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.

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.

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.

MagnaFlow has a line of universal high performance mufflers in many different shapes, diameters and lengths. The muffler and pre-silencer we got from MagnaFlow are made from high-quality polished 304 stainless and feature a perforated core wrapped with stainless mesh and ceramic wool. Note how the perforated core is straight with no obstructions to the flow.

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. Mufflers like the Walker Ultra Flow, Thermal, A'pexi, Borla, Edelbrock or MagnaFlow are examples of good, low-backpressure mufflers with an absorption design. Many Pre-made exhausts like A'pexi, Tanabe, Greddy, Borla, MagnaFlow, Thermal or HKS also have mufflers of the free-flowing absorption design.

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. A Walker Magnum Glass-Pack is a good muffler to use as a resonator. 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. Don't ask how we know this.

Pretty on the outside, but performance-hampering on the inside, this is a typical louvered core muffler. Although this is a well known brand of muffler featuring a stylish, 4-inch tip and spiffy, polished stainless steel construction, the louvered core is not the bet solution for optimal performance. The louvers cause considerable turbulence, making for more backpressure.

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 ass 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.

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.

BUILDING IT
First you must buy mandrel bends from any of the aforementioned suppliers. The mandrel bends and tubing are made in mild steel, or if you want to get fancy, many companies also make them out of 304 or 409 stainless. Of these choices, 304 is more desirable as it is more corrosion resistant and can be polished to a mirror finish. It is also more expensive. 409 stainless is more rust resistant than aluminized mild steel but it cannot polish and turns brownish purple with age. If you use stainless, be sure you have your muffler shop use a proper stainless welding rod.

Next, select your muffler and pre-silencer. It is usually best on a streetcar to get the longest mufflers that will fit under your car for the quietest exhaust note. If you stick with perforated core stuff, it will not cause any increase in back pressure and no loss in performance. A perforated core tip will also help your exhaust be quieter.

Next, find a local welder, fabricator or muffler shop that is willing to work with these mandrel bends instead of crush bent tubing. Look under the car and figure out how to lay out the exhaust system using cut sections of the mandrel bends. Cut sections of the bends and piece them together, tack welding them first until the position is finalized, then once the final configuration is made, seam weld the joints using a MIG, or preferably TIG welder using the proper welding rod. If you can help it, do not use a gas or unshielded arc electric welder.

MagnaFlow makes pre mandrel bent sections of tubing out of 304 stainless that can be cut up to make your exhaust system.

To make a real sano all-stainless system, MagnaFlow also sells stainless exhaust hangers.

We fabricated a 3-inch exhaust for a 400-hp Sentra SE-R with the help of Mike Saiki from Motivational Engineering, and Wayne Zotti of Speedway Muffler in Gardena, Calif. An abrasive cutoff wheel is usually the easiest way to cut sections of mandrel bend for your exhaust system.	Saiki then test fits the sections under the car for fit, trimming and adjusting as needed for best fit.	Saiki then tack welds the parts in place using a MIG welder.
Wayne Zotti then seam welds the joints of our exhaust system in place on the bench with a TIG welder and using stainless welding rod.	Our high-flow MagnaFlow cat is welded in place with Turbonetics quick-release V-Band clamps so it can quickly be replaced with a straight pipe when the car is run at the track.	All done! Our new system is nicer than just about any on the market and flows well enough for our 400-hp turbo engine. It is also reasonably quiet and is low profile enough not to scream "give me a ticket please." Note the big hole of the tip extends through the whole exhaust system; no rice-boy stuff, all business and all performance. When we paint the car, we will send the system to the local polisher to shine it up.

Once you have the basic shape laid out, you will need to attach hangers and flanges. Most good muffler shops stock these. If at all possible, use the stock exhaust hanger locations; this will minimize the vibration transmitted into the car.

Next, you will want to degrease the new exhaust and paint with heat-resistant paint, like VHT or Thermo-Tec if the system is aluminized or bare steel. 304 stainless can be left bare or taken to a local plating shop and polished. 409 stainless can be left bare, but can't be polished.

If you want to get fancy, you can box your system out and send it out to be ceramic or thermal barrier coated by Swain Technologies, Jet Hot or many other companies. Most large urban centers have coating shops that can do this. These extra steps are worth it when it comes to having a sano and long-lasting finished product.

Finally, relax and enjoy the power. With careful fabrication, you can built a system as good as, if not better than what you can buy pre-made. This sort of system can be built using the resources available in just about any town.

TO CAT OR NOT TO CAT
Whatever you do, do not remove or gut out the catalytic converter on your street machine. The monolithic, straight-through design of modern three-way catalytic converters is usually quite free flowing on most modern imports, producing at the most, only a pound or two of extra backpressure. A gutted cat can actually hurt power as the empty box can cause flow stagnation, which effectively shortens the length of the moving gas column in the exhaust pipe. The empty box can also reduce important flow velocity. This can be felt as loss in bottom-end power.

Because of these factors, it is not unusual for cars to actually gain power with the addition of a cat. If every last bit of power must be extracted, as in real, off-the-street sanctioned racing, then the cat can be removed and replaced with a length of pipe the same diameter of the rest of the exhaust system, not simply gutted to a power robbing shell. A full race turbo or nitrous oxide system can benefit from removing the cat when racing levels of boost or nitrous are being run. Boost or nitrous flow levels you would run on the street on pump gas are not enough to warrant cat removal for performance gain.

If you need to change your factory cat for a larger, high-flowing one, Random Technology and MagnaFlow make replacement cats with 3-inch or even larger inlets and outlets.