A velocity stack is like a tuned exhaust in a way except it relies more on having a long column of moving air that likes to keep on moving towards the inlet valve even after it's closed. This builds up some pressure to help force air past the valve next time it opens. In other words, it uses the inertia of a column of air to ram into the cylinder. Like the old Dodge Ramcharger engines or the Mercedes 300SL used. BTW, that's why the 300SL engine lays over at a 45 degree angle. It's to give room for the inlet manifold length. These engines were tuned for around the 6000 rev mark and used a manifold somewhere around 30" long but our engines run twice as fast so only need a velocity stack half as long which would be...errr...let's see....30 divided by 2 gives...ummm...I'm not sure but I think it's longer than in the photo

Sorry guys, I can't help myself sometimes

But what a stack like those in the photo will do is stop most of the fuel that sometimes spits out the carb. CL team race engines like the Oliver Tiger use a very long stack because they can't afford to waste any fuel.