[Diesel_Dave]

Correct, a non-lean burn fuel injected gasoline engine will adjust the fueling based on the O2 sensor readings, so as to keep the AFR right near 14.7:1.

On a turbodiesel with fixed geometry turbo decreased backpressure will give you a higher AFR. On a turbodiesel with a variable geometry turbo, it depends on how the controls work. In many cases the VG will adjust to keep the AFR the same, albiet at a lower exhaust manifold pressure, so the pumping losses go down.