[Big Dave]

There is an enormous amount of experience out there with hopped-up Cummins B-series engines. A 4BT3.9 is nothing but a "12 valve" Cummin 6B5.9 with two cylinders missing, so all that experience is applicable.

A LS2 makes 405 HP. How many minutes of full power do you suppose that a LS2 actually see in its service life You can get some serious tickets by driving 175 MPH.

The diesel is what pushes a 'Vette over 40 MPG. You could put an itty-bitty gas engine in it but I don't think 40 MPG is possible without competition-grade hypermiling. The diesel, with its higher efficiency does it easily.

The propane/nitrous is there for bursts of acceleration. I specify nitrous to avoid having to put a bigger turbo on the engine (and accept more turbo lag) for maybe fifteen minutes of raw power a year. Its simply "turbo logic." With a turbo, a small engine can have a "Jeckyll and Hyde" personality. Economical most of the time, but powerful when needed. How many turbo cars are at full boost much of the time? They'd grenade themselves.

The little Cummins is straight mechanical and I don't have to mess with electronic controls.