A secondary, though not insignificant, function of the exhaust system is to reduce the noise from the exhaust flow. This function is performed by the exhaust muffler, without which the exhaust system would be extremely loud as anyone who has attended a motorsport race event would attest. However, the way the muffler reduces noise can cause flow restrictions and can increase backpressure, both of which has a detrimental impact on engine breathing and engine performance. This is most noticeable in stock mufflers that use intricate flow paths and baffle plates to reduce exhaust noise by slowing and reversing the exhaust flow. The result is a very quiet exhaust but at the cost of a large increase in backpressure that robs the engine of power.
Performance Muffler Design
A muffler designed for better performance would have neither intricate flow paths nor baffle plates. Instead, they would have a straight flow path. As a result, these types of mufflers are often described as straight-through or free-flow mufflers and usually contain noise absorbing material such as stainless-steel mesh or Fiberglas while the inner pipe has either small holes or louvers through which the noise can pass and be absorbed by the noise absorbing material.
There are a few important considerations in the design of a straight-through performance muffler.
- First, the diameter of the inner pipe should be the same as that of the rest of the exhaust. A step up to a wider diameter pipe would create a low pressure area in the muffler, which would cause turbulence. This turbulence would impede gas flow and would result in backpressure. And, obviously, a step down to a narrower pipe would be a restriction and, as we've discussed in power basics, flow restrictions are what we need to eliminate in order to produce more horse power.
- The second consideration is the amount of noise reduction we require. A straight-through muffler provides better performance by producing less backpressure at the expense of inferior noise reduction. Consequently, we may need a much longer muffler to achieve the required noise reduction. This can be accomplished by using several shorter straight-through mufflers or a resonator between the catalytic converter and the main muffler to reduce exhaust noise; or by using a single long straight-through muffler. A word of warning though: a straight-through muffler still produces backpressure; it just produces less backpressure than a stock muffler and a longer straight-through muffler will produce more backpressure than a short straight-through muffler. The result is a trade-off between performance and noise reduction.
- Another consideration is the use of small holes in the pipe or louvers through which the noise can pass into the noise absorbing material. From a performance view point, we'd rather go with small holes instead of louvers. Louvers protrude into the exhaust flow and result in a greater disturbance to the flow, which means more power reducing backpressure! But, having opted for having small holes in the inner pipe, we must also note that having fewer holes will produce less backpressure but will also result in less noise reduction. Here again, we have a trade-off between performance and noise reduction.