A Discussion of Flow, Resistance, and Resonance

A device inserted in the exhaust system designed to reduce the noise of combustion, the muffler connects from the exhaust manifold/exhaust pipe interface into the tailpipe.

The Muffler's Role

The exhaust system of an automobile engine holds three major responsibilities: conducting the burned gases to the rear of the car and into the air; converting harmful emissions into carbon dioxide and water vapor by way of a catalytic converter (introduced 1975); and deadening the sounds of combustion. The reduction of combustion noise is achieved by passing exhaust gases through a muffler.

The Prevention of Backpressure

A key muffler design concept involves minimizing the amount of backpressure. Backpressure denotes resistance to free flow, in this case the flow of exhaust from the engine. Mufflers ideally reduce backpressure to allow the burned gases to be forced from the cylinders. In general, the higher the engine revs, the more backpressure that accumulates.

If backpressure is not properly reduced, the pressure of those exhaust gases will slow down the power stroke of the pistons, significantly reducing an engine's power. For a given car speed, the loss in power increases very fast with the increase in backpressure. While the precise design of a muffler varies between manufacturers, there are two general approaches to muffler structure:

  1. Straight-Through Muffler.
    A straight path for the gases extends from the front to the rear of the device. A central pipe is surrounded by sheet metal shell exactly three times the diameter of the pipe. The space between the outer and inner shells may be open, or in some cases, filled with steel wool or various other sound-deadening materials. This design reduces more backpressure, but is also the noisiest.
  2. Reverse-Flow Muffler. A curved, s-shaped path for gases has the advantage of saving space. The size and shape of the muffler's chambers influence the noise level and backpressure. This design reduces noise most effectively, but does not reduce backpressure as well as Straight-Through Mufflers. There are three important components of the reverse-flow system:
    1. Helmholtz Tuning Chambers (HTC). Similar to blowing across the neck of a soda bottle, the frequency of the engine's explosions causes resonance in the exhaust pipe. The HTC is designed to absorb resonance and noise volume by calibrating chamber volume, tuning tube size, and gas temperature in such a way that these sounds are inaudible.
    2. High Frequency Tuning Chamber (HFTC). While the above HTC apparatus primarily affects the lower-frequency sounds, this tuning chamber reduces the sound level of the higher frequencies (audible as whistling) present in the exhaust system. These frequencies can be caused by: exhaust flow past a sharp edge in the system; venturi noise in the carburetor; and friction between the forceful exhaust flow and the pipes.
    3. Reversing Crossover Unit (RCU). Effective in removing or reducing mid-range frequencies. The number and size of holes determines the amount of crossover (s-shape).

Muffler Replacement

Typically, rusting within the exhaust system makes it necessary to replace the muffler/tailpipe interface. If the car is used frequently for short trips, replacement may be necessary every 20,000 miles. The tailpipe is held in place by clamps and straps and is directly connected to the muffler. To replace a muffler and tailpipe, simply remove these attachments.

The tailpipe often rusts to the muffler, which complicates the task of separation at the joint. If the parts are intended to be used again or recycled, an amount of penetrating oil should be applied to the joints before their removal; otherwise, these areas may be cut apart. Hacksaws have been used for muffler removal, though power driven tools accomplish this task more efficiently.