the power. What kills the 5 speed is trying to shift it quickly. IMO the more power you make the more a auto makes since. I will be the first to admit that the TH400 is the most cost effective solution. I really like the overdrive of the stock auto and the lock up converter. If our new tricks to the Z auto work like I hope I think it will be worth the cost. We now have a triple lock up converter for the Z trans, so that should hold there ;-)