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Originally Posted by beef_bourito
And as for the exhaust, lets say it's an open engine bay car so there's nothing to burn and there's not turbo.
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This is generally speaking as there are exceptions to everything:
As was already stated ideally you don’t want any back pressure, it always has a negative impact on performance. However, sometimes back pressure is tolerated to improve the scavenging effects of the exhaust system.
During the exhaust stroke the exhaust gas is pushed out of the cylinder into the exhaust pipe. As the exhaust gas flows through the exhaust pipe it carries momentum. After the exhaust stroke the exhaust valve closes, but the fluid do not stop flowing because of its momentum.
Since the fluid is still flowing through the pipe but no new exhaust gas is being added the exhaust system pressure briefly drops near the exhaust port. Now if the exhaust valve opens again before the temporary drop in pressure disappears it will take less power to drive the next exhaust stroke. Lowering the exhaust system pressure allows the exhaust gases to more readily flow out of the cylinder.
To maximize this savaging effect one needs to maximize the fluid’s momentum. Momentum is dependent on both mass and velocity. Mass is more or less a constant since there is a limited number of fuel sources available, nor would it really be practical to try and change. Velocity on the other hand is directly affected by the exhaust pipe diameter. Larger diameter pipe will have a lower velocity, while smaller diameter pipe will create a higher velocity for a given flow rate.
Since the smaller diameter pipe can impart more momentum to the fluid it would seem to be the better choice for maximum performance. However, the smaller diameter pipe requires more power to maintain the same fluid flow rate. Or another way to look at it is the smaller pipe creates more back pressure.
So a balance between back pressure (pumping losses) and scavenging (fluid inertia) must be found. As well, if the pipe diameter is fixed then it will only be optimized for a single flow rate (read: engine speed). For an engine speed less than ideal the back pressure will be low but the scavenging will also be too low to be useful. At an engine speed above ideal the back pressure will quickly increase nullifying any of the scavenging effects. This allows the exhaust system to be tuned for
high-end or
low-end power, but not both.
For the case of no exhaust it can theoretically be treated as a pipe with an infinite diameter. So the back pressure would be low, but there would not be any exhaust gas scavenging.