fuel efficiency changing from small blocks to big blocks?

A bigger factor is if the engine is running at or near it's peak torque output, which indicates best cylinder filling and peak efficiency. A well-optimized large displacement engine can operate at or near it's peak torque over a very broad range of engine speeds. This can be especially helpful for an application where you change engine speeds often and don't have the benefit of a gearbox to re-match the engine to the load at the different engine speeds. This is a good description of a powerboat doing, say ski tug duty where you are frequently accelerating and decelerating. In these applications, the large displacement engine can have an efficiency advantage because of being tuned for a very wide torque peak. The larger displacement allows some peak power to be traded away in this fashion. The faster the boat comes up on plane, the better for efficiency as the drag penalty for a planing hull that is not yet planing is very high.

The 26-28 MPG Chevy Corvette is a good example of a vehicle with a large displacement engine that because of it's wide torque peak can be surprisingly efficient when in high gear. A small displacement engine would not have the torque at 1500 rpm to pull this little hat trick off.

Jimbo

 

In response to the OP, my explanation is correct. Unless the gear ratio in the drive is changed, or the prop is changed, the engine efficiency will decrease.

 

YellowJacket,
Thanks for the explanation I believe I'm getting it. Correct me if I'm wrong. In the example of the vette running the motor in overdrive at lower RPMs but opening the throttle more allows the motor to run more efficiently. The example of the propeller, by increasing the pitch and slowing down the motor and prop, one is "harvesting" the excess torque of the motor.

 

The point is that by minimizing the intake manifold vacuum you are reducing the intake pumping loss.

(Intake pumping loss) = (RPM)*(engine displacement)*(manifold vacuum)

The equality should really be a proportionality.

By going to a bigger displacement without changing prop or reduction ratio, you end up increasing both engine displacement and manifold vacuum.