Turbo Boost and FE

[GasSavers_Hal9000]

Good points all of them, but there's something else to keep in mind and the water pump analogy points it out well. It's not always just about max efficiency. Any engine has certain conditions where it really performs. For the big block V8 its low rpm heavy load conditions. For my little D15 honda engine, it's light load, moderate rpm (3000 or so) and for my 2.3L cosworth mercedes, it's heavy load, high rpm (6000 plus). None of these engines are guaranteed to be more fuel efficient than the others except within the conditions where they work the best.

Unless you drive under a very narrow set of circumstances (Over the road truckers do that, but few other drivers do) you're going to be moving in and out of the most efficient range of the engine in question. You can either try to adjust the car and the engine so that the max efficiency occurs in the range of the most common driving conditions, or you can try to widen the efficiency range of the engine to cover more driving conditions (but like a torque curve on a dyno graph, you'll end up with lower peak efficiency over a wider rpm range). You can do both of these things with a turbo depending on how you build the system and size the turbo.

The problem of a turbo either spinning up early and running out of gas, or not spooling until late in the RPM range is a dead issue. The turbochargers designed within the last decade are leaps and bounds ahead of what most people realize and are used to. The old standby garret t series turbos are such old technology at this point that I think they should be scrapped altogether by the garage tuner crowd. They have major performance limitations compared to the new generation turbos which can perform over a wide variety of driving conditions, rpms, and throttle loads.

In terms of the small engine vs. large engine in the same car. You're only presenting half of the argument. Put a 4 cylinder into a chevy pickup and it's not likely to get any better mileage than a V6. But the V6 chevy pickups definitely get better gas mileage than the V8s... (and in case you haven't driven a full size pickup with a chevy V6, I have. Believe me they have just as much get up and go as the V8 models.) Similarly, if you put a V8 in a honda, I'm willing to bet you won't get better gas mileage than the stock 4 cylinder. Ditto for a big 6 cylinder. All this is because to move a certain mass at a certain speed it's going to take (setting things like gear ratio and wind resistance aside) a specific amount of power. Change the mass you move, and you change the power needs and the best engine for the job. If you can accept that premise, then you can move on to how to extract the needed power in the most efficient manner.