[mx6mat]

These theories capture the basics.

The fact is there is an optimum diameter that is primarily dependent on the velocity of air in the pipe, the friction in the pipe, the density of the air. For fluid flow in any pipe there is a value obtained from charts using this information called the darcy friction factor. The optimal condition is where the velocity in the pipe is the fastest but still producing layered steady flow(laminar flow). The pipe will give the least resistance and consequently the losses in the pipe will be minimal.

The difference in pressure between the engine and the exhaust is negligible when using different size exhausts. The volumetric flow rate through the exhaust doesnt change that is why when you decrese the diameter the air flow has to be faster. In the perfect system the gases exiting the engine will arrive at the collector at the same time. There shouldnt be any lag other wise there will be an imbalance in the fluid flow at this point in the system and the gases will be traveling faster in one pipe than the other to reach the collector at the same time or you may get air flow in the opposite direction back into the engine in the worst case. The optimum case has to be when The air arrives at the collector allowing the maximum amount of air to flow through the exhaust.

Fluid flow can be described in 2 possible ways. Laminar or Turbulent or chaotic. In turbulent flow the friction losses in the exhaust are greater there fore this will result in a drop in pressure of the lenght of the exhaust and hence decrease the velocity of exhaust air flowing and hence the volumetric flow rate. ( ie less air exiting exhaust). At the point just before the flow becomes turbulent the flow is perfectly laminar (layered) and this gives you least pressure loss through the exhaust. Therefore greatest flow rate.

The next step would be to minimise the pressure losses due to friction in the pipe by lowering the darcy friction factor. This can be done by choosing a smoother material or polishing the inside of the exhaust )if possible. The only other way is to make the exhaust as short and as straight as possible.

Welcome to your first lesson in engineering fluid flow.

As for having less torque. I suggest this is due to the fact that because you have reduce the restriction in the exhaust, the gas will suck out of the cylinder faster and the total force on the piston at the begining of the stroke will be less untill the engine starts revving fast enough that the escaping air actually aids the piston on the up stroke and gives a higher negective pressure is the inlet opens to suck more fuel in.

The collecter also provides the smoothest possible transition into the main exhaust pipe. AT there would be the highest losses if not done properly.

Of course this description has been essential for a NA engine but the principles for the exhaust of a turbo engine are essential the same except the turbo becomes huge restriction in the system. Hence the efficiency of your turbo becomes another variable in the system. (turbine size, fiction losses in the turbo[ie ball bearing turbo] ) Allowing the gases to escape faster allows the turbo to work at its optimum.

A better exhaust hence increases efficiency as you would expect.



This is all off the top of my head from theoretical point of vue. Some of it my not be 100% so feel free to correct me if you know better but Im sure it is pretty close.