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.
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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.
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