From http://jalopnik.com Velocity stacks are the lace teddies of high-performance engines. So when Chicagoland's Fluid Motorunion wanted aluminum horns for their project (a BMW M3, pfff) they ran to the AutoCad and created a mockup. Then, they carved out these nasty beastys in three dimensions.
The shop did all the conceptualizing, designing, cutting and assembling in-house — using 6061-grade aluminum, a part-magnesium alloy used in both automotive and aircraft construction. An Aquamist water-methanol injection system runs from a tank at the rear to a manifold that leads to eight direct-injection nozzles. These nozzles sit at the tips of eight arms that reach over the stacks (those are those gothic, flying-buttressy things). The designers wanted it to look mean. Mission accomplished.