I will be running roughly 400 hp (with N20) through my drive train this summer, I have extra cash on hand, and I was wondering if my rear end/transmission (A4) goes out, which one will most likely go first. I am planning on eventually having roughly 700 hp (with N20) down the line, so if I have to replace these, I want them to only have to be replaced once. Eventually (year or 2) I will be getting a 383 in there, with roughly 450 hp NA + 200 or so shot of N20. What would you suggest I get for a Tranny/rear end if it does go? Can I get a higher stall that would be more suitible for say a 450 hp engine even though I will only be having 300 na hp (plus 100 shot when I do use it) or will it be way too unstreetable.

 

Because you're running with an auto, not a standard, your odds are better for the rear end lasting longer, but if you start spraying N2O, you will definitely need slicks/DR's, so then you'll still be taking a pretty big risk with the 7.5" 10-bolt. I think it's safe to say you WILL need to upgrade to a 12-bolt or a 9" rear end in the not-so-distant future.

Tranny wise? Well, behind 300 - 400 HP, it'll hold for awhile, but don't be surprised to find it throw in the towel at some point too. Depending on what your purpose for the car is (@ ~450 HP, I doubt it's gonna be a daily-driver anymore?? ) you may want to consider swapping in a built TH350 or TH400 in place of the 4L60E. Unless you can find a reliable tranny builder who can build a 4L60E to stand up to the abuse you plan to put it through, your chances are better for getting a TH350/400 to take the power, and parts are more readily available and cheaper for them too.