At our coop in town, where I work in the spring and fall, we have a few terragators and they have 3208 CATs and 555 Cummins. All of them have straight pipes, including an old manure spreader made by Loral with a 3208 in it and a 5" straight pipe that I use in the fall. It sounds pretty decent but I think it comes nothing close to an I-6 with a straight pipe. Both engines are rated at 250 HP. I know they have had tons of problems with the 555s and the 3208s are ok, just short on power, and you have to be very fussy with maintence so that they will last.