Changes in the Wind

For componentry so heavy on twists and turns, an exhaust system’s job is pretty straightforward: to deal with the residue of the combustion process. From the combustion chamber and out the exhaust valves, through the manifold or turbocharger, and then out the pipes, those hot and nasty gases and particulates-what the combustion explosion couldn’t incinerate-are sent packing.

But so is noise. The exhaust system has to control sound, too, a job that will become more important as governments begin more tightly regulate truck noise. Creating a system that can manage noise and combustion byproducts quickly and efficiently is a challenge. The various bits of piping have to be accommodated by the vehicle’s body and frame, too. It only takes a few too many elbows or silencing baffles to restrict exhaust flow to a point where the engine is working harder than it should to purge those gases. When that happens, power and fuel economy-and, ultimately, engine life-suffer. Rest assured, truck manufacturers reach the best compromise possible in their standard exhaust system specs. But you may have a special application of some sort, or you may simply want to search for lower restriction in hopes of improving fuel use or performance. The truck OEM may have options that suit you, or you may travel to an independent shop in the aftermarket to have a system custom-designed. Especially in the latter case, you would do well to understand the basics of exhaust system design first.

RESTRICTIONS

Getting rid of a diesel’s combustion waste efficiently demands that the exhaust system-from the turbo outlet on back-impose as few restrictions as possible. Those restrictions are collectively called “back pressure,” measured in inches of water or, more likely, mercury. The measurement changes from engine model to engine model, and will change again as the exhaust system itself is modified. It’s impossible, incidentally, to have zero back pressure unless you have the exhaust exiting directly from the turbo. A typical measurement-and maximum-is three inches of mercury.

When you have more than the engine’s predetermined restriction limit, the piston takes on more work as the engine pumps harder to sweep the cylinder clean after combustion. If the engine’s working harder-to provide the same amount of output-it’s using more energy to create the power that the driver’s foot is asking for. So fuel economy suffers.

It doesn’t stop there. Various internal temperatures go up, combustion suffers, and the engine is overfuelled-and maybe creating a little smoke. That can cause carbon buildup on the injectors and on the inside of the muffler and other exhaust system components. With fouled injectors, you get progressively less efficient combustion. With a fouled muffler, you’re just increasing restriction again. You’ll see a performance loss within three or four months. In fact, when mechanics are faced with a low-power complaint, they usually look first for signs of this kind of restriction.

A larger muffler-or none at all for those who choose a straight-pipe option-will reduce back pressure while also being more effective in controlling noise. Back pressure will also be chopped by minimizing the bends and elbows of the piping, if you can do it, because a straight path for those gases is the one of least resistance.

In fact, it’s unlikely to be worth the significant expense of redesigning your exhaust system to minimize back pressure. You can’t make enough difference, even if you manage to reduce back pressure from three inches of mercury to two, for example. That’s a huge reduction, but in performance terms your drivers will only notice the change if their driving style is flat out in every gear at every moment.

For trucks and truckers who drive rather more sedately, the difference will be invisible.

REDUCING NOISE

The science of noise is complicated, but remember two key points: one, the human ear is a sensitive instrument, able to hearing noises at levels from zero to 120 decibels (dBA); and two, that’s what’s called a “logarithmic” progression that actually represents an increase of 1 billion times.

A typical truck with straight stacks decelerating with the engine brake on will produce a noise measured at over 101 dBA. In the same mode, a truck with an inexpensive aftermarket muffler will typically produce as much as 90 dBA, and with a high-quality OEM silencer it will be around 84 dBA. Note that the difference between a truck operating at 90 dBA and another at 100 dBA is twice the noise. Normal conversation, by comparison, is 70 dBA, a private office is 50 dBA and a soft whisper at five feet is 35 dBA.

Local noise laws demand quiet, but it’s also a matter of public image. It’s not only the outside world that’s affected. Exhaust noise enters the cab as well.

The kind of muffler you have will also affect restriction. Most mufflers use a combination of baffles and perforations to deal with low- and high-frequency noise, respectively. You can buy aftermarket mufflers that just use perforated pipes, doing away with baffles entirely, and thus reducing back pressure significantly. It’s a matter of mild controversy, however, and it’s worth noting that truck makers do not, as a rule, offer exhaust systems with perforation-only mufflers.

An interesting twist on silencing is the ongoing development of the so-called “active” muffler. It’s a microprocessor-controlled device that “reads” the engine’s noise and then creates so-called “anti-noise” to cancel out the original sound using microphones and speakers. One of the advantages is that there’s no muffler and thus less back pressure. THE SHAPE OF THINGS

The most common configuration on heavy tractors is the vertical muffler with vertical outlet running up the rear corner of the cab. It can present clearance problems, but it keeps those nasty gases far above the noses of motorists and pedestrians. Usually it’s a single pipe and muffler, but with bigger engines the vertical arrangement often uses dual stacks. There’s only one pipe out of the turbo, but when it splits into two, restriction will drop if the muffler and piping are the same size as with a single stack. One thing to look for with this design is the relative strength of the bracketry that supports it.

The other main design choice is a frame-hung horizontal muffler and outlet pipe. It solves many clearance problems and saves some weight because of less piping and smaller mounting bracket requirements, but it emits exhaust fumes at street level and is exposed to road debris and snow. Trailers will stay clean, however, unsullied by exhausted carbon.

Then there’s the horizontal frame-hung muffler combined with piping to a vertical outlet, often used on dump trucks and midrange machines with van bodies. With only pipe running up the cab’s rear corner, instead of a fat muffler and heat guard, space and weight are saved. Again, however, there’s exposure to road debris at very close proximity.

In terms of exhaust piping, a five-inch diameter is standard on heavy trucks, three or four inches on lighter vehicles. That’s all very standard stuff, but beware of on-the-road repairs in which smaller piping may be used. It’s not unheard of to see four-inch pipe replace a five-inch spec, resulting in a significant increase in exhaust restriction.

AFTERTREATMENT

Only a few medium-duty diesels, and no heavies, require exhaust aftertreatment by way of catalytic converters to meet federal and U.S. particulate emission standards. Mounted in front of or behind the muffler, and sometimes integrated into the muffler, they’re expensive at the time of the truck’s purchase and expensive to replace.

Things will change, however. With more stringent U.S. emission regs coming in 2002, both medium and heavy-duty engines will probably need some new sort of exhaust aftertreatment device onboard. Diesels will likely be equipped with exhaust-gas recirculation devices, which will not only cost more but will also increase corrosion, oxidation, and acidic contamination of the engine’s lube oil.

It remains to be seen how this will play out, but one thing’s certain: truck exhaust systems will only get more complex.