[This article first appeared in Pontiac Enthusiast Magazine. Permission to publish it on the Internet was granted by the magazine and the authors to fellow Pontiac enthusiast and fellow webmaster, Eric Douthitt (The Pontiac Garage). It now appears here exclusively through the courtesy of Eric, Jim and Tom Hand. ]
Several years ago, we decided to reduce the sound level of Jim Hand’s 1971 LeMans wagon to an "almost stock level" while maintaining its drag strip performance. This goal was possible because as early as the late 1980s, all major muffler manufacturers offered high performance components that provided the capability to do so. Just like other performance components for Pontiacs, the choice of the mufflers was a personal one. Various mufflers offer radically different sounds, but the goal was to approach the traditional "Pontiac GTO sound"; this is what drove the final decision to use dissipative-type mufflers on the wagon. The testing performed is summarized with this article. Additionally, it is important to point out that most muffler manufacturers have made changes to their product lines since this testing was performed so it would be wise to contact them for their most current information regarding sound and performance.
Stock exhaust systems were designed to fit a particular chassis, meet durability and cost requirements, satisfy noise level standards, produce a particular sound and most importantly carry, the exhaust gasses from a stock engine. Luckily, there are components available that can carry higher volumes of exhaust gasses. Pontiac’s performance cast iron or aluminum manifolds, aftermarket headers, crossovers, high flowing exhaust pipes and mufflers can be used in various combinations to handle flow. In pursuit of a quiet high performance system, a number of tests were performed at Kansas City International Raceway using Jim Hands, 1971 LeMans station wagon. At the time of the test (1995), the wagon ran a relatively mild 455, equipped with a 9779041 (RAIV) camshaft, Rhoads lifters, 1.65:1 rocker arms, 1971 #96heads with increased airflow capability, 9.9:1 compression and 1.75" diameter Hooker headers. The drivetrain consists of a Turbo 400 transmission with a 2,700 RPM (flash stall) Conclusively, the average performance for the wagon during the 1995 season was, 12.38 seconds at 110+ MPH (best time in late 2000 was 11.3299 at 117.24 MPH).
Various mufflers crossovers and exhaust and tail pipes were compared to determine which produced acceptable performance and noise control. This article is a summary of the components tested, the results obtained and the recommendations based on these results.
Mufflers are expected to reduce the transmission of exhaust noise and they must do it without significantly restricting the flow. A muffler is a "sound filter" and its performance varies with the frequency of sound. There are two basic types of mufflers. One is a dissipative muffler. It contains a series of perforated tubes or chambers that are surrounded by a sound absorbing material. The sound (acoustic energy) is absorbed by this material and dissipated as heat. A familiar example is a straight through "glass pack" muffler.
The second is, a reactive muffler or one which the geometric shape, style and quantity of chambers determines its sound reduction capabilities. In a reactive muffler, the acoustic energy is reflected back toward the source, preventing portions of that energy from leaving.
Many mufflers use a combination of reactive and dissipative technologies. When properly designed and assembled, dissipative reactive and combination mufflers can reduce sounds levels while allowing high volumes of exhaust gasses to flow through them.
Muffler Test Results:
To allow muffler variations to be more accurately determined on the LeMans wagon, a "single muffler" exhaust system was fabricated. By forcing all of the exhaust through a single muffler, its effect on engine performance and sound was magnified. The charts showing backpressure, sound levels and performance provided by different muffler's are shown in charts 1 through 4. After analyzing the results, the large bodied Walker #17749 Dynomax muffler was chosen. It’s quiet, flows well and is relatively inexpensive. The muffler body is 20 inches long and it has an oval cross section that’s roughly 9.75 inches wide by 4.25 inches thick. This muffler has offset 2.5 inch diameter inlet and outlet connections.