Carb theory: What made the spread-bore designs so effective?

[mikedc]

I'm talking about the Quadrajet & Thermoquad, compared to the earlier Holley & Carter square-bore designs.

The spread-bores ran well with good gas mileage. Were the gains mainly coming from the staggered barrel sizes itself? Or was it mainly other factors in the design, or factory tuning? The Thermoquad had the cooler-running plastic bodies. They both had other differences from the old Carter (Edelbrock) design but the basics were similar.


Holley has been selling a spread-bore variant of their traditional square-bore carb for years. I don't have experience with those. Do they offer much gains (mileage & part-throttle) over the standard square-bore series?



Holley also sells the 'Street Demon' model (totally different from the old racy Demon carbs). It's sort of an aftermarket version of a Thermoquad, complete with plastic body. But it's crammed onto a square-bore intake footprint. Not many people run those but the buyers seem to like them.


I realize that other factors mostly drive people's carb choices. The different intake manifolds. The spread carbs were uncool smog-era items and the aftermarket neglects them. Traditional Holleys work well on the dragstrip and they have a cool image. Edelbrocks are cheap & simple and they don't leak. Etc.