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Old 03-18-2008, 08:14 PM   #8 (permalink)
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Hi Different...,

That was an interesting article, but its not about tuning an exhaust so much. Its about the aerodynamics of the fluid flow in the pipe, and how big an exhaust pipe can disrupt this flow, by allowing multiple flow modes that interefere.

Tuning an exhaust is about the reflections that the pulses in that article describes. These pulses and reflections add and subtract at the exhaust (or intake port). Lets just talk about th exhaust for simplicity. The exhaust valve opens and a pulse of waste gas leaves through the valve. This pulse will travel along within the exhaust system tubing until it hits something that has a different cross section shape. When that happens some of the pulse will reflect from where the cross sections change shape, or discontinuity. Now you have a dimished pressure pulse continuing out the exhaust system, and a smaller one headed back to the valve. As the valve is closed, the pulse reflects off the valve and starts to cause a vacuum at the valve opening. This is when the valve should be opening if the exhaust is tuned. The near vacuum at the valve then cause the exhaust gasses to be sucked out more completely and quicker.

In race cars there are no mufflers. The discontinuity is the change from the pipe to the open air. ISo the exhaust is just a pipe. The length of the pipe is set to the above condition described at the maximum power RPM ideal for the specific type of racing. In F1 the pipes are really short. In Nascar they are somewhat longer due to the pushrod nature of the engines. Take off the pipes from these engines and they would have considerably less power.

The speed of the pulses is and is not like waves on a string (an excellent way to envisions what is going on, btw). Waves on a string have a constant propagation speed set by the tension in the string. Exhaust pulses do not, as the exhaust waves are cooling as they travel and that changes their natural propagation velocity, since the gas density increases. So, this type of tuning is mostly done emperically, as computation is very difficult for such non-linear phenomena.

BTW, any shape pressure pulse can be described as a sum of sinusoidal waves. So, figuring out a solution for sinusoidal waves, and adding them together is the usual way engineers work on such problems.
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