FAQ

What is the difference between mandrel bent and pressure bent? What does CNC mean?

• Most factory exhaust systems built through the 70’s were pressure bent. Pressure bending is when a tube is placed into a die and a hydraulic ram, which has the other half of the die, or the shoe, is then pressed together to push the tube around the radius. Nothing is inside the tube to prevent the tube from collapsing or buckling, and the machine is completely manual-the operator indexes the pipe by hand according to an angle gauge in the end of the pipe. Not a very precise method of bending, but quick and inexpensive.
• Mandrel bending is when a set of balls, called a mandrel, is inserted into the pipe during bending. These balls are positioned and pulled through the pipe so that the bends are not deformed and maintain a close to perfect shape throughout the length of the bend. The pro’s are that the tube can be bent on a much tighter radius and still flow well due to the pipe being round and not collapsed. The machinery is expensive for a good quality CNC mandrel bender with the appropriate tooling.
• CNC bending is when a computer controlled set of motors indexes the head of the machine and controls the position of the tube and the rotation of one bend in relation to another. Essentially, the machine does all the work. The tube is cut to the appropriate length, inserted in the collet, and the button is pressed to start the cycle. The operator does not touch the part until all the bends are complete. CNC mandrel bending is the best method for producing exhaust systems that flow and fit well. With a CNC mandrel bender, you can also bend thin wall tubing so that it weighs less. Being stainless, it will not corrode so thin wall tubing is not a concern for durability.

Why is Stainless better than other materials for exhaust systems?

• Stainless is an excellent choice for exhaust systems due to its corrosion resistance. Starting in the early 90’s all OEM manufacturers went to 409 stainless for the increased durability and corrosion resistance-the EPA mandated that exhaust/emissions systems be warranted for 5 years minimum. 304 stainless, specifically, the Everlast grade that Stainless Works uses exclusively, will not rust or corrode over time (unless exposed to excessive amounts of road salt). Unlike aluminized steel, it will not rust from the inside out due to the corrosive fluids produced during combustion. All of Stainless Works 304 Everlast systems have a lifetime guarantee against defects in materials and workmanship.
  

What is the difference between grades of stainless (304, 321, 409…..?)

• Without getting into the specifics of metallurgical properties, 409 is what is typically used on OEM applications. It has fair corrosion resistance, is magnetic due to the high metallic content, but is a less expensive material than 304. 409 is not an ideal material for a performance/restoration exhaust system because it will turn color (brownish/rust) from being exposed to the atmosphere without ever being run. For OEM’s who never care what the exhaust system looks like, it is a decent material that will last long enough to get past the warranty period. 304 stainless has much better corrosion resistance and a lower metallic content than 409. 304 will not turn color from sitting exposed to the atmosphere, and will only discolor slightly from getting extremely hot. The best thing about 304 is that you can maintain it with a scotch bright pad if you ever spill oil on it or have it discolor. Stainless Works only uses Everlast 304 stainless, which is a premium mil spec grade of stainless that is slightly more expensive than standard stainless, but is processed much more closely and is more consistent.

Does stainless expand a lot more than aluminized steel. I have heard that a stainless system will grow 1 ½” in length when it gets hot. Is this true?

• In a word, NO! If stainless systems expanded as much as some olds wives tales would lead you to believe, then how would so many OEMs use it and not have problems? The key is to look at a property of the material called the coefficient of thermal expansion (COTE-Geek terms for how much the material will grow when it gets hot). For mild steel the COTE is:

6.3 X 10-6

For stainless, the COTE is:

9.4 X 10-6

· So, to calculate the change in length we use the following formula:

=(COTE)(Length of component)(change in temperature)

Looking at an exhaust system that is 84 inches long, and whose temperature will change 600 degrees F, on average, for mild steel:

change in length=.3157 inch.

For stainless: =.473 inch

As you can see, the difference over a 7-foot long exhaust system is negligible (it amounts to .0019 inch per inch). 

What core material do your mufflers use? Do they get louder over time? Stainless Works uses either stainless wool or mild steel packing for our mufflers (depending on whether you buy an aluminized or stainless muffler or exhaust system). Our stainless mufflers use 304 stainless shells, packing, and cores, and like every Stainless Works product have a lifetime guarantee. The wool does not blow out over time like fiberglass or other packing materials.

How do your systems differ from a concours correct factory exhaust system?

• Stainless Works prides itself on building systems that fit and perform better than the factory systems. A concours type system will be pressure bent like the original, which impedes flow, and will be routed a little differently than our systems. Our systems are designed to fit and perform perfectly. In many cases we improved on the fit of the factory exhaust system. Being CNC mandrel bent ensures that it will fit well every time.
  

Will I need to cut or weld my exhaust system to install it?

• Stainless Works Factory Fit exhaust systems are designed to be installed with a minimal amount of fitting on the car. You may have to trim the headpipes to length for a perfect fit-we do installs at our shop and have seen massive variations from one car to another; so in certain applications we leave a little length on the headpipes so that you can trim them for the desired fit on the car. Trimming is easy with a common hacksaw-just trim a little at a time and be very conscientious of how much you are removing. You do not have to weld the system in place, but if you never plan to remove it, a tack at each joint is not a bad idea. We also recommend using either red high temp RTV or muffler cement in the joints for a leak free seal, and use plenty of never seize on the clamp bolts. Some of the “Hot Rod” systems we sell do require cutting and welding-see our application guide for details.
  

If I want to weld my system together in stainless, what tips do you have?

• We sell mufflers, bends, straight tubing, etc-everything you need to weld a system together. For cutting stainless, a hacksaw or a chop saw with a fiber blade works well. When using a chop saw, make sure to mount the part in the vise very securely-if you pinch the blade with the part the blade will disintegrate. If you cut with a hacksaw go slowly and try to cut as straight as possible. After cutting, sand the pieces so they fit flush using a belt sander and 80 grit or coarser belts. Welding can be done with either a MIG or TIG. For a mig, use stainless wire and gas specially for stainless welding (AGA gas sells this under the name Stain-mix). For tig welding use 308 rod and make sure to get good penetration. It is usually a good idea to tack everything together under the car and final weld it on a bench to get good weld coverage all around.
  

What type of core do you use?

• We make our own core material in house. We own a machine which punches spiral louvers in a piece of tube. The spiral louvers direct the sound into the packing and help control noise very well. Straight through hole perforated. tube does not do a lot in the way of lowering the DB level of an exhaust system.
  

Since the core hangs in the airflow stream, is it restrictive?

• We recently did some dyno testing at Wheel to Wheel in Michigan on Mark Stiewlows new 400 CID, 600 HP engine. With S-tube Stainless Works mufflers and complete exhaust system, we were down 1 hp at peak, but made more average torque and horsepower than with no mufflers. With a chambered muffler that is approximately 6 DB quieter than our first muffler, we were down about 15 flywheel HP compared to open headers. For a very quiet, streetable muffler, those numbers are excellent!! In Marks words, “I would trade 6DB for 15 HP any day of the week. I have had other exhausts that were so loud you couldn’t hear yourself think. These make great power and sound excellent!” Obviously, the tales of perforated cores flowing 50% less than other cores is not true!!
  

Will polished stainless turn color?

• It depends on how hot you get the exhaust. On a street rod we built last year, the owner put several thousand miles on it and the pipes turned a light champagne color. We have had headers turn dark gold when the motor was not timed and/or jetted properly. The nice thing about stainless is that is it ever does discolor, you can renew the luster with Blue-Away and Simi chrome polish. Unlike chrome, it cannot permanently discolor.

Do you recommend coating stainless to reduce under hood temperatures?

• We built a custom set of headers for a T-70 Lola that was being converted into a “street” car. Due to the tight confines of the engine bay, the owner wanted to try coating the headers to reduce the under hood temps. The owner was fanatical about measuring and recording all engine operating data-he had thermocouples on everything! After coating, he did not see one degree change in the under hood temps.
• Another issue is that all coatings will stain if they get oil spilled on them. Bare stainless is nice because maintenance is easy-a scotch bright pad and WD40 on mill finish, blue away and Simi chrome on polished.