Charon
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Thanks for the pictures. Those reflections make it difficult to tune an engine with straight pipes, because at some engine speeds they help the engine to breathe, while at others they hinder breathing. Carburetor jets are fixed, so the mixture that is right when the engine is "on the pipe" is wrong at other speeds. It is actually possible, if the planets align just right, to have air blow back out through the carburetor, then be re-inhaled. On each pass the carburetor mixes more fuel, so the mixture can go horribly rich at certain speeds. These effects help explain the "dip" at mid-speeds on some torque curves.
Aftermarket exhaust makers usually promise increases in horsepower. But they don't mention the rest of the story. Horsepower is the product of torque and speed. To increase horsepower one or both must be increased. Speed usually cannot be increased very much without shortening engine life, so torque is left. The problem is that torque cannot usually be increased over the whole torque curve. Think of the torque curve as being made of a piece of string. If you pull it up at one speed, it goes down at another. Aftermarket pipes usually manage to increase the torque just a bit at the top end, where they can advertise horsepower, but the decrease at the other end usually tends to make the engine less "driveable." Same thing happens with "race" cams, but that is another subject.
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