Greeting everyone!
My first post here, I´m from Argentina.
Yeah, i know, yet another Q4 transmission post, we might be able to make it a sticky !

Well, i have my engine pretty much ready, forged rods and pistons (i can hook you up with pistons here, i bet they will be cheap for you, paid U$S320 for the set), upgraded cams, ported and polished cylinder head, Holset HX35 turbo, MSII standalone, 550cc/min injectors for gasoline and 1200cc/min injectors for methanol. I know it is not that necessary for this power rate, but want to play on the safe side, dont want to melt or crack anything.

Well, lets cut to the chase. I am modifying the gearbox to fit the upper and lower strengthening plates, and trying to find a way to fit a third plate to protect the lower part of the casing, good luck with that, ill post pics when ready...
What else would you recommend to upgrade? gears 1-3 ? a friend of mine blew the 5th doing 250kph... but i cant tell wether it was due to the shafts splitting apart, or the gears just not being able to withstand the power (he had no strenghtening plates whatsoever).
So i was thinking on fabricating spur gears 1-4 and leave the 5th helicoidal, that way i can rest assure it wont be the gears that break and i can goof around in 5th gear not coping with the grinding. Of course, if i could skip this, it would be great... gears do not come cheap!
What about the rear differential? any issues there?
Viscous coupling? Haldex? Cardan? I´ll be fitting a 5 puck ceramic disc clutch with properly rated pressure plate.
The reason im worried is, i will use the car for occasional drag racing (amateur), and theres no way i can find a spare Q4 gearbox in Argentina. So... gotta play safe.
Thanks !

And please, if you can explain the certain reason why 3rd gear blows up, and maybe a solution, it would be great! Thanks!

I'm afraid I can't offer any specific help as I'm not familiar with the Q4. But the reason for your friend's 5th gear failing could be identified by the mesh pattern on the gear teeth. For example if the wear pattern is near the edge then the shafts will have moved.

Sounds like a great project, keep us all posted please.

3rd gear fails because of the way it was constructed. I've read that the integrale lot have had this issue and the way they upgrade 3rd gear is to use 3rd gear from the coupe 20vt gearbox as it's the same and constructed slightly differently.

What happens to the gear?