Well I had the HEMI and tuned it with the Diablo 91 octane performance tune with the performance transmission tune. This would not be ideal for towing, when I picked up my project 240sx I installed the towing tune for everything. Ive heard of issues with all tuners, from my Dad who's been working with Chrylser for over 30 yrs for some reason they dont like to be modified, and some are mor picky than others. Changing small things like tire size and base tunes arent going to give much issue, but once you start changing shift points, which mine did without a hitch, it can cause issues. Google it and you'll see numerous threads about it. I gave it a shot and was lucky to not have a single problem, other than the occassional transmission over temp warning which was not related to the tune, and my HEMI ran like a mad man! I was chirping my 32" MTRs in 2nd at times. Ive got my DiabloIntune for sale as I sold my XK, not sure if it'd work with the 4.7 but Im only asking $300, its $380 new and mines mint.