You can get performance chips, computer upgrades (like programers), and fuel injectors. I've got a buddy with a 4.0 and he installed a chip and had his computer tunned and now has as much, if not more, power than a stock 5.0. Everybody swears he has a V8 when they ride in it. Depending on which 4.0 you have, you can expect to gain 20 to 50 hp by upgrading. My best suggestion is to avoid Ebay when it comes to chips, etc...too many generic ones that don't perform. Believe it or not, you can order a lot of high quality upgrades through Auto Zone or Advanced Auto Parts stores. Most can order power programmers. Spend the extra money and go with a respectable brand and you will be happy with the outcome.
