The Strong Silent Type

Continued from page 2

Crossover Test Results:

By reviewing the data in figures 8-10 (below), 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 crossover, 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.


Fig. 9

Fig. 10

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 the track 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. 


Dr. Gas/Pro Motorsports

Torque Technologies 912-242-0691

Walker (Dynomax, CVX) 800-767-DYNO

Supetrapp 216-265-3461

Borla 805-983-7300

Copyright © 1997-2009 Bill Boyle. All rights reserved.
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