The difference between the off-road H-pipe (left) and X-pipe (right) is obvious. Although this X-pipe is a prototype, BBK/Brothers said they're going to make the weld line in the middle less apparent with their final product. That way the pipe will look cleaner. The end connections are ball socket ends which make for a leak-free exhaust. While the tubing size is the same (21/2-inch diameter), the airflow through the X-pipe's crossover is obviously smoother. Does it matter? We'll find out soon enough.
Between the headers and the mufflers on a V-8 Mustang is the intermediate pipe, more commonly known as an H-pipe. One of the first modifications many Mustang owners do is to replace the stock H-pipe with an aftermarket version, either replacing it with aftermarket high-flow cats and bigger pipes, or removing the cats entirely with a pipe devoid of cats, known as an "off-road" pipe due to the fact that it's not legal in areas with smog inspections. Choosing which type of pipe to run on your car is not as simple as deciding legal or illegal H-pipes, because there are also cat and non-cat X-pipes, which differ from an H-pipe at the crossover.
An X-pipe's supposed advantage is that because the bends are smoother at the convergence, the flow and scavenging are both better, and hence the X-pipe is worth more power. We wanted to find out, so we decided to test 'em both on the dyno. We also wanted to see any differences between the two in both versions: legal and off-road. For our test, we used two cars, both of which belong to the 5.0 Mustang family. Michael Johnson's '90 GT is mostly stock, but has a Holley SysteMax lower intake and camshaft. He tested the cat pipes. For the off-road pipes, we used Matt Rawlins' '92 LX, also known as the Silver State LX. It has gone under the knife a little deeper than Johnson's, with the additions of TFS Twisted Wedge heads, a Cobra intake, 1.7 rockers, and some other goodies as you'll read later. Rawlins got to test the off-road pipes. Read on to see which one made the most power.