Re: 366 gmc milage questions?
High compression gasoline , turbocharger gasoline, schoolbus--not good bed fellows.
Mostly it's about duty cycle. A pickup might take 50hp to push it down the road at a given speed and a bus might take 100hp. On hills or an any acceleration you are going to see 100%--sometimes for several minutes. A pickup might need only 40% on the same hill. Going across Wyoming in a bus into that wind, you might be at 80%+ for hours or 100%. It is designed to take it. The compression was lowered, the displacement is reduced (A 460 down to a 470 or a 454 down to a 366), the heads have reduced port and valve sizes to increase low rpm airspeed and improve low rpm cylinder filling. Due to the duty cycle, the exhaust valves are usually sodium filled to help take heat away from the head. The sodium turns to liquid when it gets warm and circulates heat up the stem. The cooling passages in the heads are generally larger, the water pump, radiator hoses and radiators are larger. Usually the distributor shaft is thicker because it drives a larger oil pump. There may be a mechanical governer built itto the distributor as well. The blocks usually have added webbing and the cranks are steel. I don't know if the 366 has all of these, but I've had a 361FT and a 413 truck engine and they were both like this.
It might be tempting to try and squeeze more out of the gas engines in these, but I wouldn't. If you increase the compression ratio, you may induce detonation that you won't notice in a bus. In a car or truck you drive around using 20% so if it has detonation, you are rarely at full power so it doesn't matter. And on a newer engine there is probably a knock sensor to retard the ignition. The best thing you can do with a bus motor is maintain it.
I think 6mpg is what those get. In a high load application they get 1/2 what a diesel gets. The difference is less in a low load application.