Chris,

I basically agree with your first paragraph. SU or Weber or whatever, the larger the throat, the less air velocity you get for any given vacuum condition, the less well fuel will get pulled out of the jets, and the less well the fuel will atomize.

Volvo didn't pick 1-3/4" SUs because they couldn't get anything larger. Unlike Webers (which have replaceable fixed venturis in various sizes), SUs are a variable venturi design. The piston lifts only as far as necessary to maintain optimum flow velocity, in theory at least (perfect they ain't). Going to larger SUs just means the piston won't lift very far.

I don't understand why using multiple motorcycle carbs feeding a D-jet type plenum would accomplish anything. One carb of suitable size would be sufficient. The VPD supercharger essentially works like that -- the only reason a single 2-barrel Weber is used is because it's readily available and extremely tunable.

--Phil