Everything was the same in the swap even the computers? Chrysler recommends Premium fuel for the 6.4, is that what you use especially when towing? My buddy has a 2021 with a 6.4 and 8-speed and I get MUCH better fuel mileage with my 5.7 than he does. BTW, put 1,000 miles on it today towing, not issues and I figured I should break it in the same way I did when I first bought the truck...LOL [/quote]

I use the entire 5.7 top end and cam as that induction system makes more tq per CID than the 6.4 top end. I run 6.4 SRT8 injectors and stock 5.7 computer and the fuel trim never adds more than about 10%. You would like it as there is no MDS when you do it this way. The real 6.4 trucks have EGR, less compression and less TQ than mine and definitely use more fuel, mine uses almost exactly as much fuel as my stock 2009 5.7 half ton. As for fuel I run premium when towing and regular when not towing, the only time I hear pinging or see it on the scanner (even with 87 octane) is when it is very cold engine and ambient temp, once it warms up a minute or two it quits, it's counter intuitive and I don't have an answer (maybe the timing is way more agressive when very cold?), I have built a couple for customers this way and theirs are the same (2009, 2011, and 2012). I just feel like with that much compression I should have to run premium even though my ears and knock sensor say it is ok. It is also much more pleasant to drive than a 5.7 as it don't downshift nearly as much now with the 6.4. [/quote]

Interesting! As soon as they tell me the warranty is done I will have to start playing...THANKS!