We left the car at MagnaFlow for about a week, and when we came back we found that after some preliminary testing, the guys had built a really trick system consisting of 2½-inch tubing from the header collectors into the X-pipe, 3-inch out of the X flaring into 3½-inch tubing running for about 26 inches before necking back down into a pair of 3-inch mufflers with turn-downs. The theory behind this design is that it will keep the velocity of the exhaust gasses moving quickly through the headers into the X-pipe to maximize the scavenging effect, while the larger-diameter tubing ahead of the mufflers prevents the gasses from stacking up as they pass through the mufflers to avoid excessive backpressure in the system.

    Sounds good, but would it work? To find out, we tested three exhaust system configurations on MagnaFlow's in-house Dynojet. For a baseline, we ran the car with open headers and saw 333 hp at 6,300 rpm and 304 lb-ft of torque at 5,200 at the rear wheels. Next, we ran a 2½-inch bolt-together system consisting of a BBK short off-road H-pipe designed to fit the company's full-length headers connected to a set of race-type 2½-inch welded mufflers with turndowns. Our Mustang's carbureted 302 didn't like this combo, as power fell to 323 hp at 6,300 rpm and 295 lb-ft of torque at 5,200. The increased backpressure also richened up the air/fuel mixture by about two carburetor jet sizes. Finally, we swapped on the stepped X-pipe setup and were impressed to see power levels equal to the open headers: 335 hp at 6,300 hp and 302 lb-ft at 5,200. That's a gain of 12 rear wheel horsepower, although it turns out we weren't really giving anything up or gaining anything extra from the open headers. So the car's happy, and our ears are even happier!