I own a fleet of KW T-800 and WL-900 tractors all Cummins powered with 525hp N-14's and 600hp ISX's. The ISX is a very good engine with a ton of power,but as good as it is it can't hold a candle to the N-14's when it comes to durability. After being in the business for over 40 years,the Cummins N-14 is the best,most durable truck diesel engine I have ever seen.In fact,other than the odd leaking accessory drive seal or a cam follower box leaking a bit at around 700,000 miles,we have never had one that needed a rebuild before a million miles. BTW/I never owned a CAT (1693-TA,3408-TA or any version of the 3406) that went much over 700,000 miles without a catastrophic failure.