[Bonanza Bob]

Welcome. You are correct on the first three Newells with the steerable tag axle (ZF suspension). All Newells beginning with #697 had the ZF suspension with the steerable tag axle. Most 2004's also had the Detroit Series 60 500 HP engine. I show that #723, a 2005 model may have been the first Newell with the ZF suspension AND a CAT engine and it was a 525 HP C-13. Although there may have been some, I don't know of a 600+ horsepower Newell prior to about #750, a 2005. It was about #760, a 2006 when 600+ horsepower engines became typical in Newells.

As to the adequacy of 500 HP in a coach with almost a 60k pound GVWR, I will leave that to owners in the 2004-2006 range to add their input but will add that the Detroit had the Allison transmission and at that time the 600+ horsepower engines were all mated to ZF, either 10 or 12 speed, auto shifting transmissions. In long steep mountain grades more gears and more horsepower is a good thing so it depends on the percentage of driving you are likely to do on long steep mountain grades. In other conditions, the Detroit is certainly adequate. It just won't go up a long steep incline as quickly as a higher horsepower, higher torque engine with more gears.

I currently own a 2003 Newell Quad slide with DD and Allison and am considering upgrading to a newer Newell with steerable tag and Cat engine with  the ZF suspension and transmission.  On paper it is a lot of money for steerable tag, new suspension and transmission with a bigger engine, is it really worth the investment.  Are there any issues with the ZF transmission and bigger Cats?

Thanks.