Q: gas mileage difference 383 stroker motor 400 hp or ls3 computeried 400 hp same car

The gas mileage attained in either of these applications will depend heavily on your driving habits as you may already know. Either of these will also depend heavily on the type of intake system you are using to arrive at your 400 hp number.

When you say 383 stroker motor, I would assume you are speaking of an earlier generation 350 Chevrolet with the 400 c.i. crank and rods arriving at the 383 CI. Also, are we assuming both of these options would have the same intake manifold and fuel injection? As you may know this will also have a large impact on the type of camshaft, ignition timing, valves, etc. which will all affect fuel economy.

Since there are a lot of factors at play that determine horsepower and gas mileage numbers, it is very difficult to project which will do better in terms of fuel economy. However, the convenience and automatic adjustments that a computerized fuel injected motor offers are going to be much more accurate than a carbureted motor of the same horsepower.

Without knowing more detail, I would say from a big picture standpoint, the LS3 motor will do much better on fuel.

If you’d like a second opinion on this, have the car inspected by a certified technician, such as one from YourMechanic.