Muffler Types
Once, a typical performance muffler used multiple baffles inside it. The exhaust gas was forced to meet blank walls inside the muffler, making its way out through holes punched in the tube. Often it would then have to squeeze through even more holes before it could continue on its way. Each section of the muffler allowed expansion and pulse reflection, decreasing noise. These mufflers were reasonably quiet, but very restrictive to flow. This type of baffled muffler is still currently fitted to some new cars.
Next on the scene was the 'Turbo' muffler. Even this muffler dates back to the 1960s, where it was first used on the turbocharged Corvair Spyder in the US. The Turbo muffler doesn't use baffles to block off flow and so it has less flow restriction. Instead of using baffles, the Turbo takes the exhaust gas on an S-shaped path through the muffler. The gas enters the muffler, travelling straight down to the other end of the muffler, where it is forced to turn through 90°. It then heads back in a different tube the way it has come, before it gets told to turn around again. Finally, it flows out of the muffler.
The benefit of the Turbo muffler is that it is effectively three times longer inside than outside! The disadvantage is that each of those 90° turns causes a flow restriction. Some Turbo mufflers use little curved internal guides that are designed to help the gas make these U-turns, but restriction is invariably caused. The Turbo muffler works by allowing the gas to expand through chambers at each end of the muffler and through holes ("perforations") in the tubes that carry the gas.
The best Turbo mufflers use bends external to the muffler body to turn the gas through 90.
Finally, we reach the most modern type of muffler. The straight-through muffler uses a single perforated tube that takes the exhaust gases straight from the inlet to the outlet. The exhaust gas can travel through the muffler with almost no restriction at all. The gas expands through the holes in the pipe, causing the silencing. Most straight-through mufflers use holes that have been punched cleanly, but poor designs have a 'louvre' type where there are projections of lots of little bits of metal into the gas flow.
These are the three basic types of mufflers but there are also variations on the designs. Some sophisticated straight-through mufflers uses two chambers, with the exhaust gas expanding through into the second chamber after it has squeezed through the perforations in the main tube. Others use a 'dog leg' design, with a central open chamber and offset inlet and outlet pipes that pass through their own respective chambers.
Comparing Mufflers
A genuine straight-through muffler outflows any other type. When compared with the same length of empty pipe, a good straight-through design flows 92 or 93 per cent of the maximum possible. That is exceptionally good, and can be compared with the poor flow of a traditional Turbo reverse flow design that is typically down at 59 per cent. Those mufflers using a 'dog-leg' internal design with offset chambers have a flow of about 65 per cent, while traditional baffled mufflers can be as low in flow as 38 per cent. These figures are the result of extensive muffler testing on a flowbench.
The amount of noise developed with a particular design is very dependent on the car to which the muffler is fitted. When used with a resonator or on a turbo car, straight-through designs can be very quiet. In fact, in a muffler comparison carried out on a Holden VR Commodore V6, the two straight-through designs were the quietest of all the mufflers. The restrictive Turbo designs were amongst the noisiest - so much for the theory that you cannot have a quiet and free-flowing muffler! In a second test carried out on a Nissan 200SX turbo the same applied - the quietest and best flowing mufflers were the straight-through designs.
However, no less than four of the small 3-inch straight-through mufflers that were measured as being very quiet on the Nissan 200SX were latter fitted to a Holden 5 litre V8 Commodore VS using twin 3-inch exhausts. The power gain over the standard system throughout the rev range was excellent, but the noise was unbearable. We suggest that the only certain way of ascertaining the noise level is to actually fit the muffler to the vehicle. Obviously it helps if you can find someone else with your type of car who has already done that!
Selecting Mufflers
Inspect all mufflers that you are thinking of buying. Look at the construction of the muffler - well-made mufflers are often quieter mufflers because the company has done a good job in all aspects of the design. Using a torch or the sunlight, carefully examine what you can see of the inner workings of the muffler. If it is a straight-through design, there should be no welding dags or other bits of metal protruding into the path of the exhaust gas. Even a small lump of weld can make a big difference to flow. If it is possible, look through the perforations in the tube. If you can see white fibreglass packing, you should be concerned. Fibreglass is an excellent silencing material, but the fibres can easily get sucked through the perforations by the exhaust gas flow. To prevent this, there should be a layer of stainless steel wool up against the holes to protect the fibreglass.
When you look through the muffler, make sure that the tube size doesn't decrease within the muffler. At one stage a very well-known 2½ inch muffler actually had internals that were closer to 2-inch, perhaps explaining its poor flowbench results! The larger the muffler canister, the more likely it is to be effective at silencing. The Japanese aftermarket exhaust manufacturers are probably the best in the world at producing very quiet exhausts with excellent flow, and one common denominator of their systems is the use of large mufflers. You want the body of the muffler to be as large as can be fitted under the car. However, the muffler shouldn't have big flat panels, because these can vibrate and create resonances. Instead they should be stiffened by being curved or even by having the manufacturer's name deeply stamped in them.
Most mufflers are made from either aluminised mild steel or stainless steel. Some one-off or low-budget designs use only painted mild steel and should be avoided. Mufflers are prone to corrosion because the engine produces acids that cause the pipe to rust from the inside out. This is especially the case if lots of short trips are undertaken. Stainless steel mufflers use one or a combination of two types of material - 304 or 409 grades. Of the two, 304 is the more expensive and is a similar grade of material to that used in kitchen sinks. The lower grade 409 is easily picked because it attracts a magnet. Both types of stainless steel can be polished to a high lustre, but 304 is more durable with mufflers made from this material having warranties of up to 10 years. In comparison, aluminised mild steel mufflers last four or so years, depending on the type of trips undertaken.
An exhaust is much more likely to be quiet if it uses three major components - a catalytic converter, resonator and muffler. Each of these performs a silencing function and so the noise level will increase if any is left out. Note that even if you wish to reduce restriction to a minium, a straight-through well-designed resonator will cause almost no impediment to flow while at the same time substantially dropping noise levels.
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