Not ever having a diesel GT is there that much difference in fuel consumption to warrant the work involved in that swap ? Al
I would guess that the diesel will burn no more than 60% of what this 18hp Onan gulps up. With the hours I use this tractor, I'll likely save $100 to $150 in fuel per year. Being I have $300 invested in the engine, the cash layout will pay itself back in 2 to 3 years. Is it worth the effort....maybe not, but this is really more about the fun I have with projects like this more than the economics.
But, the water cooling is a plus for me. Using it in the silage pit during the 100+ degree heat, the Onan got hot enough to vapor lock. I could hear the fuel boiling in the carb after it shut down. The water cooling will keep things cooler so I won't be shutting down to vapor lock. As for power, the Onan has more than what I need. I would actually have been happy with a Kubota Z482 twin cylinder, which has almost 14hp and been an easy fit without any major parts being moved. But I lost out on 2 such engines, then got this one for the $300 complete with ALL parts for it to run....radiator, hoses, & complete wiring harness, and even the hour meter.