I signed onto this site out of curosity. To introduce myself - I am a long time racer and engine builder and presently I build the VW powered engines for the Bruce Meyers "Manxter" SCORE off-road racing team. I have designed exhaust systems for Cosworth and Hart Formula II engines operating in the 12,500 rpm range and our present VW engines operating in the 6,000 range. I drove Formula II cars for seven years and have won two SCCA driving championships. I have an engineering degree and I reside in the off-road capital of the world, San Felipe, Baja, Mexico.

The site that I logged onto had questions and answers about exhaust sizes and formulas compared to the computer programs. I personally use the Desktop Computer program only as a guide to check camshaft profiles. It gives you absolutely no help in selecting exhaust lengths or dimensions.

In selecting or building an exhaust system, when in doubt, go small in diameter and long in length. The speed of sound of hot gas is 1,100 fps plus, but not practical for the engine builder. The optimum speed of exhaust gas in an engine seems to be 700 fps so that is the figure to shoot for. Visualize a lake with a certain diameter and depth of water and watch a leaf float downstream. Follow that leaf until it reaches the river at the end of the lake and watch it accelerate dramatically as it reaches the narrows.

An understanding of horsepower and torque is required to design an exhaust system. Horsepower is the result of torque X RPM. Maximum volumetric effeciency will always occur at the crossover point of horsepower and torque. Since torque drops off faster than horsepower after this point, increased rpm will continue to make power after the torque begins to drop off.

Don't go designing an exhaust system that will make the most power between 6,000 - 8,000 rpm if your engine isn't capable of high rpm. A fat, but baaad looking, exhaust system is like the leaf floating in the lake - slow gas movement except at very high rpm. A smaller, but modest looking, system is like the leaf when it reaches the narrow stream. The same amount of water flows through the large lake as flows through the narrow stream; the laws of physics dictate that the velocity has to increase drastically for that to occur; therefore the diameter of the pipe controls the speed of the gasses, independent of the length.

The length of the exhaust pipe controls the RPM of maximum power as well as the width of the power band. The shorter the pipes the higher the RPM of maximum power and the narrower the power band. Longer pipes lower the RPM of maximum power and widen the power band considerably so if you need driveability go with longer pipes and smaller diameters.

Mean Dean Moore

Welcome Dean. This subject has been cussed and discussed in painful depth on the board. Do a search on 'header design' or some such and you will find the threads.

With your great resume, I am looking forward to your contributions and insight.