gas mileage Mechanical vs Vacuum Secondaries

kielbasa said

I've always heard that vacuum secondaries get better mileage, wondering if anyone knows how true that statement is?
Just curious, I've been trying to figure out why that would be the case (assuming steady state driving with a constant throttle opening)...

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As noted in the answers... Carbs can come a variety of ways, from non-adjustable emulison bleeds to adjustables in 3,4,5 steps... If there is a flat road, and a steady right foot, and two identical carbs (meaning both same size throttle plates, same booster design, same manufacturer, same either non-adjustable, or adjustable and tuned by someone that knows how to tune...) there will be no difference in milage in the adjustable tuned ones, and minor advantage in the vacuum secondary if we're talking about Holley's because of the sizing of emulsion bleeds. PVCR does nothing in steady state, and any increase in size will mean a more sensitive power valve. Not more fuel per se, but more sensitive. If you change any of the parameters the whole "apples-to-apples" deal goes out the window. IMO, vacuum are for engines with cams of moderate duration. DPs are better suited for engines where horsepower is a top prerequisite rather than economy.