice compact close ratio 6-speed if it can hold its own:D

 

So far, I have hammered the trans with repeated runs in the low 13 second range with nitrous and the trans has given me no trouble. My guess is that my engine is producing about 210 lbs./ft. of torque at it's peak, maybe a little bit more, but the trans is fine. I, and several others, have also driven our cars hard all day long during track events and the same holds true. Any problems with the tranny will likely appear when we begin to use drag radials on the car. I do have a few ideas as to strengthening the trans and those will be tried next spring. I can tell you, for sure, that the trans is an Aisin model(Toyota Co.).

CRH

 

a lead mazda tech told me that it could take up to 350 hp, after that he was not sure, i do not know about the torque though. He recommended keeping the stock clutch cause it was a weak link but then again, said the transmission could be had for like $750 if yours broke, so who knows?

 

Mazdamaniac has also pointed out that he believes the factory fuel injectors to be able to flow enough to support about 300 h.p. so that figure of 350 you mention sounds in the ballpark. Using 350 as a figure the torque rating would be about 234 lbs./ft. on the input shaft. I would be interested in finding out, which I may soon do, exactly where the weak link is in the transmission itself. Perhaps the case doesn't have enough mass to support high levels of torque but the main/carrier shafts do. In that event a stronger case with some cryogenically hardened gears may suffice for most uses. I do have a major issue with the bearings used in the entire tranny as they are noisy as hell, which is not a good thing. I figure if I get around to working my magic with the trans I may be able to raise the performance of it a bit. Quiet running, smoother shifting, and greater strength, those are my goals with it. That price of $750 isn't too bad considering for another few hundred bucks I could lay my mods on it and still market it at a reasonable price. Shepherd Racing builds a really nice trans, albeit for the DSM's, for about $1200-$1600, plus $80 shipping.

CRH

 

charles nice to have you back, the guys at puerto rico are using this trans with some mods for 20b conversions and 650 hp monsters.

 

some transmissions in the 8 have a weak link. the mainshaft rentention ring (best way i can describe the piece, it is located just outside of the transmission housing, inside the tailshaft extension (?). it is about 2.5 to 3 inches in diameter, approx 1/4 inch thick, that holds the mainshaft in place) well anyway, mine shattered, and when i had it taken in, the transmission was stuck in 6 th gear. my service writer let me know that mazda had a new part number for this, leading me to believe that the part had been redesigned beacuse of other failures.

 

Having worked at a motorcycle parts shop in the past I can tell you that a new part number doesn't necessarily mean a part upgrade, although I wouldn't necessarily discount the idea either, given your story. I currently work on an assembly line for Ford and part number changes happen frequently without a change in the actual part in this industry, too. Thanks for the info, lurch, as that'll help me in the near future.

rotarycrazy, those mods you mention, are they available for public purview or are they described anywhere? I am wondering if the mods on the RX-8/Aisin trans are to interface it with the 20B or for strengthening purposes.

Thanks guys,
CRH

 

So, I am not crazy.


I've always thought mine had too much bearing whine. A number of people have tried to explain it away as "proximity of driver to the case" and "more NVH in favor of 'performance'". 10K of break-in helped, and redline fluids helped some more. It's liveable but it's still more vocal than I'd expect. Do you think this is just an aesthetic issue or does it have some implications for long-term reliability?

 

This may seem incredulous but many moons ago I was a competitive speed roller skater. During those years we hated the Japanese bearings on the market because they were noisy and had more friction than the Swiss or German bearings that were much more costly(like the Fafnir brand). I would venture to say that there is a direct correlation between my experience then and what we RX-8 owners are experiencing now, especially because the noises we heard and hated were the very same ones we are hearing from our transmissions. When you get 8-10 guys blazing around a 100M track at top speed you can bet the bearing noise is quite audible through the floor. Guess what it sounded like? The RX-8 transmission idling in "neutral". That noise is carrier/mainshaft bearing noise. Guess which bearings I will be specifying and cross-matching when I get around to playing with my transmission?

I think the issue is mostly aesthetic because I have been beating the Hell out of my transmission for two years without a problem. Whether two days of track racing or every weekend of nitrous-aided drag racing this trans has dealt with it all all long as I wasn't trying to "bang" shift it. I still don't like the noise and noise equals friction. If I am going to offer rebuilds I might as well also upgrade the internals or else what's the use?

CRH

 

a guy I was talking to swaped a 3 rotor turbo in and is running 650hp/xxx TQ and runs mid 10's with the full interior 8 and drag tires. He said while they did a little work to strengthen 2nd, 3rd and 4th gear, they also modifyed the shift forks and have no problem with the stock 6 speed trans in the rx8.

 

It seems that mazda gives out some technical service bulletins only for european dealers and others for only US market dealers. For European market there was a bulletin about the noise in 6-speed tranny when off gear, it was to do with the extra gear for reverse and not bearings. After swapping the gear with a redesigned item the noise stops. Do you guys have that bulletin? if not, I think I could post it here.

 

I agree -- a car that revs like silk to 9000 rpm should have a silky transmission to match.

 

I've rarely ever seen a used RX-8 tranny sell for $750. A new one through the Mazda Motorsport racer discount program is $2100 + freight. The dealer price is considerably higher. The individual parts are so expensive that any kind of serious failure is almost guaranteed to be less expensive to replace rather than have it rebuilt by someone else.

The Italian Cup RX-8 racing series (NA stock long-block engine with intake, exhaust, and PCM mods) are seeing synchro, countershaft, and input shaft failures on factory transmissions, they now modify them.

 

Great info, Team and MrBungle. Thanks.

CRH

 

The reverse counter gear makes sence. The main input shaft has a sealed ball bearing and seperated just past the main shaft counter gear. The counter shaft runs on a noisy needle bearing in the front. The reverse counter gear rolls on a sleeve bearing and is in constant meash with the counter gear and the reverse gear on the main shaft. Please post the Euro service bulletin or direct me to It. THANKS. My trans is out on the bench, nows the time.

 

sorry guys, Im no longer with mazda I could probably get the bulletin number/date if that would help? Also if I remember it correctly replacing the gear with the upgraded item did not help every time. The tranny looked and measured ok, but there was still the noise even after replacement

 

On this note: My friends Porsche Cayman S gear box is much louder then the 8's.

 

:D :D :D
lol, no it was just time to move on

 

About 280-300 foot lbs is the "safe cutoff" point based on spline section strength.

That doesn't mean you can't run 400 foot lbs without breaking it, only that the life will drop pretty quickly, maybe as quick as you drop the clutch pedal in first.....