Problems with big, long stroke engines run with original gearing are cam timing, internal friction and pumping losses at high vacuum.
A big engine running at high vacuum loses a lot just trying to suck air past the throttle blades. In my experience a 454 V8 gets 12 on a good day in a 3/4-ton pickup. A 350 in a similar truck will get maybe 15. That's a 25% improvement. But put a big trailer behind it and the 454 will get 8.5 plus while the 350 will get about 8. Under full load the pumping losses fall to near zero.
So what else is going on? There's cam timing. An engine whose torque peak is 2,000 RPM running at 3,000 rpm will be a gas hog no matter what else you do. My 235 want from sub 10 mpg to almost 13 mpg with a change of camshaft to one with a torque peak of 2800 RPM; no other changes. I had similar results with a motorcycle engine.
Then there is the issue of internal friction, and inertial losses from reversing piston direction twice every revolution both associated with long-stroke engines. Compared to say a 283 V8 a 292 is a very long stroke engine. It was designed to run in the range of 2,000 RPM give or take a bit. The small-block was designed to produce more power in a compact, light-weight package and will run happily at 3,000 RPM. My 235 running on flat ground at 2000 RPM has oil temperature about the same as coolant temperature. At 3,000RPM on the same day and road the oil temperature climbs to 275 deg after a while. Coolant temperature remained the same. Where did the extra heat come from? Burning gasoline.
So if one wants a flexible, fuel efficient engine, it should run at about it's torque peak at cruise speed, it should be as close to zero vacuum as practical, it should have minimal piston speed and greatly reduced internal friction. That's not a 292 I-6 running at 3,000 RPM on the interstate. Sounds like a turbocharged 4 cylinder with direct injection and a 10-speed automatic transmission. Well how about a compromise and a 305 Vortec V8.
