Diesel engines, often blamed for disproportionately high levels of toxic emissions, may find their reputation salvaged somewhat by new and improved exhaust filters that claim to reduce emissions by about 25 percent, with the added benefit of increasing fuel economy. Several automotive manufacturers with a history of offering diesel models have introduced similar systems that combine high-powered catalytic converters with diesel particulate filters. Volkswagen's BlueMotion turbodiesel lineup (sold mostly in Europe and South America) and Mercedes' BlueTec diesels are two such examples. In many cases, these OEM components and car models use weight reduction and other streamlining techniques to improve overall efficiency.
Improved exhaust efficiency is critical to getting the most out of your car, and there are even ways to improve an older car (assuming it's in generally good condition -- a performance exhaust system won't work miracles on a total jalopy). A mechanic can usually order identical original replacement parts, but there are also alternatives from aftermarket performance parts suppliers that often give the car a facelift (at least, as much as the car's underbody can be spruced up, anyway). There are several frequently-mentioned benefits of performance exhaust systems. Aftermarket manufacturers are well aware that the auditory experience is critical to a pleasurable drive, and it also lets nearby drivers know that a little extra coin was spent on the car, too. Most manufacturers offer a variety of systems that range in sound from a subtle throatiness to an all-out roar. A good aftermarket performance exhaust can improve throttle response (how fast, and smoothly, the car reacts to the pressure applied to the gas pedal) and give a boost in horsepower. Some systems can even improve fuel economy. Performance exhausts are able to provide this power boost because they're less restrictive than stock exhausts. If the engine can push out more air (which also means it can take in more air) that helps the engine make more power. But contrary to what you might think, a totally free-flowing system will be counterproductive. Aftermarket exhausts are engineered to provide just the right amount of back pressure so that the engine doesn't end up running at sub-peak output. That's why it's important to choose one designed and optimized for your specific vehicle. And before you go ripping out your exhaust pipes, make sure the system you want is street-legal and won't cause your car to fail its emissions test.
Understand, too, that a real performance exhaust system cannot provide mind-blowing benefits for a trivial investment. Beware of cheap products like clip-on or screw-on "performance" exhaust system components or exhaust tips that claim to give the look of a full performance system. They won't help the car's drivability and they may even hinder it by adding extra weight and wind resistance. And besides, those gaudy tips really aren't fooling anyone.