Vacuum And tuning Help

dgc333 said

Because 4bbls are used in performance applications where the goal is to have as close to zero intake manifold vacuum at WOT as possible they are rated at a pressure drop across the carb of approx. 1/2" Hg which is close to where they are used. 2bbls are typically not used in perfromance applications and some restriction is not as important and are rated at a pressure drop of approx. 3"Hg.l.

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Dave the numbers are correct, but the reason for the differnce is not. Back in the day, before 4 bbl carbs, when a 1 or 2 bbl carb was put on a flow bench to determine the cfm rating (as you said) a 3 inch pressure drop was the standard used. When 4 bbl carbs came into being, the same flow bench could not flow enough air to get to a 3 inch pressure drop, with the increased air flow capability of the 4 bbl carb. So rather then build a lot of larger flow benches, it was easier/cheaper, to just change the measurement standard to 1.5 inch pressure drop. Just a FYI :)

If you put a 600 4bbl carb on a flow bench, that could develop a 3 inch pressure drop, it would flow a ton more then 600 cfm.