How To Replace Exhaust Tubing

| December 12, 2008


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For many truckers, the occasional faint scent of spent diesel fuel is a pleasant reminder of their occupation and the mechanical power they command. The fragrance is much less appealing, however, when an exhaust leak opens, filling a cab with noxious fumes that foul clothing, carpeting and even cello-wrapped sandwiches.

Cab and sleeper interiors are particularly susceptible to exhaust leaks because most of the system’s plumbing is routed just below the floorboards. Left unattended, these problems only get bigger, adding more noise and odor to a driver’s working environment.

Flex tubing is an exhaust system’s highest mortality part. It usually lasts one to two years, about half the life expectancy of solid tubing. Flex is designed to isolate vibration of components, such as the turbo and mufflers. Over time, though, its accordion-like structure corrodes and stiffens, eventually cracking because of the constant movement of the connected parts.

One big contributor to premature flex-tube failure is misuse, says Clark Lewis, head of product management for Donaldson in Minneapolis. “If an exhaust system isn’t properly designed, truck makers will sometimes use flex to take up the misalignment [of solid tubes],” he says. “Any bends in the tubing will limit its ability to flex, causing it to crack in a very short time.”

Another factor in the longevity of flex tubing – and all exhaust components – is metal quality. Most original-equipment parts, from turbo to tailpipe, are now made from aluminized steel, a combined metal that lacks shine but offers good corrosion resistance. Stainless steel and chromed cold-rolled steel are popular for exterior pipes. Chrome is slightly shinier than stainless, but it doesn’t last quite as long, typically rotting from the inside out. The shortest life cycle is that of plain cold-rolled steel, used for low-end aftermarket solid tubes and some mounting hardware. Galvanized steel, the material of cheap flex, is only slightly better. But in using it, you risk the additional problem of galvanic corrosion, a chemical reaction that occurs between dissimilar metals when they’re clamped together.

Obviously, the purpose of an exhaust system is twofold: route engine gasses to an appropriate location – downwind of the driver – and quell the roar of combustion. These goals, however, are not equally important in the minds of some owner-operators. A portion of the trucking public likes the sound of high-decibel horsepower, preferably delivered through dual straight pipes the size of culverts.

This sort of customization, featuring monster stacks with or without any sound attenuation, is impressive on the truck-show circuit, but it’s not necessarily good for engine performance – or public image. Many in the loud-and-proud crowd use single short mufflers to subdue some of their trucks’ volume. Although the technique might get them within arm’s length of federal vehicular sound regulations – 85 dBA above 35 mph (measured at 50 feet) – it can also raise exhaust backpressure to unacceptable levels, cutting into power and fuel economy.

All original equipment mufflers and most aftermarket replacements have flow ratings indicating their exhaust-handling capacity, measured in cubic feet per minute. The numbers, available from suppliers and manufacturers, enable truckers and shop personnel to closely match mufflers with specific engines, all of which also have flow ratings.

Unfortunately, the process of getting the correct replacement muffler isn’t always exact, says Dale Zuhse, account manager at Fleetguard/Nelson in Stoughton, Wis.

“A person working at an aftermarket parts counter might look in a catalog and choose the first product that seems to match a trucker’s requirement, in terms of inlet and outlet diameter and physical size,” he says. “There could actually be eight or 10 mufflers with those dimensions, but each one has a unique intended application. It’s important that the parts man understands the difference and does a little research. That doesn’t mean he must use the OE product, but he should at least know whether the muffler he’s selling is for a single or dual system, and he should know the engine model and horsepower. He should also know if the original setup had any unique features. This information is going to be critical as we move into catalytic mufflers.”

Given the risk of misfit, it’s probably lucky for truckers that mufflers usually last a long time: four or more years when mounted vertically, as they are on about 90 percent of Class 8 trucks. Horizontally mounted mufflers, often called “weed burners” because of their proximity to the ground, last about three years.

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