I read somewhere that the 6 speed transmission is capable of handling upwards of 700 ft lbs of torque. The weak spot in the 6sp is most certainly the syncros. The second weak spot is the concentric slave cylinder. The CSC housing is made of plastic from the factory. This causes issues when a stronger clamping clutch is used.
I had my trans replaced for syncro issues before I went TT. I still have the stock clutch and I am putting down 474 hp. So far no issues after 5000+ boosted miles.
I have no experience with the automatic. I have read about a couple of 7AT failures on the forum and the reason seems to be that the higher gears can't handle the added torque. But I will leave further explanation to those with more experience.
The durability of the engine has been dissucssed here many times. Here is the short story.
These engines are strong. The durability is all in the tune (to a point)
Many feel that the safe point is right around 500 WHP on a stock block.
In my opinion, go with a 6MT if you don't want to worry about the trans and just build the bottom end of the engine. That would probably allow 600 WHP all day long without worry.
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