psi vs. cfm
03-18-2003, 01:34 PM
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psi vs. cfm
ok, why is it that 5psi on one turbo is not the same as 5psi on another. psi is the force that a fluid exerts on its containter. i realize that psi is pressure and cfm is flow. whereas pressure can be measured in torrs, pascals, kg/cm^2, psi..., but cfm is mesured in ml/sec or whatever. like pressure is mass/volume and flow is volume/time. i'm just trying to make the connections. i know i'm on the right track, i just haven't finalized the thought process. smile.gif
03-19-2003, 01:14 AM
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If you've got 5 psi boost, that means that the static pressure is 5 psi in the intake manifold or wherever you're measuring it. You can get a flow from pressure if you know the density of the fluid (the density of air changes with temperature), and the geometry and surface conditions of the flow channel(s). When you go from one motor to another, it is the flow channel that changes. Putting pressure on your intake manifold is basically pushing the air through it--the rate it can flow at depends on the size and conditions of the escape route.
What this means is that 5 psi in one motor will be different than 5 psi in another, given the same air temp, because they have different inlet configurations. The size and roughness of the inlet runners and ports, the condition and lift of the valves and valve seats, all that piping and plumbing will affect the flow. You have friction anywhere the flowing air (and fuel when it's added) touch stationary surfaces, as well as the questions of laminar/semi-laminar/turbulent flow effects, so small differences in the curves and surface roughness can have significant effects.
If your blower has a flow rating, it is usually based on a test setup which measures the pressure and airflow right at the outlet. This is a decent benchmark for comparing different turbo models but does not translate directly to real-world performance. The plumbing will affect what you end up actually getting out of it.
[ March 19, 2003, 08:19 AM: Message edited by: Ego ]
What this means is that 5 psi in one motor will be different than 5 psi in another, given the same air temp, because they have different inlet configurations. The size and roughness of the inlet runners and ports, the condition and lift of the valves and valve seats, all that piping and plumbing will affect the flow. You have friction anywhere the flowing air (and fuel when it's added) touch stationary surfaces, as well as the questions of laminar/semi-laminar/turbulent flow effects, so small differences in the curves and surface roughness can have significant effects.
If your blower has a flow rating, it is usually based on a test setup which measures the pressure and airflow right at the outlet. This is a decent benchmark for comparing different turbo models but does not translate directly to real-world performance. The plumbing will affect what you end up actually getting out of it.
[ March 19, 2003, 08:19 AM: Message edited by: Ego ]
03-19-2003, 05:43 AM
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Laminar is DEFINITELY the wrong word there, you are looking for "linear". Pressure versus airflow is either a lograthmic or exponential formula, I cannot remember which. The basic premise is: increased pressure never returns an equal amount of air flow. Pressure increases give diminishing returns, until the point where more pressure results in zero additional flow.
After that point, the flow can actually be hindered by further increases in pressure. Fluid dynamics is a bitch like that wink
[ March 19, 2003, 12:44 PM: Message edited by: Red ]
After that point, the flow can actually be hindered by further increases in pressure. Fluid dynamics is a bitch like that wink
[ March 19, 2003, 12:44 PM: Message edited by: Red ]
