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ZBUM's View on Backpressure
I'm sick of people using the term "backpressure". All they are doing is repeating a term they've heard before. The following is my take on the subject of exhaust gasses and flow....
backpressure is a myth and a misnomer (sp?).
The truth is....
Exhausts are hard to optimize for a wide RPM range. Since most engines make most of their power in the upper RPM range, most exhausts are optimized for lower RPMs to help with low end power.
Exhaust gases come out of the engine in pulses. Depending on how long the valves are open and how many RPMs the engine is turning determines how much air and how fast it comes out. But once it's out, it has to stop since there isn't anything else to force it any longer. At least, not until the next cycle.
the exhaust between the exhaust valves and the turbo is highly pressurized. And since pressure acts in all directions equally, the engine has serious back pressure.
As for the size of the exhaust thing, remember that the exhaust is actually a series of pulses. A good exhaust will create an even flow of air. This is hard to do over a large RPM range though. So most exhausts are optimized for lower RPMs since engines typically can make more power at upper RPMs to compensate for the exhaust that's made for the lower RPMs.
Think about it. The more RPMs, the more air that flows. The more air that flows, the more space it takes up.
So why does the larger exhaust create a lower RPM loss? The pulsing action is magnified. So instead of one smooth flow, the air is started and stopped and started and stopped. Thus, the energy is used to start and stop the movement. That's why the low end loss.
And to answer the question "why doesn't it matter on turbos?"
Because the turbo takes those gases and smooths them out.
When the gases hit the turbo, they are pressurized because it takes energy to spin the turbo. And once the turbo starts spinning, the energy of the turbo is very smooth coming out of it. So the size of the pipe doesn't matter as long as it is big enough to flow the volume of air coming out of it.