Pipe Music

Another incentive for upgrading your 4x4's exhaust system involves simple economics. Performance exhausts provide horsepower and torque increases without breaking the bank or waylaying your vehicle for weeks on end. With routing that follows OE pipes, installation is simple. And, depending on the vehicle, dyno testing indicates increases in ouput as high as 32 horsepower and 32 lb/ft of torque.

 

How is such a dramatic improvement possible with simple plumbing? Some stock systems actually throw up barriers to free-flowing exhaust gases, impeding their flow. Bends in the pipes decrease the diameter and restrict flow; pipe diameters may be incompatible with the driver's need for power

at specific points of the rpm curve. All this creates too much backpressure, leaving spent fuel in the combustion chamber and reducing the healthy intake of air and fuel for the next cycle. But it's not just a simple matter of increasing volume and eliminating backpressure. According to Scott Stutler, DynoMax's national sales manager, some level of backpressure is integral to the dynamics of an efficient exhaust system.

"You have to reduce backpressure and create a strong exhaust flow," he says. "If that pressure is reduced too much, you lose the scavenging effect." In other words, it's important to maintain a

balance between velocity and volume.

Scavenging occurs when exhaust gases exit the combustion chamber, and a vacuum forms. As the intake valves open, that vacuum essentially sucks fuel and air into the chamber. That's a benefit in off-road conditions where speeds are low but the requirement for torque is high.

"At higher rpms, everything's running at a higher velocity," explains Stutler. "The system isn't as dependent on the scavenging effect at high rpms as it is at lower revs."

One misconception in choosing an exhaust system is that bigger pipes translate into bigger performance. The key is to figure out where on the rpm curve your engine spends most of its time. "Since 4x4 exhausts need increased torque at low rpms, smaller diameter pipes are the best choice," Stutler adds.

The general rule is to go up by 1/4- to 1/2-inch larger than stock for the best horsepower increases. DynoMax provides a guide to the optimum pipe diameters for specific applications based on engine displacement.