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Exhaust X-Pipes

This simple mod nets more power and great sound

It's a proven fact that joining the exhaust pipes of multi-cylinder engines improves power and torque over a broad rpm range. A "tuned" set of exhaust headers (with pipes of calculated diameter and length) actually assists sequential firing cylinders to scavenge spent exhaust gases, making room in the cylinder for an undiluted, fresh charge of fuel and air. The velocity of exhaust gases in one header tube causes a low-pressure area in an adjacent tube, literally sucking the exhaust out of that cylinder. The more cylinders, the greater the effect. The late and great Smokey Yunick built several eight-into-one header collector systems that showed increased power on the dyno. Unfortunately, such a system is virtually impossible to package on a typical sedan type of vehicle.

Early H-Pipes

That which works with headers also works downstream with dual-exhaust systems. Unconnected dual exhausts on V-8-powered vehicles act as two independent four-cylinder engines and exhaust systems—and we all know what four-cylinder engines sound like. Back in the '60s, performance engineers at the Big Three car companies discovered that running a balance tube between the two branches of the exhaust system (subsequently called an H-pipe) would broaden the torque curve as well as cut down the interior noise caused by resonance in the exhaust system.

Fast-forward 30 years to the '90s, where a revolutionary development in exhaust technology has taken the balance-tube H-pipe to the next level. Instead of connecting the branches of a dual-exhaust system with a restrictive 90-degree connection, the X-pipe union simply flows the two branches together without changing the direction of exhaust flow, and one side can easily draw from the other for a sizeable reduction in back pressure.

Many companies are now producing X-pipes for most popular performance cars. They are typically available in off-road versions (without catalytic converters) or as 50-state legal models with proper emissions equipment. Most are designed as simple bolt-ons that require less than an hour to install with basic hand tools. The photo shown here is an X-pipe for a 1999 Mustang Cobra. It's an off-road version that deletes two of the car's four catalytic converters but retains provisions for the oxygen sensors.

For those who have vehicles where no X-pipe is available, a custom X-pipe can be made by any competent muffler shop by purchasing the X-pipe union and bending up some exhaust tubing to make it fit. If you are fabricating your own X-pipe, a tip is to place the X-pipe union as far to the rear of the car as physically possible—it will make more power that way.

More HP

What can you expect from this modification? A major performance-car magazine did a test on a 350 small-block Chevy engine and the results were fairly spectacular. Torque at 3,000 rpm jumped by 20 lb.-ft. and horsepower increased by 13. These are increases in the useable part of the rpm range and should not only result in quicker acceleration but better fuel economy as well.

What we haven't commented on is the sound. Connecting the branches of the dual exhaust in this manner makes for the sweetest-sounding V-8 you can imagine. Instead of hearing the firing impulses of two disconnected four-cylinder engines, you hear all eight cylinders out of each pipe. What a difference!