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Mass Air Flow

  1. Sep 21, 2009 #1
    This inquiry is in relation to internal combustion engines. I am trying to figure out how to maximize the mass of air going into my engine. Because the more air that can get into the cylinder, the more power the motor can make. Now I've been researching Bernoulli's Principle and as I understand it: If the velocity of a fluid increases then the pressure decreases and visa versa...... So if this is true then the actual Mass of air flow over the course of 1 min would be the same correct? Or am I missing something?

    I am trying to understand how this works because in engines there are intake ports. These ports allow air to flow into the motor. Some people say large ports are the way to go because they flow more cfm but others swear by a smaller port which increases port velocity. From what I think I know, the more cfm would mean more air mass regardless of port velocity. The way I look at it is that if this head flows 300cfm and this other head flows 250cfm then the 300cfm head would induce more air mass and therefore make more power...

    Though there is this gentleman who had a completely stock 600cc Honda motorcycle. He took it to they dyno and ran it for a baseline. Then he reworked the intake ports by actually adding some material to increase port velocity and took it back to they dyno. He increased hp throughout the rpms and I believe saw a peak gain of 12hp. Nothing else was changed and this was SAE corrected which Factors for ambient air temperature.

    Anybody who can help me to understand and solve my self made dilemma I would greatly appreciated any feedback you can provide me. Perhaps I'm not taking into account some variables?
  2. jcsd
  3. Sep 21, 2009 #2
    Also some variables to consider are that air does have mass and therefore has momentum and inertia and that the whole process of the engine sucking in air does take time. I don't know if this adds more difficulty in solving my issue but these are certainly things that must be accounted for. Also valve timing effects how much air gets into the cylinder and so forth.
  4. Sep 22, 2009 #3
  5. Sep 23, 2009 #4
    In general, yes, more air flow will mean more power. However it isnt as simple as that. Sonic velocities at the inlet will be the limiting factor (choking). This is when you have so much pressure difference between the inlet and cylinder that the air flow speeds up to the speed of sound, after this point has been reached you can make the inlet as big as you want and it wont make a difference to how much charge you get in the cylinder.

    Making the inlet smaller means that it will choke more easily. Making very large inlets means you eliminte the pressure pulses. So you've got to match the port size to the inlet size. Too big or too small will harm performance.

    The air flow is not uniform, there are pockets of higher pressure (density) air that are caused when the inlet valve is closed suddenly. This pocket of higher pressure air then rebounds off the closed port and back along the inlet manifold, and then rebounds back.

    The key is to open the valve when this high pressure (density) pocket of air is about to hit it. This then allows more charge to enter the cylinder.

    However its not as simple as that, becuase the timing for opening is differnt at different engine speeds. This means you need to cut the inlet length to correspond to the rpm that you want to highre charge at.

    Shorter inlet trumpets will mean less time to bounce and will suit higher rpm, longer inlet trumpets give the air longer to bounce and will cause a gain in low end torque.

    Without knowing exactly what your friends engine was like before or what he did, its impossible to say why he saw a power gain.

    Also you need to be very careful with your thinking, don't think that becuase something worked in this case it holds true for all engines.
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