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A number of people have commented that they have blow their gearbox after turbocharging their engines.
I am trying to establish a reasonably safe boost setting for use on track days, that won't blow the gearbox.
My car was recently measured making 155kW at the rear wheels at 11.5psi. I have since calibrated it to run to 14psi. Apart from putting my road worthy street car in with the outright racing cars in the Victorian rules, I suspect the gearbox would not last long in racing use. Conciquently, I will reduce the boost for track days.

Could those who have had experience with the 6 speed gearbox, please tell me what engine output they have run during track use and how long the gearbox lasted?

Check out miataturbo - search, as i assume the question is well discussed.

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The majority of gearboxes suffer failures because the engine is twisting on its mounts. It's pushing one way and the PPF is pulling the other. Obviously the gearbox takes the main part of the force and this causes internal failures. Most of the turbo racers have now gone over to the solid mounts and some have made up torque bars as well. This is what the guys in the US have been doing and most are running the stock 6 speeds.

One post on the Miataturbo blog said the 6 speed is bullet proof to 300rhp (225kW). Others claiming higher outputs where also running it.
Another thread said the "twisting" problem was rectified on the 6 speed by the extra ribbing.
Bottom line was a lot of people where braking 5 speeds repeatedly, then fixing the problem by fitting the 6 speed.
Locally, I know of a leat one person with straight cut gears in a race only car making 212kW because he broke the 6 speed. However, I am well short of this.

It also depends on the driving style.

The 6 speed could be fine for something with say 250kw if it's dríven well and with some sympathy.

But if you want to do hard launches and be somewhat rough with it, it might only take 150kw to break one.


(random figures here, just guessing at power output)

Yep, it all comes down to how you drive it.
If you drive it fairly normally most day and give it the occasional thrashing, you wont see many issues.

As suggested, the twisting (whole engine/box etc assembly) can cause premature failures, it can also make the car extremely hard to shift quickly no matter if you're running the 5 speed or the 6.

More solid mounting will help in a track car, but will likely be unbearable in a road car, particularly if it's daily dríven. If you thought your convertible did a bit of vibration and general road feel now, drive onw with solid mounts, you'll go back to loving your stocky!

In summary, and purely my opinion, if it's a track car that rarely sees serious track use, leave it til it breaks - there is a good chance it wont...

Lots of good road cars have survived a long time woth over 200rwkw... It's not the peak power or torque that kills a gearbox, it's using them all at 10/10... Easy on the changes when driving like a maniac may solve half the box problems in the world!

Most of the turbo racers have now gone over to the solid mounts and some have made up torque bars as well.

From all the people I have spoken with who have broken gearboxes it was solved with harsher solid mounts to stop the twisting.

I currently run these mount, avoid the higher duro they are unnecessary.

http://www.miataroadster.com/awr/awr_motor_mounts/awr560170/i-409021.aspx

There is an increase in low rpm vibration, however once under way you cannot notice a difference. I'm totally used to it nowdays.

Hope it helps.