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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.
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This sits well with me because if you consider that the definition of peak torque is when the rev speed is at the optimal time for filling the cylinders with air. ie. It below peak, it has too much time to fill the cylinders, above peak, it doesn't have enough time to fully fill the cylinders.
A less restricive exhaust would allow quicker exiting of the gas from the cylinders, allowing more time fill the cylinders with air thus making anything below peak EVEN LESS efficient...... Holding that thought, would that mean that the peak torque would be moved up the rev range?
The scavenging effect would throw another spanner into the already complex theoretical base as well.
Question through, does exhaust gas eddie like water does? Ie if you look at a river flowing, you will see that the water around the banks is moving slower than that in the middle, this is because the bank, not being perfectly smooth causes turbulance thus causing some water to stagnate. This is actualy a good thing for flow becuase this stagnant water basicaly acts as the 'the pipe' for the water in the middle becuase what is going to cause less surface resistance, the bank or the stangant water...interesting effect. Anyway, does this occur with exhaust gasses? I believe it does on the intake side of things. I read about some research by NASA that found that a slightly rough pipe flowed better than a perfectly polished and smooth one because of the 'eddieing' (A point to remember when you take your head to get ported and polished).
