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%. 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 becoming crushed. If you are making your own exhaust you can buy pre-made mandrel bends from Magnaflow, Burns Stainless, Kinsler or Bassani. The huge speed parts mail order emporium Summit Racing stocks both the right kinds of mufflers and mandrel bends. It is better to uses a smaller diameter mandrel bent pipe than a larger diameter crush bent pipe. Remember that velocity is just as important as backpressure.
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| Mandrel bends from Burns stainless (above)are available in exotic metals like 321. Bends from Magnaflow (below) are available in 409, an excellent choice for street cars. |
Pipe Diameter Guidelines
Some basic exhaust pipe diameter guidelines for stock or bolt on modifcations, non-turbo cars are as follows:
1500cc-2000cc motors- 2 inch
2100cc-2500cc motors- 2.25 inch
2600cc-3000cc motors- 2.5 inch
Add ¼ inch to the pipe diameter to optimize for NOS use. NOS increases exhaust gas volume. Remember that this may be too big for optimal operation off of the bottle. For turbo motors 3 inch is the minimum size pipe that you would want to run, even for the smaller motors. For turbo motors larger than 2.5 liters, the biggest tubing (usually 3.5 inch) that you can find is appropriate. It is almost impossible to have too big of an exhaust on a turbo car.
How is a Good System made?
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 two 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. For those who want the best, want a light system or have a race car, 321 stainless is the tubing of choice. 321 has good high temp strength so you can use thin tubing, about 0.035” thick for light weight. It must be tig welded and have good tight mitering before welding with back purging so it’s not something most local muffler shops can deal with although a good welder/fabricator will have no problems working with it.
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| Magnaflow makes an excellent kit called a Hot Rod Kit that includes 409 stainless mandrel bends, stainless hangers, stainless 02 sensor bungs, a flex joint, and rubber isolators. The Hot Rod kit has everything you need to make a killer exhaust system, just add stainless mufflers and you are good to go! |
Next select your muffler and pre-silencer. It is usually best on a streetcar to get the longest, highest internal volume 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 backpressure and no loss in performance. A perforated core tip will also help your exhaust be quieter.
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| We chose a Magnaflow muffler for our project exhaust. It has a 3" perforated core, 304 stainless construction and a 4" tip. |
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. Do not if you can help it, use a gas or unshielded arc electric welder.
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| The mandrel bends are cut to shape the exhaust to the desired configuration. |
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| A mig welder is used to tack the cut segments in place under the car. The hangers and flanges are tacked together afterwards. Tack welding is used because if you make a mistake, you can easily grind the tack welds off and re-position stuff. |
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After the exhaust is mig tacked together under the car, it is removed and the final welds are made with a tig welder.
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| Our completed, awesome 3" all stainless SE-R exhaust. No one makes anything like this with a fit this good on the market! It cost less than a pre-made system as well. |
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| The custom made, shorty, Burns race muffler, thin wall 321 stainless exhaust system on our time attack car saves about 40 lbs off of a typical street type exhaust. The Burns muffler is suspended in a floating cradle mount that allows the exhaust to move without stressing the thin tubing. |
Attach hangers and flanges; most good muffler shops stock these. 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 and wont polish.
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| Magnaflow makes these cool stainless headed hangers. |
Making stainless flanges can be a hassle if you want an all stainless system. We use aircraft stainless quick release V-Band clamps instead of flanges. These can quickly be removed with a single bolt and are a race car cool product. Be sure you use high quality V-Bands because there are many inferior weak parts on the market. Burns Stainless sells high grade V-Band clamps.
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| Burns Stainless V-Band clamps are quick release and are easier to deal with than making some stainless flanges. |
If you want to get fancy, you can box your system up 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, you have built a system as good as if not better that 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
What ever you do, do not remove or gut out the catalytic converter on your street machine. The monolithic, straight through design of modern 3-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 will 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 a loss in bottom end power.
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| Magnaflow and Random Tech make large diameter high flow cats so there is no excuse for polluting anymore. |
Because of these factors, some of the cars that we have worked on over the years have actually gained power with the addition of a cat. As the number of vehicles on our highways grows every year, we must all do our part to manage pollution. 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 on, full race turbo, Naturally Aspirated or a huge Nitrous system can benefit from removing the cat when racing levels of boost or nitrous are being run. Boost or Nitrous flow levels that you would typically run on the street on pump gas are not enough to warrant cat removal for performance gain. In short the only time you should remove a cat is for off highway racing events.
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| If you remove the cat for racing purposes, then don't gut the cat, it will lose power. Instead use a pipe the same diameter of the rest of the exhaust. |
If you must remove your cat at a race, replace it with a straight pipe of the same diameter as the rest of your system. Don’t put a gutted cat in for the reason of screwing up exhaust gas velocity by creating a huge turbulence inducing dump chamber in the middle of your exhaust. If you need to change you factory cat for a larger high flowing one, Random Technology and Magnaflow makes replacement cats with 3-inch ore even larger inlets and outlets.
So in short remember, more flow=more go!
Sources
Magnaflow
Burns Stainless