Strong Silent Type

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"You can build a quiet exhaust system that performs almost like open headers."

By Jim and Tom Hand

Jim Hand contacted me in November of 1999 and inquired if I would be interested in featuring the exhaust article, "The Strong Silent Type" on the Pontiac Garage. The article has been re-created with permission from Pontiac Enthusiast magazine.

The Strong Silent Type

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) Continental torque converter, 3.55 gears, and McCreary 28x10 inch DOT tires. The total race weight is 4,100 lbs. with a full system and driver. Running a closed exhaust system exclusively, 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.


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.


The ultimate exhaust system should be able to flow as much exhaust and let the engine run as well as an open set of headers does. Therefore, tests were conducted at the drag strip to compare the wagon's performance with open headers to its performance with various exhaust and tailpipe combinations.

A mandrel bent 3.0 " diameter system with 3.0" Walker #17773 Dynomax mufflers and 3.0" tailpipes (available from Torque Technologies ) was tested against open headers and against a system consisting of the same 3.0" exhaust pipes and mufflers but using only 2.25" tail pipes. The temperature of the exhaust gasses at various locations was also measured to provide additional data.

The temperature measurements showed that exhaust gasses cool significantly before they reach the tail pipes. The larger diameter exhaust pipes and mufflers dissipate heat, causing the volume of the exhaust gasses to be reduced. This characteristic enables smaller diameter tailpipes and in some cases, even a muffler with a smaller inlet and outlet to be used. The benefit of this heat loss was apparent when we contrasted the back pressure values of the same muffler mounted in two different locations.

The charts showing the results of the performance tests are given in Figures 5-7. One chart also demonstrates the heat loss affects the back pressure measurements between the front mounted and rear mounted Walker #17749 mufflers.

From these tests, it was noted that the Torque Technologies system, with the 3.0" Walker mufflers, produces performance essentially equal to properly tuned open headers. It was also demonstrated, that the use of 2.25" tailpipes, caused only a small reduction in performance.


A crossover, which is an interconnection between both sides of a dual exhaust system somewhere in front of the mufflers, has been used by automotive manufacturers to reduce sound levels and increase performance. A crossover lets the constantly changing pressure in both pipes equalize by letting each half of a V-type engine simultaneously use both sides of the dual system. In other words, the crossover helps minimize the negative effects of back pressure. Most crossovers look like an "H"; however, another type of crossover has been recently introduced that looks like an "X".

A 3.0" diameter X type crossover (from Dr. Gas/Pro Motorsports) was fitted to the LeMans wagon. Due to their superior sound level reduction, the Walker #17749 Dynomax mufflers were used instead of the smaller case but larger diameter inlet and outlet #17773 units. Using the #17749 mufflers and Flowmaster's 3.0" to 2.5" reducers, performance comparisons of the X type crossover and the original H type were made.


By reviewing the data in figures 8-10, it can be seen that when comparing the H and X types, the X crossover not only reduced 60 foot times and quarter mile times but increased trap speed as well. Functionally, the difference between the X and the H is, the X type forces the exhaust to cross between both sides. Based on the performance increases we noted with the X cross over, the scavenging forces created by the gasses crossing over in this way appear to be stronger and more effective across a wider range of engine speeds.

The X crossover is available in various combinations of sizes. In fact, Floyd Hand has a 1966 GTO coupe, equipped with a 455 engine and drive train that is very similar to the wagon. Times in 1998-99 were as quick as 11.70's at 114+ with the X crossover and full exhaust system. Floyd uses an X type crossover with a 3.0" diameter inlet and 2.5" diameter outlet. This configuration eliminates the need for reducers when 2.5" diameter components are used after the cross over.

Subjectively, the GTO and the LeMans wagon are both quieter under load with the X crossover but no wide open sound level measurements have been taken. Whereas conventional dual exhaust systems have a characteristic rap or low rumble when the engine is under load, the X type crossover has a higher pitched smoother note.


1. Always use a crossover of one kind or another. Our tests indicate that the X type gives additional improvements over the H type.

2. Mount the mufflers as far to the rear of the chassis as possible. An important characteristic of the exhaust system (behind the headers) is its ability to dissipate heat energy. Heat loss brings with it, gas volume reduction, enabling smaller mufflers and pipes to be used without penalty.

3. Always us the largest case muffler that you can fit under the chassis possible. The larger internal volume allows additional acoustical energy to be absorbed ,dissipated and eliminated.

4. Unless an engine is in the 500+ horsepower level or run at very high RPM, the maximum tailpipe size required for minimal power loss is probably 2.5" diameter. When the exhaust pipes and mufflers drop the temperature significantly, the volume of the exhaust gas is reduced and tailpipe sizes is not as critical.

5. When you must adapt various pipe sizes, always use long tapered cone reducers, such as, those available from Flowmaster. You can also use a crossover that has reduced pipe sizes built into it.


It's enjoyable to be able to drive to the strip and run respectable times in a quiet fashion. Both the wagon and the GTO coupe provide the owners with good driveability on the street. The cars are ready to race quietly at thetrack with no dirty and time consuming work on the exhaust systems. Best of all, little or no performance loss is noted with the full exhaust system. We hope this summary of the tests performed, allows you to select components that provide an exhaust system for your Pontiac that yields similar benefits. SOURCES: Dr. Gas/Pro Motorsports 801-571-6097 Torque Technologies 912-242-0691 Flowmaster 800-544-4761 Walker (Dynomax, CVX) 800-767-DYNO Supetrapp 216-265-3461 Borla 805-983-7300