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Modern Trucking Techniques
Exhaustive research
Exhaust systems protect you and your truck; here's a look at the latest technologies

By Paul Abelson
Senior Technical Editor

Your truck’s exhaust system accomplishes three main tasks. It provides a path to move heat and gas away from the engine; it quiets the multiple explosions – actually rapidly repeating controlled burning – resulting from the combustion process; and it moves noxious gases away from the occupants of the cab.

As you know, your engine produces power when fuel ignites in each closed combustion chamber. As the fuel-air mixture burns, its volume expands, building pressure in the cylinder. The heat and pressure force the piston down the cylinder, turning the crankshaft that delivers the newly converted energy to where it does useful work: the drivetrain at one end and accessory drives at the other.

Note that it is converted energy, not created. Energy cannot be created or destroyed. It is converted from one form to another, or dissipated and absorbed. In an engine, latent energy in fuel is converted to heat.

The intense heat is what expands the fuel-air mixture to create pressure. About 40 percent, give or take a few points, is converted to mechanical energy. About 30 percent is absorbed through the engine into the cooling system.

In the cooling system, the heat is transferred to the air stream flowing through the radiator or radiated from the engine to the air under the hood. After forcing the piston down, the remainder of the heated gas flows past the exhaust valve, into the exhaust manifold and on to the turbocharger. There it does useful work before flowing out the exhaust pipe or pipes. Between the turbo and the pipes, a muffler lowers the sound energy created by the expansion of the gas.

That's the functional description of the exhaust system. Like any system in a truck, it is a result of compromises. By understanding those compromises and making appropriate modifications or adjustments, you can realize more useable power and greater fuel economy.

The turbocharger is an air compressor. By spinning at high rpm, the compressor side takes in huge volumes of air, forcing far more into the intake manifold than an unaided – normally aspirated – engine could take in. The compressor is on the same shaft as the turbine, which drives the compressor by drawing heat energy from the exhaust and converting it to the mechanical energy that spins the shaft. The more heat that can be retained in the gas when it reaches the turbine wheel, the more power the turbocharger will have to move more air.

Insulation is the key
There are two types of devices to keep heat in the exhaust system, at least up to the turbocharger. Exhaust components can be wrapped or covered with high-temperature insulation or painted with insulating compounds.

At The Truck Show, Las Vegas, American Diesel and Gas (1-877-427-9800, introduced Q-Shield insulation kits for big bore diesel engines. Using pre-cut and shaped aluminized fiberglass, the kits cover intake plumbing, the charge air cooler, exhaust manifold and tubes and the turbocharger to retain heat.

Testing done on Detroit Diesel bus and truck engines showed significant increases in fuel economy as well as horsepower increases. Q-Shield kits keep air cooler in the intake system, and, by keeping heat in the exhaust, they help control under-hood temperatures.

Another approach to temperature control on exhaust manifolds, pipes and turbochargers is the use of ceramic and metallic coatings. Turbocharged racecars use special coatings to retain exhaust heat in the manifolds going to turbochargers. A Michigan company, KFEi (1-888-207-6210, offers ceramic metallic thermal barriers and liquid ceramic insulation. These treatments insulate the manifold, pipes and turbocharger.

These companies make fuel economy claims based on fleet tests. However, the tests were not certified by the Society of Automotive Engineers or the Technology and Maintenance Council or an independent third party. They may be accurate or even understated, but they were not done according to the approved procedures.

Back pressure refers to the resistance to the flow of fuel within the exhaust system. It takes energy to overcome back pressure, energy that could be used to propel the truck or, if not used, would increase fuel economy.

The compromise with mufflers is that in order to bring sound levels under the limits set forth in Federal Motor Carrier Safety Regulations (Sec. 325.7) and the Federal Code (40 CFR 202.20), fuel flow must be interfered with. Interference induces back pressure in mufflers. One of the major challenges facing muffler manufacturers is to reduce noise without increasing back pressure, at least not too much. Most “stock” mufflers use baffles and perforated tubes to create alternative gas flow paths so that high and low frequency sounds tend to cancel out each other.

Walker Heavy Duty ( makes the Mega-Flow Noisebraker, designed to reduce noise to compliant levels with minimal back pressure. Tests with a Cat C-12 engine indicate 4 percent mpg improvement or more.

Spiral Turbo Specialties, ( uses a metal spiral inside the muffler tube. Their fleet tests produced fuel economy improvements of 6 percent and more. Other benefits claimed for the quiet performance mufflers are higher horsepower, increased torque, lower pyrometer temperatures, quicker turbocharger spool-up and sound levels within one to two decibels of stock mufflers.

Anyone who has ever used a compression brake knows the machine gun sound it can create. More and more jurisdictions are prohibiting compression brakes. Enforcement is usually based on sound, not actual use. Donaldson Co. ( developed the Silent Partner muffler, designed to reduce engine-braking noise by 50 percent or more.

Herman Miller is fleet manager for a large discount store chain, a former chairman of TMC’s Engines Study Group and currently a member of its board of directors. His was one of the first fleets to use Silent Partner.

“Our trucks are a rolling billboard, with our name running 30 feet on each side of our trailers,” he said. “Anything our trucks do reflects directly on us. Those are our customers out there.

“If we run through a mountainside town with the Jake Brake on, we sure can make heads turn, and that’s not always good. But for safety, I want them to use the Jake and leave the service brakes fresh for any emergency.

Donaldson’s Silent Partner lets me do that without annoying people with the noise. Even better, we’re not seeing any loss of power or fuel economy. As long as we don’t annoy people, no one objects to our using our Jake Brakes.”

Straight stacks look great, especially on show-quality trucks; but un-muffled stacks are an annoyance to the public.

That’s why truck builders offer under-cab mufflers with “Y” pipes to split the exhaust. You can have the look of straight pipes without the accompanying noise. With the increasing number of teams on the road, a good muffler helps a driver get a good night’s sleep while the truck is rolling.

Exhaust systems don’t need much attention, but they do need some. When a muffler burns through, the need for maintenance is obvious. But other components – tubes, clamps, elbows and flex pipe – all deteriorate over time. Stainless looks great, but because of cost considerations, aluminized steel is more commonly used in exhaust systems.

It offers better protection from corrosion than mild steel, without stainless’ price penalty.

You should inspect your exhaust system at every PM, every oil change. Remember, exhaust gas contains acids that attack the system from the inside. Look for any softening of the metal and listen for leaks. They can be quite dangerous.

Flex pipes isolate engine-induced vibration from cab-mounted components like mufflers and stacks. Because it is a stainless steel spiral almost continually in motion, it is subject to failure. It is located under the cab, so any leaks can allow exhaust into the sleeper. (See related article on Page 88.)

Exposure to exhaust can contribute to and cause a variety of problems. Safety Currents Express, Vol. 3, No. 12, June 16, reported on a study by the University of Montreal indicating people exposed to carbon monoxide and high noise levels suffered significant hearing loss at high frequencies, especially those with 25 years of occupational noise exposure. Truckers were among the workers identified.

Your exhaust system comes with your truck, and your truck will work quite well with a factory system. But just as many hot rodders start their modifications with the exhaust, truckers can get noticeable performance and fuel economy improvements by upgrading their exhaust systems. But stock or aftermarket, remember that all exhaust systems need maintenance.

Paul Abelson may be reached at