Direct fit or universal mufflers
 

Most premium-quality performance mufflers and pipes today are made of stainless steel (usually aircraft-quality T-304), but there are lesser quality grades of stainless that sell for less. Aluminized steel is another option, but it does not have the durability of stainless. Even so, aluminized steel is much better than plain, uncoated or painted steel.
Most performance pipes are formed with mandrel bends which do not narrow the diameter of the pipe where it curves. This reduces unwanted restrictions and keeps the exhaust flowing freely. Most premium mufflers and systems are also plasma or TIG welded to maintain the strength and corrosion resistance of the stainless steel. Some manufacturers also use a process called bulge forming or hydroforming to form steel sheets into molded shapes using extremely high fluid pressure.
Performance mufflers are available in both direct-fit or universal configurations. You will find the greatest selection in universal products, which can be adapted and made to fit almost any vehicle. But the main drawback of one-size-fits-all mufflers is that they sometimes require cutting and welding pipes, and fabricating hangers and mounts.
When the stock muffler is removed, the existing pipe may have to be cut, shortened or lengthened. If the pipes are getting weak, they may also have to be replaced to complete the installation. New hangers are often necessary to support the muffler, or existing hangers may have to be repositioned to provide proper support and keep the system from flexing or rattling. Clearance may be another issue, especially with large oversized muffler cans that are not the same size, shape or length as the original muffler.
Making modifications to a stock exhaust system raises another issue. It may complicate matters if you ever want to go back to a stock muffler. This may not seem like much of an issue when your are installing the muffler or exhaust system. But if and when you sell the vehicle, you may want to remove the parts you installed and replace them with the original parts. So the fewer modifications that have to be made to install a custom universal-fit muffler, the better.
Direct-fit mufflers usually do not require modifications or changes because they are designed to replace the stock muffler perfectly. Most are a simple bolt-in installation that connect to existing pipes and have mounts that line up with the stock hangers. The major drawback with direct-fit custom mufflers is limited availability for less popular models.
Sound quality is something that is harder to control with universal-fit mufflers than direct-fit mufflers, which have been engineered for a specific vehicle application. Different engines sound differently and produce different frequencies and harmonics in their exhaust systems. So don't expect a universal muffler to sound exactly the same on different vehicles. At the same time, some performance mufflers are tunable in that they have removable baffles. You can change the sound by adjusting or removing the baffles.