Every time I hear this crap that our engines need back pressure I just shake my head. It is just the dumbest thing around. The Otto cycle engine does not need any backpressure. In fact the less backpressure the better. A suction in the exhaust is even better as it will help pull exhaust out of the cylinder so the engine will not have to use power to push it out.

Check this out, airplanes fly. They do this by managing flow dynamics also known as fluid dynamics. When you increase the velocity of a fluid its pressure decreases. When this decrease in pressure is above the wing, the pressure under the wing pushes up lifting the plane into the air. When you decrease the velocity its pressure increases. When air is drawn through a venturi in a carb the pressure drops and that is what draws the gas out of the bowl and into the air stream. If you look at a venturi you would think that it is a big restriction, but it causes the pressure to drop. A venturi is basically an airplane wing in a circle.

Now with an exhaust system you want the lowest pressure you can get so the engine works less hard pushing the exhaust out. So, it is all about flow management. A small pipe will cause the flow to be faster and it travels down that pipe in pulses. The faster it goes the less the pressure is especially after the pulse wave. If you get to big of a pipe the velocity slows down and the pressure increases. So a large diameter pipe can in fact result in a higher pressure in the pipe and cause the engine to work harder to push out the exhaust and you loose torque.

Do you want a visual aid? If so, light two candles. Place them about 6? apart. Now blow between them. You will notice that the flames lean toward the air that you are blowing between the candles. This is because the pressure between the candles dropped and the higher-pressure air is rushing to move toward the low-pressure air pushing the flames over as it passes through.

Now there are always compromises. Your engine runs at different RPMs and different loads and you will want the smallest diameter pipe possible without restricting high flow at high RPM and loads. At some point a small diameter pipe will become flow restrictive and will need to be larger, but the trade off is loss of low load velocity and increased pressure and you perceive this as loss of low RPM torque. A short straight pipe is best but you have packaging problems. If you have to have bends, the bends should be a larger diameter then the straight sections because you want the flow to slow down to reduce turbulence in the bend, but this really adds huge cost to the system. Now if you really want something slick, have all your bends take on the NACA duct peanut shape for the best possible flow. Mandrel bending prevents a smaller diameter then the straight and is usually the best you get.

It is all about managing flow velocities without becoming flow restrictive.

With headers designs, if you can decide what RPM you want the most power, you can tune your primary tube length so that the lower pressure portion of the pulse wave of one cylinder is passing the end of the primary tube that has the exhaust valve that is just opening so that primary tube will have the lowest pressure possible. This however can only be tuned to a specific RPM range and you also have to worry about packaging problems. Usually headers with big fat primary tubes will perform worse then ones with smaller tubes.

Now when you hear someone say that you need back pressure for low RPM torque they are exactly opposite of what is really going on. You want the least pressure possible in the pipe at low RPM (or any RPM) and larger pipe increases pressure because the velocity is much lower.

None of this applies to turbo systems. That is a whole other chapter on exhaust theory.
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Gadget