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recita cosi:
At this point you may be tempted to ‘reflange’ the header’s end – don’t. I’ll explain why below.
After you’ve put the header back on, you’ll want to take care when attaching the exhaust coupling. Make sure you’ve got a nice tight seal and that you’ve slid enough of the coupler over the header. Without the flange, it will be much easier for the coupler to come off. If you haven’t figured it out yet, the purpose of the flange is to give the coupler something to ‘bite’ to.
At this point, you may be wondering how cutting the flange off the header improves performance? There are two theories as to why this is so.
The first theory postulates that by shortening the combustion chamber / tuned pipe distance, you’ve shortened the length of time it takes the pulse wave to reflect back to the combustion chamber. Now the motor gets ‘on the pipe’ at a much lower RPM. This theory makes sense except for the fact that by getting the motor on the pipe at a lower RPM, the motor would loose high RPM power. This is not the case. High RPM power stays the same. Additionally, many people have replaced the stock coupler with a longer one, keeping the length of the exhaust system the same. To me, this last point discredits this theory.
The theory that I feel makes more sense is what I call the “Reverse Venturi Effect” theory. By removing the flange, the exhaust system is now a consistent diameter all the way from the motor to the pipe. If you are familiar with fluid dynamics, you’ll know that gas moving through a pipe will experience an increase in pressure and a decrease in flow rate if the circumference of the pipe increases and just the opposite if the circumference decreases (the venturi effect). I believe the flange causes a reverse venturi effect that messes up both the exhaust flow and pulse wave at low RPMs. Removing the flange eliminates the reverse venturi effect and allows the motor to realize its full potential.
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