The Velocity stack may be alternately called stub stack, air ram, or ram horn. The factory called it "venturi" when they were incorporated into the air cleaners on the later MGA Twin Cam engines. For most applications the intended purpose is to smooth out the flow of air entering the intake of the carburetor. Reducing turbulence here may result in a small power gain (but not nearly as much as most people might like to imagine). When you see these on a race car, and the horn is more than 2 inches long, it is also being used to tune the length of the intake runner for maximum air ram effect within a specific range of engine speed.

At 14:40:18 UK time 05 September 2005, Carl Heideman of Eclectic Motorworks wrote:

I've dyno tested stub stacks and velocity stacks for articles in Grassroots Motorsports and Classic Motorsports magazines and can hopefully shed some light on this.

Several companies make velocity stacks, which replace and eliminate the air filters. Most of the manufacturers are copying other products improperly and therefore produce velocity stacks that look cool but don't work. I have only found one good manufacturer, TWM, that makes proper units that do work. Moss, APT, and TWM sell them in two versions, short and long. The long ones give you about 2HP more at the wheels, but won't clear the brake booster on a late MGB. The short ones will, and give you about 1 HP at the wheels. The TWM units are extruded aluminum with a rolled radius around the bell. Cheaper units (that have no effect or cost HP) are stamped with a sharp edge around the bell.

K&N and APT make stub stacks, which fit inside the stock or aftermarket air filters. I've found that they usually give about 1 HP at the wheels--not great, but every HP counts. Additionally, they will give a smoother A/F graph on the dyno, which translates into smoother running on the street. I highly recommend them for any car.

- Carl