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THE MUFFLER
The key part of your exhaust system is the muffler. The muffler is the can at the end of your exhaust whose main purpose in life is to make the exhaust noise quiet. To be the whisper-quiet device most car owners demand, a typical stock muffler must have an intricate, labyrinthine flow path to help slow and cool the hot, vibrating exhaust gas. It contains baffles that cause the exhaust flow to reverse direction and intermix. These are great for reducing noise but are not so great for flow. The twists and turns the exhaust must endure in a stock muffler are restrictions that cause excess backpressure. You can run in a straight line faster than you can run in a tight, fun-house maze, right? The same goes for your exhaust gas.
To produce the most power, an exhaust should have minimal restriction on the exhaust flow. Restriction hampers the burned exhaust gases from exiting your engine, causing some charge dilution with the incoming fresh fuel air mixture. If all the exhaust gas cannot escape from your cylinders, it dilutes the flammable power-producing intake mixture that is trying to come in. The diluted mixture does not burn as well as a pure mixture.
This causes a loss of power. You don't feel so energetic at a packed club with lots of cigarette smoke, sweaty bodies and hot stuffy air right? Neither does your engine.
With greater restriction, backpressure is generated, making the engine work harder to pump the exhaust out of the cylinders. That work could be used to turn the wheels instead.
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BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Some stock mufflers and exhaust systems have up to 18psi of choking, power-robbing backpressure. In direct contrast, a well-designed, high-performance street exhaust system typically has about 2 to 6 psi of backpressure. For an interesting comparison, an un-muffled straight pipe on a real racecar usually has 1 to 3 psi of backpressure.
To get the least amount of backpressure, most of the good, high-performance mufflers available today have what is called a straight-through design. These mufflers quiet the exhaust by absorbing high-frequency vibrations in heat-resistant packing, usually consisting of stainless-steel mesh and heat-resistant ceramic fibers. All Remus exhausts are straight through designs, unless noted otherwise.
They typically have an inner core that is straight-through with no baffling at all, much like a straight pipe with many small holes in it. The pipe is louvered or perforated when it passes inside the muffler's shell, allowing sound energy to pass through the holes but leaving the exhaust gas flow unimpeded. You can see straight through these types of mufflers. The louvered or perforated core is usually wrapped with either fiberglass wadding (hence the old-school term, Glass Pack) or, in the better mufflers, stainless-steel mesh backed by ceramic fiber to help further absorb the sound.
On straight-through mufflers, the longer the muffler and the bigger the can, the quieter it is. The length usually has no effect on backpressure, just noise output. These absorption type mufflers work in the same manner as the silencers used on guns. If a silencer had baffles that impeded bullet travel, you would definitely have problems! The same is for a perforated core absorption muffler, straight through, no baffles, no restriction, and no backpressure.
It is best to avoid straight-through mufflers that have a louvered core. Many old-school glass packs suffer from this design. Some spiffy polished stainless and big tip mufflers on the market also have these. The louvers generate quite a bit of backpressure because they stick into the exhaust stream and create considerable turbulence. Even though these mufflers are a straight-through design, they can have more backpressure than a stock muffler.
When buying a straight-through muffler, look for one with a perforated core if you are interested in producing more power. A good, properly sized, perforated-core straight-through muffler will add only about 1 to 2 psi of backpressure to your exhaust system.
An old-school type performance muffler, which is still very common in speed shops, that has seen better days is the Turbo Muffler. This is a less restrictive version of a stock-like reverse flow muffler. In the old days, these were well flowing mufflers, but now the new-jack perforated core, straight through absorption types, has superseded them, Many old-school domestic shops will try to sell you one of these as a hot set-up, but they should be avoided, just like the louvered core glass-pack.
A disadvantage to the straight-through muffler is that it is often louder than a reverse-flow muffler. Usually a straight-through muffler needs a small sub muffler or resonator to keep the exhaust quiet. A resonator is usually a small, perforated core glass pack placed somewhere in-between the catalytic converter and the main muffler. Like the main muffler, the longer the resonator, the better a sub muffler will be for noise reduction. Almost all of the pre-made performance exhausts feature resonators.
Some good performance mufflers are only available with a semi-universal 2.5-inch inner diameter. If you have a smaller engine that requires a 2-inch pipe, it is still OK to use a main muffler with a slightly bigger inside diameter. This larger step up in diameter at the very end of the exhaust system won't hurt performance and sometimes can help it slightly.
When designing your own custom exhaust, it is important to remember to make it as quiet as possible. Loud might be cool to you, but remember that a too loud exhaust is perhaps the number one harassment ticket given to performance enthusiasts by your friendly law enforcement officer.
TIPS ON TIPS
What about the tip? Big tips do nothing for power but can dress up the back of your car. Some like to install ridiculously large tips on tiny stock mufflers or on stock cars. Take this too far, and you may find pictures of your car featured on Web sites like riceboypage.com or beaterz.com.
If you are on a tight budget, save your money and don't get a big tip until you get the performance exhaust system (and maybe the engine) to go with it. This advice may preserve your dignity. Some big tips feature resonated cores, which quiet the exhaust's note by a few dB. You can spot these with their usually perforated or mesh inner pipes. These big tips are actually functional and the discerning eyes of the world of pipe fashion may actually forgive the use of a resonated tip. Maybe.
On an entirely practical note, remember that cops like big shiny tips too. Nothing tells a cop pull me over and bust me like a big *** tip.
THE EXHAUST PIPE
To save costs, your typical stock exhaust uses small diameter, crush bent pipe. Crush bends are easy to make in mass production. However, crush bends can reduce the flow of a pipe by up to 50 percent. Your typical exhaust system made by the local neighborhood muffler shop is also crush bent. The best exhaust systems, like most Japanese pre-made exhaust systems, come with mandrel bends.
Mandrel bending is done by a special machine that uses a non-crushable insert, or mandrel, that goes into the pipe while bending to prevent it from being crushed. If you are making your own exhaust, you can buy pre-made mandrel bends from MagnaFlow, Burns Stainless, Kinsler or Bassini. The huge speed parts mail order emporium Summit Racing stocks both the right kinds of mufflers and mandrel bends.
It is better to use a smaller diameter mandrel bent pipe than a larger crush bent pipe. Remember that maintaining velocity is just as important as reducing backpressure. All Remus exhausts are mandrel bent,
PIPE DIAMETER GUIDELINES
Some basic exhaust pipe diameter guidelines for non-turbo cars are as follows:
1,500cc-2,000cc engines : 2-inch
2,100cc-2,500cc engines : 2.25-inch
2,600cc-3,000cc engines : 2.5-inch
Add half an inch to the pipe diameter to optimize for nitrous oxide use because of the increased exhaust gas volume. Remember this may be too big for optimal operation when you aren't on the bottle. For turbocharged engines, 2.5-inch is the minimum size pipe that you would want to run, even for the smaller engines. For 2,000cc and bigger engines, 3-inch works well, and for bigger engines the biggest (usually 3.5-inch) you can find is appropriate. It is almost impossible to have too big of an exhaust on a turbo car.
Buy a straight through muffler now: Remus universal muffler Remus Sports label Remus Power Sound Remus Wild Label 
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