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What you are forgetting is that there is more than one way to downsize an engine other than physically.
A turbocharger mated to a late-closing intake cam will see your fuel efficiency increase, guaranteed. The only sticking point is that the turbocharger has to be more efficient at compressing the intake air than the piston during the early part of its stroke.
Pistons are efficient at high compression levels, but superchargers (whether they be turbo or belt-driven) are often more efficient at low compression levels. Mazda used a Lysholm supercharger on their Miller-cycle Millenia engine, but just as efficient of a turbocharger could be found (drivability would go down, though).
The Miller-cycle decreases the effective volume of the engine, making up the capacity difference with a supercharger. Mazda also downsized their engine, but they were interested in total efficiency.
Also, I don't understand why a turbocharger doesn't decrease pumping losses, at least on the intake stroke. If the intake manifold is above ambient, doesn't that mean the absence of pumping losses?
- LostCause
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