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My gearbox died last night. Started as a bad rattle in 2nd (like gravel in a blender) that got a lot worse, tried to limp it home in 1st/3rd, very quickly these gears degraded and I got it towed. It barely moves in neutral now. My guess is that 2nd broke and quickly destroyed the rest of the box.

What are my options? Car had a 5 speed I'm guessing the 6-speed isn't as strong - S15 and IS200 boxes aren't know for their strength - and the 5 speed seems well matched to the turbo's torque and midrange. Should I just get another NB8A box? Are they known to not handle extra power or was I just unlucky? AVO turbo kitted engine running 7psi, tuned for and run at up to 14 by previous owner.

6 speeds are tuffer than the fives.
Only problem is you'll endup only using 4 of those gears (usually 1, 2, 4, & 6)
OS-Giken make a gearset for the 6 spd as the gears are from an Aspen core (same as the S15 and possibly skyline).
Having said that I could only get 3rd and 5th in the Giken, and they're the gears I don't use.

Commiserations on the killing of your first box

Bazz

In order of cost and strength:
1) Quaife transmission
2) Quaife gearset (requires professional assembly)
3) OS Giken gear set (6 speed only)
4) 6 speed NB8 SE/Mazdaspeed gear box
5) 6 speed NB8 gearbox

Hope his helps. I've been in your position way too many times...

*edit* OS Giken gear set added to list (Thanks Woo)

I've owned the car for a week!

If I put another 5 speed in, is it inevitable that it will die from this much torque? I really don't need 6 gears - I can understand why Mazda did it with a high-revving n/a engine, but the turboed engine is a different story. Will also have a shorter diff ratio. Speedo compatible?

stefan wrote:I've owned the car for a week!

Yep, mine went after a week with 230rwhp

stefan wrote:If I put another 5 speed in, is it inevitable that it will die from this much torque?

Anything will inevitably wear down, but the stock one is only $300 or so from a wrecker. Look at it this way, for an equivalent 6 speed at $2,000, you could have 6 spare 5 speed boxes (I still have one left LOL).

stefan wrote: I really don't need 6 gears - I can understand why Mazda did it with a high-revving n/a engine, but the turboed engine is a different story. Will also have a shorter diff ratio. Speedo compatible?

Not sure why your diff ratio will change if you are leaving the crown and pinion alone? Speedo compatability is dependent on your speed sensor plug (electronic on NB8s I believe).

Again, happy for anyone to correct me if my memory or information is incorrect on this.

P.S. I'm on gearbox number 4.

boyracer should have an SE gearbox or two lying around...

I seriously doubt any wreckers in Tas will have an mx5 box - not all that common down here. Will have to buy from interstate; they seem to be $600-700 online.

What I mean re: diff ratio is that I will have a short diff ratio than a car that originally came with the 6-speed.

The best gearbox is the one running Redline shockproof (sp?)

You don't need a turbo to break a gearbox, the GM did one some time ago. I replaced the 5speed with a 6speeder, kept the 4.1 diff, and revelled in the extra pickup from the new ratios. Revved a bit harder on the cruise, but not so much to worry me.

Some of the racing turbos up here were going through gearboxes quickly, but that stopped when they switched gearbox oils.

Good luck

I am running a 6 speed with a 4:3 diff and love it. Yes it revs out quick and it is great, it is much quicker to change a gear than to wait for it to rev out with a 3:6 diff(stock nb8b aus spec).

I more than likely treat it much more harsh than you do with probably more power/torque also.

And as a side note I use 5th gear regularly. It is the same ratio as 4th in a 5 speed so that is my normal cruising gear.

Cheers, Tim

zoomzoom wrote:And as a side note I use 5th gear regularly. It is the same ratio as 4th in a 5 speed so that is my normal cruising gear.

Yup, 5th is the new 4th!

orx626 wrote:

zoomzoom wrote:And as a side note I use 5th gear regularly. It is the same ratio as 4th in a 5 speed so that is my normal cruising gear.

Yup, 5th is the new 4th!

mx52nv wrote:*edit* not sure where the OS Giken gear set fits in the above list... or if anyone has actually put one in a NB8 box...

They suit the 6 speed only so slot them in at number 3

Oh and btw I have them in my 6 speed (Ozimoto can supply, you fit)

Also the SE box carries the same part numbers as the NB8 box and components according to Mazda, so work that one out

I still have the original gearbox in my SP, with a short shifter and it is putting out close to 200RWKW, or even more if I turn the boost up, and I haven't had any problems with my box.
I do run Redline Shockproof oil in it though.

The car has done a number of track days as well as skid pan and drifting days, with no problems at all.

How do you treat your gearbox?
If you are a little sympathetic with it, it should last a very long time, even with a turboed engine.

Do you double-de-clutch when changing down?
This really helps with the longevity of the box as you aren't taxing the synchros all the time, which can lead to wear.
Do you try and change gears without the clutch?
This is a big NO-NO, even if you think you know how.
Do you constantly do very fast upchanges?

I wouldn't go for the 5 speed box, the 6 speed is a newer design and stronger than the 5 speed.
Having said that, the 5 speed is half the price of a 6 speed.

The trouble with buying a used box, is that you really can't tell how good it is until you've installed it in the car.

I've only owned the car for a week so I can't vouch for its previous treatment. I think I treat it pretty well myself.