Originally Posted by Frankie

If we're talking about the same exact engine, if you have more RPMs, you're going to use more fuel. The vacuum differential between the pressure in the manifold and the outside pressure changes based on RPM. With more RPM, you'll see less pressure in the manifold, thus the engine is going to suck harder and use more fuel. That fuel is gonna come from either the jets in the carb or the fuel injectors, but it's still gonna come out. You can't raise RPM without consuming more fuel than at low RPM.

Think about a garden hose. It's always open the same amount, but if you only give it so much pressure, by turning the spout on halfway for example, only so much water comes out. The higher pressure the water, the more water is going to come out of the same hole. Same goes for air sneaking past a slightly open butterfly. When you have more pressure coming in, due to a pressure drop in the intake manifold, you're going to be able to squeeze more air in.

When you WOT an engine, do you think it gets maximum air instantly? Nope. You've gotta wait for the RPM's to build up before it realy starts to max out the throughput of that venturi. That's why carbs have accelerator pumps