July 31, 2002

Exhaust systems: more complex than they appear
by Jim Kerr

The exhaust system on your vehicle is probably the last thing on your mind, unless there is a loud rumbling sound from the rear of the vehicle or something is dragging while you drive. The manufacturers, on the other hand, spend a lot of time and money putting those pieces of tubing beneath the floorpan. Deceptively simple, there are few parts to a complete system: manifolds, catalytic converter, muffler, pipes, and hangers, but if it isn't designed right, your vehicle drives like a brick. Get it right, and horsepower appears magically. What are the secrets?

Modern exhaust systems need to do four things: remove the exhaust gases from the engine as efficiently as possible, reduce the noise of the exhaust pulses to acceptable levels, reduce harmful emissions in the gases, and direct the poisonous gases away from the passenger compartment. Any series of pipes can direct the exhaust away from the passenger compartment, so this is the simple part. Getting those gases out efficiently is a challenge.

To make horsepower, the engine needs lots of air and fuel in the cylinders, but unless the exhaust gases are removed, the air/fuel mixture can't get in. Most manufacturers use cast iron exhaust manifolds bolted to the engine to direct the exhaust gases out. Some vehicles are using steel tube type manifolds to allow the exhaust gases to flow easier. Although these "headers", as the aftermarket calls their high-performance steel tube manifolds, do allow more flow, they have some drawbacks. Cast iron manifolds are easier to manufacture than steel tube ones, and their durability is very high. They also don't "ring" like steel tube type manifolds do. Anyone who has driven long distances on the highway with steel tube "headers" can appreciate the quietness of cast iron manifolds.

One last advantage of cast iron manifolds is their ability to hold heat. Catalytic converters need to be hot before they start working to reduce emissions, so they are placed close to the engine so the exhaust gases can warm them up (somewhere in the range of about 600 degrees F). Cast iron manifolds keep the exhaust gases hot until they reach the catalytic converter, so the converter starts working quicker.

Converters have gone through several design improvements over the years. Honeycomb ceramic cores are coated with rare metals (platinum, palladium, and rhodium) to cause chemical reactions as the exhaust gases pass over them. Today's converters have more but smaller holes for the gases to pass through. Reactions occur faster and there is less restriction in the exhaust system. With a catalytic converter or the vehicle, exhaust noise is already reduced, so mufflers are designed to tune the exhaust note for a pleasant sound.

Some vehicles sound powerful; great in a sports car but not in a limousine, so mufflers are designed for the image of the vehicle. Some Nissan cars use a muffler that provides both. A spring-loaded flap closes passages in the muffler at low flow for that quiet sound, but step on the throttle and the higher exhaust flow opens the flap for improved performance.

Improving exhaust flow and performance is accomplished by designing a system with few corners in the pipes, less restrictive mufflers, and smooth transitions throughout the complete system. Aftermarket high performance exhaust manufacturers build many "cat back" systems. The exhaust system from the rear of the catalytic converter to the tip of the tailpipe is changed with low restriction parts. This has no effect on emissions, but can make the exhaust note too loud for some drivers.

Mandrel bends on the cat back system pipes allow maximum exhaust flow through bends in the system. These smooth, full-diameter bends are too expensive for original equipment applications. Original exhaust pipes are manufactured with equipment that "necks down" or reduces the diameter of the pipe where it bends. Cheap but not as efficient.

The most innovative exhaust system award on a production vehicle has to go the Honda's CBR954RR motorcycle. The 954 cc four-cylinder engine uses a computer-controlled titanium rotary valve in the exhaust pipes to change the path of exhaust flow at different rpms. At low rpm, all four cylinders exhaust through one tail pipe. At middle rpm, each pair of cylinders exhausts through separate mufflers. At high rpm, all the exhaust pipes are connected together for maximum flow through both mufflers. The results? 154 horsepower and lots of torque too!

It may be a long time before we see this type of maximum performance exhaust system on passenger cars, but the possibility exists. There is much more to exhaust systems than meets the eye.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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