The old style of performance muffler is the "turbo" muffler. These are just reverse-flow mufflers with optimized airflow. They reduce noise well, but their flow is poor compared to even chambered mufflers. Nowadays, most el cheapo mufflers are turbo mufflers. There are a few name brands that still produce turbo mufflers, so make sure you know what kind of muffler you're getting before you buy!
Next up the food chain are chambered mufflers. These replaced turbo mufflers as the high-end performance mufflers and are a fairly old design. They try to maintain a somewhat-straight exhaust path whilst also having resonating chambers to cut down noise. These are generally louder than turbo mufflers and around as loud as straight-through mufflers and flow a tad better than a turbo muffler. Their quality varies a lot because there are all kinds of wild chambered designs. Here are a few examples:
Flowmaster super series muffler. Probably one of the better chambered mufflers.
A crazy Corsa chambered muffler. The long, thin chambers are designed to reflect sound back just right to cancel the sound waves out.
Hooker aerochamber muffler.
The most modern and best-flowing muffler design is the straight-through perforated core design. Several companies only produce these types of mufflers nowadays, such as Magnaflow, Apex'i, and HKS. These mufflers have a straight pipe inside them that has perforations which allow sound to pass through and be absorbed by fiberglass wadding/stainless steel wool packed in the can. Of these, there are several different designs:
-louvered-core mufflers. These are often a cheap alternative to the high-end perforated-core mufflers. They use louvers instead of drilling out perforations. Sounds fine, except that the louvers create turbulence in the exhaust flow, meaning they're very restrictive.
This is what I'm talking about. Don't buy one of these.
-louvered/baffled glasspacks (aka cherry bombs). These are just bottle enclosures filled with fiberglass and using a baffled or louvered core. They have the same turbulence problem that the above mufflers do, and they are generally pretty loud. Magnaflow actually offers glasspacks with a proper perforated core. These are fine but will probably be pretty loud.
-Offset. These mufflers are straight-through but inside the can the perforated pipe is at an angle. The advantage of these mufflers is that they get straight-through flow but at a lower noise level. Unfortunately they are big and heavy so they aren't suitable for all applications.
These are your best option if you can find a place to put it and don't mind the weight, because they will have the best noise/flow ratio. Magnaflow, flowmaster, dynomax, and a few other companies offer these.
-X/hybrid designs. There are a few interesting straight-through-ish designs that sacrifice a bit of flow for sound deadening. Really, the cutaway pictures show better than my words:
SSR ultraflow X
Borla XR-1. Pretty neat design if you ask me.
-Straight-through perforated core. These are the king of flow. Their difference in flow versus a length of straight pipe is nearly negligible. They are also small and light. Unfortunately, they are also loud, but a big fat and long can goes a long way towards fixing this shortcoming. The best straight-through mufflers have stainless wadding closest to the pipe, as fiberglass will get burned out over time. If you buy one of these, get the longest and largest muffler that will fit. The bigger the can, the more sound that is absorbed, and there is no performance penalty for a longer muffler.
Here are a few good straight-through mufflers:
All of Magnaflow's mufflers are very good. I would go so far as to say downright best bang for buck, period.
Some of GReddy's mufflers are good stuff.
HKS hi-power. For $300, it better be good, and it does deliver.
Other companies that offer straight-through mufflers are Apex'i, DC Sports, and Borla.
Type of piping is a vital decision in the exhaust design process. The right type of pipe will ensure a long-lasting exhaust with good torque and power.
Material- If you want to go cheap, don't just get mild steel piping. It will get ugly and rusty almost immediately, and will rust out eventually. Aluminized steel is a better alternative and will keep decently. But really, just get stainless. It isn't THAT much more expensive and will last basically forever.
Now, there are two different types of stainless steel. 409 stainless is the cheaper option. Over time it will fade to a dull brown color and does corrode very slightly over time. It also cannot be polished. There's no structural danger, it just isn't as pretty. 302 stainless doesn't fade and can be polished to a mirror finish but is more expensive.
Bends- OEM exhausts use crush-bent tubing. Crush-bent tubing is cheap and easy to shape, and as such, cheap neighborhood exhaust shops are also going to use crush-bent steel in a custom exhaust. However, crush-bent piping is very restrictive. I have been told that a 90 degree crush bent pipe's flow is reduced by 50% when compared to a similar mandrel-bent pipe. That's a big difference. Don't cheap out here. A smaller mandrel-bent pipe is better than a bigger crush-bent pipe. Remember, exhaust velocity is just as important as absolute flow.
Size- compromise in size is very important. Too big of a pipe is going to destroy torque, but too small will choke the car off. On smaller engines, 2" piping is just fine. i4 people would probably be just fine on 2" pipe so long as they stay N/A. 2.25" pipe is also a good N/A sweet spot for both i4 and V6 people. However, if you have plans for F/I in the future, a 2.5" exhaust is a good compromise. Plenty of people here run big power with a 2.5" exhaust with no trouble. Any more and you'll lose a lot of torque while you're still N/A. If you already have a ripping turbo setup, then 3" is the size for you. Of course, if you are running a turbo, you probably already know that and know more about exhausts than me. heh.
sells mandrel-bent piping of all sizes and material at good prices. I got 6 302 stainless mandrel bends and 4ft of 302 straight pipe for around $280.
Honestly there's not too much to be said about this aside from get the right stuff. Getting mild steel hangers and flanges will defeat the purpose of expensive stainless piping. Hangers come in a couple varieties. You can get hangars with plastic bushings or pre-bent stainless hangers from magnaflow. You can then weld these to either the piping or quick-remove exhaust clamps. You can also get stainless steel rod (I think I used 3/8" rod) and bend it yourself. If you're getting a stainless exhaust done by a shop, I would buy these myself. Most shops don't have stainless hangers or flanges. Don't forget gaskets either! Exhaust shops ought to have various sizes on hand.
No, I'm not going to write a guide on welding. I don't know that much about welding, I just know how to MIG weld and that's what I do. However, there's a few things you have to take into account when you take your stuff to the exhaust shop:
-Type of welder. Make sure to ask your shop what type of welder they use. The cheapest welders are flux-core arc welders. These do not work for stainless steel, and even for mild steel they're not any good. If the shop tells you that they use an arc welder or won't tell you what kind of welder they use, I would recommend finding another shop.
Next is MIG welders. So long as you know what you are doing, MIG welders work fine for stainless. The welds won't be super-pretty and they will get slag on them that you would have to grind off, but MIG welding stainless is functionally fine.
However, best is TIG welding. In the hands of a skilled welder, TIG welds on stainless will be strong, pretty, and should be slag-free.
-Type of wire. If you use stainless steel, its crucial the welder use stainless steel wire. Otherwise the welds will be weak and rust away, which defeats the purpose of using stainless steel pipe. If you use aluminized steel pipe, you will also have to paint the welds or something, because they will rust much sooner than the pipe.
That's more or less all I've got. Hopefully if you've read through this novel you'll be well on your way to building a good custom exhaust. Happy modding!