While I love the LSx family of engines (we have 2), it's kind of like apples to oranges. Our Duramaxes are amazing because a HD 4x4 tow truck can be made to haul ass fairly easy, even though they are the size and weight of a barn. And somehow get good mileage in the process.
Things to note:
1200HP on an engine dyno normally translates to 900-1000rwhp.
A 600rwhp diesel in a 7000LB truck will outperform a 600rwhp gas engine in the same truck. Diesels have much flatter HP curves, ie - more average HP.
Race gas is $7.50 gallon and rarer than diesel fuel is.
Minor tuning errors in turbo gas engines are expensive.
I agree, a 600 horsepower 1/2 ton truck would be fun for sure. But with gas you'd pay for all that power a lot more in the fuel economy than with diesel. Best milage I've ever seen from a 5.3 truck was 17 something. That was hyper mileing on an open flat highway. Truck had free flowing flow masters, a diablo on economy, a cold air intake, and a free flowing MAF. When adding a turbo or supercharger to that youre lucky to even see the same milage as a stock truck. At least with our diesels a 600 hp truck will get better milage than when it was stock. Of course that all goes out the window if something happens with the duramax and it needs to be built. This is speaking stock long block to stock long block.
This is just my opinion and experiences, im sure lots of people will disagree. But one of the main reasons I bought a diesel was to make big power and still see around 20mpg.
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