I have a '70's era motor home that weighs about 15,000 lbs. and with a tow vehicle gcvw could get to 18,000 lbs. I am going to replace the existing power train with a modern power train. At this point I feel my options are either stay with gas and convert to the GM 8.1L Vortec with the Allison 1000/2000 transmission or convert to the LLY or LBZ with a matched Allison 1000. Given the natural low rpm torque profiles and better performance at altitude of tubo diesel, I would prefer to go to diesel even though it will be more complicated. It also is easier to avoid fuel quality problems in Central America where I will be traveling at some point with diesel vs gas.

With the existing gearing of my unit, in 5th the engine will be turning about 2300-2400 rpm at 65 mph. This seems a little high to me for a cruising rpm for a diesel engine. If the new 6 speed Allison and matched engine can be found, it would turn 1800-1900 rpm at 65mph. That seems better to me as the engine is just turning fewer rpm with less wear and should/would get better fuel economy.

Now the questions:

Is my weight too high as a constant weight/load? What are the different characteristics of the LLY/LBZ at those two rpm levels? What are the torque/economy trade offs? Is the LLY/LBZ happy at the higher rpm as normal rpm? Where can I find a downloadable copy of the torque curves of the LLY and the LBZ?

The weight of the 6.6L is close to the weight of the existing engine and I will completely redesign the cooling system. Others that have reviewed my plans are about evenly divided between thinking it is a great idea to move to one of these modern power/transmission combinations to thinking I am out of my mind. That is probably true, but I won't let it get in the way!

To make the transition, I would purchase a used engine/trany with engine and trany wiring harness and computers and anything else needed. My existing engine had a water pump failure and overheated to the point that major engine damage resulted so now is the time to make a change if I am ever going to do it. The coach is a 1976 FMC.

I thank all you knowledgable Duramax experts for your learned advise.