Mike Kojima posted on July 14, 2009 16:14
Low backpressure and high exhaust stream velocity can be achieved by running straight through free flowing mufflers and relatively small pipe diameters. The only exceptions to this are super or turbocharged motors, motors optimized for large amounts of nitrous oxide and highly modified naturally aspirated motors with huge cams. These types of engines vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all or in the case of the high strung NA motor, are very sensitive to small amounts of backpressure.
Some stock mufflers and exhaust systems have up to 20 psi of choking, power robbing backpressure. In direct contrast, a well-designed high performance street exhaust system typically has about 2-6 psi of backpressure. For interesting comparison sake, an un-muffled straight pipe on a real racecar usually has 0-3 psi of backpressure.
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| Straight through mufflers with a perforated core have the lowest backpressure, about the same as an open exhaust. With some careful thought, they can be fairly quiet as well. The noise waves go through the perforations and are absorbed by the packing. The Greddy muffler above has stainless mesh to protect the fluffy ceramic cubes from the exhaust heat while the muffler below has all stainless mesh packing. The fluffy stuff is better at absorbing high frequencies and the stainless low frequencies |
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 types of mufflers quiet the exhaust by the absorption of 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 mufflers 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 on the better mufflers, stainless steel mesh backed by ceramic fiber to help further absorb the sound.
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| A perforated core muffler has a smooth interior that flows just about as well as a plain old pipe. The perforated core muffler flows like a straight pipe but still quiets things down. These Magnaflow mufflers are a good choice for long term street use |
On straight through mufflers the longer the muffler and the bigger the volume of its can, the quieter it is. The length usually has no effect on the backpressure, just the noise output. These absorption type mufflers work in the same manor that 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.
1,2,3 Magnaflow has a line of universal high performance mufflers in many different shapes, diameters and lengths. The muffler and pre silencer that 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 through with no obstructions to flow.
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| Although this louvered core muffler looks pretty on the outside, it has a lot of backpressure. This is because the louvers make a lot of turbulence and have a thick boundary layer, effectively making the internal diameter smaller. If you use one of these mufflers, you can reduce the backpressure by running the muffler backwards and getting a muffler with a core diameter 1/2 inch bigger than the pipe. Louvered mufflers are quieter than peforated core ones |
It is best to avoid straight through mufflers that have a louvered core. Many old school glass packs suffer from these. 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 1-2 lbs of backpressure to your exhaust system. Mufflers like the Walker Ultra Flow, Thermal, Apexi, Borla, Edelbrock or Magnaflow are examples of good low backpressure mufflers with an absorption design. Many Pre-made exhausts like Apexi, Tanabe, Greddy, Borla, Maganaflow, Thermal or HKS also have mufflers of the free-flowing absorption design.