guys, he asked what size. I would put it this way. look at the total picture. are you looking for more power cheaply, the 2.55 is the best bet. OR more power for a more sizeable investment. is you do smaller than 2.55 you have to get injectors, so that's an extra couple hundred. THEN you would have to get the heat exchanger, another couple hundred. you can run the 2.55 without the other upgrades (although you get more from it if you do the heat exchanger.) if you plan to only do one round of mods, I would do the smallest and best of the best at that time and be done. if you want an increase, but don't want to drop 1k to do it, the 2.55 will get you there.