The very act of owning and driving a Mustang says something about the one behind the wheel; so, modification and personalization seem like something of a logical progression from the purchasing decision itself. Deciding on the right exhaust crossover for your application may ultimately come down to the statement you're looking to make about yourself and your pony of choice.
A true dual exhaust system, at least on a V-8, works like a pair of four-cylinder exhaust systems; the left- and right-side exhaust systems are completely independent from each other. While this might seem like a perfect scenario for flow, dual exhaust systems tend to run into problems when uneven pressure pulses from the engine "stack up" in the tubing. These high pressure areas act like a cork in the exhaust, increasing back pressure and robbing horsepower. Installing a balance tube just past the header collectors links the exhaust pipes, providing a cushion for exhaust pulses and reducing pressure-induced restriction in the system.
The X-pipe and H-pipe are both types of exhaust crossovers, named for the shape of the tubing juncture. An H-pipe is just a straight section of tube running side-to-side, connecting the individual exhaust pipes and creating an "H" shape when viewed from the bottom. You can think of an X-pipe as a pair of 45-degree exhaust tubing sections placed back-to-back. Rather than forcing the pressure pulses to expand in one direction, the X-pipe allows pulses from one tube to blend into pulses from the other, and then flow smoothly into whichever exit tube offers the least resistance. So the H-pipe reduces pressure buildup, while the X-pipe blends and redirects it.
The X-pipe does offer a few theoretical advantages over a traditional H-pipe crossover, but it depends on the pipe, the engine and the cam. Turbulence in the merge area can offset the potential gains in pulse balancing, and can, itself, pose a restriction to flow. In general, a more gradual -- say 25 or 35 degree -- bend and a larger merge area will help reduce turbulence while still preserving pulse balance. In one test conducted by 5.0 Mustang Magazine on a 240 horsepower 1992 GT, the X-pipe steadily lost 1 to 4 horsepower below 4,600 rpm; above 4,600 rpm, the X-pipe caught up and surpassed the H-pipe just below the engine's 5,400-rpm redline.
Car Craft Magazine saw quite different results when it tested an X-pipe and H-pipe on its project 1986 5.0 Mustang, which made 333 horsepower with open headers. This time, a 2 1/2-inch exhaust with an H-pipe balance tube reduced horsepower to 323, while the X-pipe exhaust system, amazingly, added 2 horsepower for a total of 335 horsepower. But recall from the last section that the X-pipe lost out at everything below 5,400 rpm; this more powerful engine peaked at 6,300 rpm, compared to the fairly stock engine's 5,000 rpm peak. So, the conclusion here is that, at least so far as the 5.0 is concerned, the X-pipe works best with engines that peak at higher rpm than the stock engine's cam setup would allow.
While most of us would like to pretend that we buy everything on the basis of making outright power, there's no shame in making a few concessions to aesthetics. H-pipes and X-pipes return drastically different results in terms of exhaust note. If you're looking for a choppier and more muscular sound, then the H-pipe's ability to keep pressure waves isolated in separate pipes will give you that classically distinct note you're looking for. Blending the pulses with an X-pipe will give you a higher, smoother and more exotic sound that works better if you have side-exit exhaust tubes. Later model Mustangs, with their high-revving, small-displacement powerplants, tend to sound a bit weird with H-pipes, but sing like Pavarotti with an X-pipe.
