Diesel-particulate filters (DPFs) needed to meet EPA 2007 emissions limits have posed a packaging challenge to OEMs who want to keep behind-cab areas "clean" so bodies can be easily installed.
Manufacturers have designed as many as a dozen exhaust configurations for each of their medium- and heavy-duty truck models, shrinking the gear to keep frames clear and to accommodate varying locations for battery boxes, and fuel and air tanks. Designs include horizontal and vertical mountings for DPFs, which double as mufflers, and for tail pipes. Vertical pipes run up the right or left side of the cabs, depending on what up-fitters and users need for various applications.
The shorter the wheelbase and the more axles a truck has, the harder it's been to design the new exhaust systems. OEMs had to lengthen some of their shortest wheelbases by 2 inches to provide enough room for the exhaust equipment and piping.
In general, conventional-cab trucks have relatively unobstructed behind-cab areas, but sometimes smaller fuel tanks were needed to make room to mount the DPFs under cabs. This is also what's been done for trucks with multiple lift axles that fill the frame between the rear driving axles and the cab.
Some low-cab-forward trucks need a few more inches behind the cab to fit everything. Long-wheelbase conventionals and low-cab-forwards could usually carry everything alongside or below their frame rails, for use with van, flat and other high-sill bodies. But deep-sided beverage and sweeper bodies need compact systems confined under or directly behind the cabs.
Smaller trucks sometimes have DPFs under the cabs and alongside transmissions. This impedes the mounting of PTO gearboxes on a transmission's right side. Dodge provides instructions on how to remove an exhaust pipe to mount the PTO. Other builders advise using a left-side or rear PTO mount.
Correctly spec'ing a truck for the application is now even more important because of the prohibition against changing the exhaust. DPFs are part of the new engines' emissions controls, and OEMs must certify them with the Environmental Protection Agency.
To keep the new exhaust system legal, a kit designed and certified by the OEM must be used. The cost of parts, plus installation after removing the original parts, will run $6,000 to $10,000.
And don't even think about scrapping the original system and slapping on a regular muffler. The truck's DPF is wired into the engine's electronic controls, which will register that wires have been cut and refuse to operate the engine. Trying to get around this by reprogramming the controls probably won't work, and if it does, it'll void the truck's warranty.