If you want an exhaust system that will give you more power and better fuel economy, Then DO NOT go with a single muffler. The faster you flow the exhaust out of an engine, the more power and fuel economy you will make. The best possible exhaust system is one where the exhaust dosen't have to go through any chambers, (such as in a catalytic converter or most mufflers) and runs through pipes with a large enough diameter to get a good flow, but still small enough to increase the velocity of the exhaust as it travels through. Most people naturally assume bigger is better, so they go for big 2.5 or 3" pipe. You need to consider how much horsepower the engine makes, whether or not it's boosted, etc. For most street applications, 2" or 2.25" pipe, either by itsself, or using chamberless mufflers such as glasspacks or magnaflow single chamber mufflers, works best, because the pipe is narrow enough that the exhaust is slightly compressed as it exits the header or manifold, and enters the pipe, which increases the velocity of each exhaust pulse as it moves down the pipe. This in turn creates a slight negative pressure wave behind each pulse as it accelerates down the pipe, which helps pull the spent gasses from the cylinder as the exhaust valve opens. Exhaust dosen't burn, so any exhaust gasses that don't exit the cylinder (as when backpressure is present) and mix with the fresh air fuel mixture cause a slower, less complete fuel burn, which in turn cause horsepower and fuel economy to suffer.
So you're thinking, so if you want to get the exhaust out faster, why not use the biggest pipe you can find? As the gasses move into a larger diameter pipe, the flow is actually slowed down as the gasses expand to fill the larger pipe. The same thing happens when you have anything in the flow path, such as a catalytic converter, or muffler with chambers that change the direction and speed of the exhaust flow, the pulses slow and begin to collide with each other, then slow down as they exit the system. Think of what happens when fast moving lanes on a freeway move into a construction zone and are diverted into a single lane where the cars suddenly slow down, it causes a chain reaction and the traffic begins to back up as all the cars slow down.
Not everyone wants their car or truck to roar like a harley davidson however, so for those that want the performance without all the extra noise, I always wholeheartedly recommend glasspacks. Sure, you can make an exhaust loud and crackly with glasspacks, but most folks don't realize that the longer glasspack you use, the more it quiets the exhaust noise. Whereas a short 14" glasspack will be loud and raspy, a really long 28 or 30 incher will have a much more quiet, mellow tone. And they are a good cheap alternative to chambered mufflers when you want a free flowing exhaust. A glasspack is nothing more than a perforated tube the same diameter as your exhaust pipe with fiberglass sound absorbing material wrapped around it between the inner tube and the shell. The exhaust gasses pass straight through from one end to the other without changing direction or flow rate, the same as in most racing mufflers.
Hope that helps clarify things a bit without getting too technical
Dave
