DeerHunter

Yeah, that makes sense. It also nicely explains why every gear except 4th was noisy after it went. It also means that gear-dependent boost targets won't really help me. I'll probably have to fiddle with rpm-dependent boost, keeping it at a reasonable level at torque peak and ramping up towards redline.

Thanks Bob. Hope you get your transmission issues sorted out on a more permanent basis.

k24madness

iTrader: (6)

Thanks that helps.

bbundy

NASSA TT2 boost level is about 285-290 ft-lbs. Autocross gets full boost setting ~320+ ft-lbs but if I data log an autocross run it never really gets to full boost. It’s ether either climbing through the revs so quickly or spinning the tires as to not fully load the engine as it passes through peak torque rpm. At Laguna I ran it at ~320 ft-lbs ~18psi.

I’m getting short life like two 6 speed transmissions a year predominantly at under 300 ft-lbs.

I'm not exactly a pansy driver though. NASA TT2 lap records on all the northwest tracks I've driven and this year 4 out of 6 Northwest Region SCCA solo events I've gotten Overall top PAX running SSM in the same car.

Im guessing gear selection in the FRS box with the different primary ratio and tooth counts on 4th gear is good for about 10 to 15% more torque capacity than a the Miata.

swimming108

iTrader: (3)

How different is the FRS gearbox from the Miata?
Can the FRS gearset be installed with reasonable amount of machining into the Miata box?
How different are the ratios?

matthewdesigns

iTrader: (8)

It does not look that bad in the new photos. I'll stick with my previous thought regarding the replacement of both halves of that affected gear pair and using the transmission. There does not appear, at least in the photos, to be any horrible smearing or damage to the teeth otherwise.

Fourth is the drive gear in this trans, and it is the one that broke, or more specifically the 4th counter gear is the one missing a tooth. Depending on how the gearset was made, you'll be replacing the countershaft if it's arranged like a stock gearset, which has the counter gears for 1-4 as one assembly, or counter 4th individually if the Quaife set is modular. I'd be happy to share a diagram for a stock transmission which illustrates that clearly.

emilio700

iTrader: (3)

Take one apart and let us know. IOW, no one here has had one apart. The folks working on my gear box have had on apart and indicate some parts may be cross compatible. Thats third hand info though.

bbundy

There was a big thread on this and the S15 Nissan box. It is a very similar AZ6 to the Miata center section looks identical with a different bellhousing and tail section. gears are all different but theory is a hybrid box could be made. Aftermarket gear sets are available for the S15 AZ6 box with claimed HP capacity in the 400 hp range with very optimimal close ratio 6 speed ratios.

Also Subaru is putting a turbo in the BRZ I think for 2015 ~280hp from the factory.

I have an S15 SR20DET 6 speed box. I'm still debating about tearing apart and seeing what can be done. I still don't think the factory gears in it are strong enough just slightly stronger than the Miata version. Same gear ratios as the BRZ. Primary gear ratio it has sends higher speed and lower torque through the secondary shaft, tooth counts on 4th make for bigger teeth as well.

swimming108

iTrader: (3)

You have at least 1 broken miata 6spd, a S15 6spd, race season is coming to an end... Is there any chance you could get enough spare time to dig into these 2 boxes in the coming months to find out if a hybrid can be made?

bbundy

No that is not 4th gear it is the primary gear set that drives the secondary shaft for every gear but 4th. There is no 4th gear set in the 5 speed box so it can't be broken. When you shift to 4th the selector ring just locks the input and output shafts together and all the drive just goes straight down the two in line shafts 1:1 and the rest of the gears and the secondary shaft just spin with no load at all being transmitted through them.
Input primary gear is one piece with the input shaft. The other side of it is one piece with the secondary shaft. So fixing it involves replacing the two shafts.

DeerHunter

From the exploded-view diagram, it seems the affected gears can be replaced piecemeal (sans shaft).



What kind of expertise is required to change out gears and reassemble? Can any competent shop do it or should it go to specialists?

bbundy

Yea replace items 7 and 8

DeerHunter

Thanks Bob. Second order of business, given that my replacement tranny should be here next week, is what gear oil to use.

• Redline Shockproof (unadulterated or mixed with something else)
• Amsoil Severe Gear SAE 250
• Something else?

Bob, I think I read you were going to try the Amsoil. Given that you just blew up another 6-speed, it apparently isn't a panacea. Do you think it helped extend life at all? How was shifting (hot and cold)?

matthewdesigns

iTrader: (8)

I know how this trans work, but I’m sure someone here appreciates your time to explain it.

When you tell someone to upshift from 3rd to the next higher position, what would you call the position where the shifter ends up? We are talking about the same thing and using different terms.

Call it what you will: direct drive, 1:1, 4th, Main drive… Mazda refers to the hub/sleeve assembly which locks the input gear to the output side as the “Hub set, Clutch – 3rd & 4th”. It also refers to the input gear as “Gear, Main Drive”, not 4th, and 5th gear as “Gear, Over Top”. If this were a highly technical discussion then using the exact nomenclature offered by Mazda would be appropriate, but given the relative lack of knowledge by the casual reader then I don’t think it’s a big oversight to refer to the broken tooth to be relative to the “Counter 4th“. We both know what they do, but everyone else reading this may not so I simplified it a bit.

Thus my admission of having seen neither the inside of a Miata trans, nor a Quaife gearset for one, and my guess as to whether the Quaife set was modular. It looks like Deerhunter got lucky and it is, based on the pic he posted, and can replace only the two items I had suggested a couple of posts ago.


Do you not trust that the guy who got it this far can handle the rest of it? Looks like they have a decent handle on how it comes apart.

Leafy

iTrader: (1)

A lot of the wear in the 5 speed being posted looks like surface fatigue. I had the book (Machine Design) on my desk so I looked, your best options to reduce that pitting/spalling is to increase the viscosity of the lubricant, so SAE250 it is.

codrus

Would you expect pitting/spalling to show up on oil analysis of the tranny oil?

I sent a sample of my 5K-mile 6-speed oil (Redline MT-90) to Blackstone just before Laguna. This was with the now-dead 2560, so only 260-270 ft-lbs of torque, but they didn't identify any particular issues with the oil. That said, iron was at > 50% of their "universal average" after 5K miles, with the "universal average" being about 24K miles.

Bob, have you ever had a tranny stay together long enough to change the oil on it and do an analysis?

--Ian

Leafy

iTrader: (1)

It should show up in analysis, and yeah as more iron.

bbundy

I've used both straight Heavy shock proof and this last one broke using Amsoil Severe Gear SAE250.

Have to push in the clutch when starting cold. The car will tend to take off in neutral when the SAE250 is cold.

bbundy

Counter 4th is the wrong term. I don't know what Mazda calls it on the 5 speed but generally every manual I've looked at calls those the Primary gears. I know that’s what is called in the 6 speed manual.

But when somebody says they broke a gear that is the 1:1 ratio for the transmission it's not possible. You can take every gear out of the transmission and still ingage 1:1. as long as the selector mechanism isn't fubar.

Steve in Bailey

Andrew, can you re-link what you are referring to? (I am too lazy to go back through all these pages ...)

Steve

DeerHunter

Here it is.