[Big Dave]

First of all, I do NOT have a 5.4 liter gas engine. I have a 7.3 liter turbocharged diesel V-8. Yup. 444 cubic inches. The same Interbational T444E engine put in school busses and Class 5 trucks.

Diesels are very different than gas pigs. Diesels almost always operate with more air than stoichiometrically required - "excess air." The engine power output is modulated by regulating the fuel input only. Same air every stroke, but fuel varies. A gas engine with its narrow fuel:air ratio modulates air input (hence fuel input) with the throttle plate.

A normally aspirated engine is inherently a resonant device. The intake/exhaust column has a natural frquency and the valves are tone generators. At the resonant frequency gas passes through the gas flow path at its maximum rate. A turbocharger on a diesel flattens that resonant peak with brute force - the compressor, so air flow varies only proportionately with engine speed. Result: a very flat torque curve. My engine idles at 650 RPM (it will idle, albeit roughly at 500 RPM) and can move the truck nicely at 750 RPM. From some dyno runs (a popular event with diesel truck types) I know that at 800 RPM, my engine make 90% of peak torque. Further with no throttle, a diesel never loses efficincy due to excessive pumping losses.

There really is no "sweet spot" (relative to efficiency) between 800 and 2000 RPM. Above 2000, the engine frictional (mechanical and gas flow) begins to rise and efficiency drops off proportionally, until (with low cetane US diesel) the torque begins dropping off around 3000 RPM.

The flat torque curve and lack of a throttle are the reasons a big displacement diesel can get good fuel economy. For a diesel, the only advantage of a small engine is lighter weight that the vehicle has to tote around. My engine weighs about 1,050 lb. Yup. A half-ton of iron.