Velocity stacks have become increasingly popular for air inlets for high performance and racing engines used in cars, motorcycles and boats. Their elliptical horn-shaped inlet smooths airflow, eliminates turbulence and delivers higher efficiency than plain straight tube end openings. This results in greater power output for these engines since they have a much easier task of gathering their combustion air.
Velocity stacks are known as elliptical horns and bell mouth intakes in the scientific realm but are called velocity stacks in the racing universe. Here is a nifty unit for a motorcycle, and the best part is that it is completely possible to knock off in the home workshop.
Measure the inlet diameter of the carburettor or air inlet. This basic dimension will determine the rounding size required in the inlet horn. This design assumes that the velocity stack will fit onto the end of the intake opening. If not, make a small adaptor, drawing from the base of the stock air filter housing. The main idea is to have a smooth transition from the end of the velocity stack into the front of the carburettor with no indentations or blockage.
Most commercially available velocity stacks are made from stamped metal. They come from drop forges with tens of thousands of pounds of force available. Another alternative would be to spin them from aluminium blanks with highly specialised equipment. Both methods are difficult and prohibitively expensive. You can rout the shape into very sharp looking black ultra-high molecular weight polythene (UHMW) panels. Purchase a 1-foot square, 1-inch thick piece of material from an industrial supplier. You will also need a 1-inch radius round-over router bit. Purchase round or oval pleated air filter elements that are about 1 1/2 inches high and about 4 inches in diameter.
Cut the sheet into four 6-inch square pieces. Drill a hole the size of the carburettor air inlet in the middle of only two of them using a wood-boring flat blade-type of drill bit. Rout out a smooth round-over in these two holes that should render a smooth horn. Put the plastic in the freezer overnight before routing because it is difficult to rout this material when the router bit is hot. Mount the horn inlets to the carburettors (assuming the motorcycle has two carburettor inlets) to check for fit. Now either make ovals or round the corners and rout the corners. On the two pieces without bores, just rout the outer edges. Reassemble the horn inlet velocity stacks to the carburettors. Drill two 7/32 holes on a concentric circle just a little smaller than the inner diameter of the air cleaner element. Drill two 1/4-inch holes in the same relative position on the air filter cover. Thread two 1/4 inch by 20 thread per inch pan head Phillips drive stainless steel screws through the 1/4-inch holes through the centre of the air filter and into the velocity stack plate.