Adding an aftermarket exhaust system is the most common first modification made to mustangs. Aside from adding horsepower, they give your mustang that well known deep rumble!
CAT-BACK EXHAUST SYSTEMS
Prices for cat-back exhaust systems range anywhere from $100 (for a GT take-off exhaust for a V6 Mustang, pictured below:)
All the way up to the higher-end of the spectrum with a Borla Cat-back system at around $850, shown below:
If you want to truly complete your exhaust system, then aftermarket headers for your mustang are a must. There are two options for headers: short tube, and long tube. The long-tube are definately preferred, and can yield great performance gains (the price difference is around $75). (but can sometimes be a little more difficult to install). Here's a picture of short & long tube headers (respectively):
Altogether, all of these exhaust product work be making your exhaust system less restrictive... and hence gain horsepower. Depending on where you live, one of the easiest ways to gain horsepower is to remove your catalytic converters (CATs). The 'CATs' do not serve any real purpose except reducing pollution. You just have to find the right mechanic who is willing to do it for you. (but beware, after removing your CATs, your 'check engine' light may come on.... that is normal, and can be fixed with a $20 O2 Sensor eliminator off eBay :) Be careful, altough while modding your exhaust and allowing your exhaust to flow easier... if you go too far, it is possible to actually lose some low-end torque due to a lack of back pressure, so choose your system carefully.
For those of you who really just want to add a little more rumble to your mustang, and are not necessarily serious about performance modifications, then the easiest modification you can make is to change out your muffler(s). A good muffler can be had for about $70, the most popular (and good sounding muffler) is a MAC Flowpath ($70):
The second muffler shown above is a higher-end, Borla muffler... which can cost over $200. I would personally reccomend the decently priced mufflers like the MAC Flowpath, or Flowmaster... because there is nearly no difference between the $70 muffler and the $200 muffler. Some people even prefer the cheaper ones.
For some real power, check out our Mustang Power Adders.