Gen1GT
June 30th, 2009, 05:53 AM
I was just reading a post I made on Clubprotege a few months back, and realized it was a pretty good explanation of exhaust theory...enjoy:
To create flow in any fluid (liquid or gas), you need a pressure differential. As long as the pressure upstream is higher than the pressure downstream, the fluid will flow. The higher pressure differential (also know as "delta pressure", or ∆P), the faster and more forcefull the fluid (gas in this case) will move. In an NA engine, the upstream pressure is created both by piston moving up towards TDC, thereby decreasing displacement (Boyle's Law) and increasing pressure, by thermal expansion (Ideal Gas Law). The exhaust valve opens before combustion expansion is complete, sharing some of that pressure with the upstream exhaust flow, which forces the gases towards the low pressure downstream area (tailpipe). The larger this pressure drop, the harder and faster the exhaust gases move towards the tail pipe. You can create a higher ∆P by either increasing pressure at the upstream area (which potentially wastes energy that could be used pushing down on the piston to create power) or decreasing pressure downstream (by having the lowest restriction in your exhaust possible, without sacrificing velocity).
As you can see, if you create backpressure in your NA exhaust system, your ∆P is less, and the force/velocity of your exhaust is less. Backpressures of 30 psi are easy, and it all depends on where in your exhaust you measure it. Thermal expansion of the hot gases creates pressure (Ideal Gas Law), but inversely lose pressure as the exhaust cools, which adds to that downstream low pressure ideal.
As it relates to a forced induction, a high pressure differential is obviously also ideal. The more pressure you have on the inlet side of the turbine and the less pressure you have on the outlet side of the turbine, means the turbine will spin harder and faster. Having zero (or a vacuum, like he said) backpressure is the best thing you can do with forced induction, because it creates a higher differential across turbine. If anyone knows anything about electricity, compare a turbo to an electric motor. Electrons only flow when there is "potential," which is a voltage drop. The higher the voltage drop across a motor, the "harder" it wants to spin. This is why high power motors are high voltage, because they require less amperes to do their work.
This theory also works for the intake side. By creating low pressure in the intake manifold by the piston moving down the cylinder, you create a pressure drop. 14.7 PSI ambient pressure wants to move towards the partial vacuum created in the combustion chamber. Or for forced induction, you create pressure at the turbo, which still moves towards the relative vacuum in the combustion chamber.
To create flow in any fluid (liquid or gas), you need a pressure differential. As long as the pressure upstream is higher than the pressure downstream, the fluid will flow. The higher pressure differential (also know as "delta pressure", or ∆P), the faster and more forcefull the fluid (gas in this case) will move. In an NA engine, the upstream pressure is created both by piston moving up towards TDC, thereby decreasing displacement (Boyle's Law) and increasing pressure, by thermal expansion (Ideal Gas Law). The exhaust valve opens before combustion expansion is complete, sharing some of that pressure with the upstream exhaust flow, which forces the gases towards the low pressure downstream area (tailpipe). The larger this pressure drop, the harder and faster the exhaust gases move towards the tail pipe. You can create a higher ∆P by either increasing pressure at the upstream area (which potentially wastes energy that could be used pushing down on the piston to create power) or decreasing pressure downstream (by having the lowest restriction in your exhaust possible, without sacrificing velocity).
As you can see, if you create backpressure in your NA exhaust system, your ∆P is less, and the force/velocity of your exhaust is less. Backpressures of 30 psi are easy, and it all depends on where in your exhaust you measure it. Thermal expansion of the hot gases creates pressure (Ideal Gas Law), but inversely lose pressure as the exhaust cools, which adds to that downstream low pressure ideal.
As it relates to a forced induction, a high pressure differential is obviously also ideal. The more pressure you have on the inlet side of the turbine and the less pressure you have on the outlet side of the turbine, means the turbine will spin harder and faster. Having zero (or a vacuum, like he said) backpressure is the best thing you can do with forced induction, because it creates a higher differential across turbine. If anyone knows anything about electricity, compare a turbo to an electric motor. Electrons only flow when there is "potential," which is a voltage drop. The higher the voltage drop across a motor, the "harder" it wants to spin. This is why high power motors are high voltage, because they require less amperes to do their work.
This theory also works for the intake side. By creating low pressure in the intake manifold by the piston moving down the cylinder, you create a pressure drop. 14.7 PSI ambient pressure wants to move towards the partial vacuum created in the combustion chamber. Or for forced induction, you create pressure at the turbo, which still moves towards the relative vacuum in the combustion chamber.