Re: More power from my AU XR8
$4k might get you to the mid to low 14's, to get into the 13's you will need a fair bit more money than that IMO as you will have to go for a stroker or supercharger.
Firstly if you have to remove the manifold then get yourself an aftermetrket one like an edelbrock. A cam change is the next thing I would look at, and having the manifold off is already a good way there. Look at a Crane 2031/2030 cam or somethign like the TFS1. To go with it, you will need some new valve springs, retainers and some roller rockers. Then yeah some decent gears in the diff will help, 3.9's if it is an auto or 3.7's for manuals. Once that is all done, get a custom tuned chiptorque or unichip and you will be close to 200rwkw and capable of a low 14 sec pass, maybe dip into the 13's.