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Thread: Air flow???

  1. #1
    ctriverguy's Avatar
    Join Date
    Aug 2007
    Ipswich, Massachusetts, United States

    Air flow???

    alright, i have to ask,as i keep reading more flow, or moves air faster... (my background in air flow was a grad level industrial ventilation class about a year ago and with that a small project outside of work. So not alot.)

    So how does one increase air flow if you have the same amount of volume (eg 1500cc) and same amount of boost (eg 10 psi)?

    I could look into all of the ventilation formulas here, but this isnt making much sense as you know your area and you know your pressure.. if those two are constant the flow into the system will be controlled by your rpms... I know, you can make the path of flow more efficient (limiting your K Factor) so it gets in and out quicker, but again, the rpms, when the cylinder closes up, control everything here...

    As for a larger diameter wheel, wouldnt that equal a larger surface to create more psi???

    I know there is ton to understand on this, so i am open to any info with some sound backing to it.. thanks guys

  2. #2

    Join Date
    Aug 2006
    Gonzales, La
    Your thinking is correct. The only way to increase the air "volume" (I think "mass flow" is a more correct term here) without raising boost, you would have to either a) reduce the temperature or b) increase the cid, VE, or rpm of the engine. It doesn't have much to do with the supercharger, really.


  3. #3
    Website may not be suitable for small children WOT's Avatar
    Join Date
    Mar 2007
    You guys are absolutely right. More air won't flow into the engine without more boost, except....

    If two SC's are at the same boost at the same rpm with the same outlet temperatures, but one of them is supposed to flow more air than the other and you make a change to increase the air flow or air density into the motor such as:

    external intercooler
    larger, smooth pipes to and from the intercooler and/or the throttle body
    different intake manifold, etc.,

    The SC that supports more flow will now be making more boost than the other SC because it will flow more air at the same rpm and as you remove the restrictions to the motor, the boost will not drop off as fast.

    Think of it this way:

    If you used a bunch of little adapters and could hook up a full sized 4” diameter fire hose to your standard hose faucet on your house and then turned it on, as long as the nozzle on the other end of the hose was closed, you would eventually fill up the fire hose and have full pressure. A gauge on the hose would show a full 45 psi (or whatever your local water pressure is). As soon as you opened the spray nozzle at the other end of the hose and tried to spray out a 2" diameter stream, water would shoot out for only a second or so and then the flow would just look like a trickle as your house faucet can no way maintain the flow necessary to keep that large diameter fire hose pressurized and spray water out of a large, fire-hose fogger nozzle.

    Now, take that same full sized fire hose and hook it up to the fire hydrant out in front of your house that you keep parking in front of and getting ticketed for, and since the hydrant is hooked up to the same municipal water supply as your house, the gauge will still read 45 psi with the hydrant turned on and the spray nozzle closed. But, this time when you open the spray nozzle you get a nice full stream exiting and the fire hose stays pressurized. That's because the fire hydrant flows more even though the house faucet and the hydrant both are at the same 45 psi static pressure with the spray nozzle shut.

    Now imagine you have both hoses hooked up as described above laying on the ground side by side: one goes to the house with a bunch of adapters, and the other is connected to the hydrant. Both are full 4" diameter fire hoses. They both are turned on, but the spray nozzles are both shut. Both hoses are fully inflated and pressurized, and the gauges on both of the hoses are reading the same 45 psi. You just barely open each nozzle a tiny bit (if that is even possible with one of those big fire hose nozzles) to spay out a fine mist. At this point, both the house hose faucet and the fire hydrant are able to keep up the flow required to produce the tiny fine mist spray. As you open the fogger nozzle more and more, the fire hose hooked up to the house faucet begins to wane while the hose hooked up to the fire hydrant is maintaining a full force spray. Now with both spray nozzles wide open, the water running out of the fire hose hooked up to the house faucet is just trickling out while the other is spraying a full high powered stream. At this point, the gauge on the fire hose connected to the house is reading about 0 to 1 psi, while the gauge on the fire hose connected to the hydrant is reading about 30 psi. Even though they both started out at the same pressure, the one that flows more will maintain more pressure (just like boost) as the restrictions are to flow are reduced (i.e. water nozzles opened, or less restriction to the motor).

    Oh, and by the way, the hose hooked to the fire hydrant eventually gets away from you and is now flopping around violently. The large brass nozzle sweeps over your head just barely missing you. It smacks into your car parked in front of the hydrant breaking your windshield and filling your car with about 250 gallons of water. In an instant, all of your Michael Bolton CD’s are ruined. But hey, was all in the name of science. Your neighbor who has been watching you over his fence all along just smiles and turns away and goes back to grilling wieners on his grill and sipping his Lowenbrau.

    Have a happy Memorial Day!
    Last edited by WOT; 05-26-2008 at 11:57 AM.

  4. #4
    Rpm should have a direct relationship to airflow on a 1 to 1 ratio, just like the fan laws, double the rpm and double the cfm. Increase the boost pressure, without increasing the discharge pressure (exhaust) and you will increase the volumetric effeciency of the motor allowing it to pump more air--think of it as a compressor. You could also think of the supercharger as a power return fan, which would have the effect of reducing the total esp (external static pressure) across the motor (pump) increasing airflow. Porting the head of the motor or installing an aftermarket cam would increase the airflow through the motor. Creating less restriction in the air intake, intercooler or exhaust system would reduce the total esp across the motor and increase airflow.

    With our motors we are concerned with getting the maximum mass of airflow through the motor for maximum horsepower. So, the more dense the air flowing through the motor the more horsepower. Heat of compression through the sc'rwill have the effect of lowering air density. Heat absorbed by the intercooler will have the effect of raising air density. Heat absorbed by introducing fuel in the intake manifold will also reduce the heat in the intake air charge and increase ari density.

    Total airflow (cfm) times air density (lb/cubic foot) equals total mass of airflow through the motor. Add enough fuel to get a ratio of 11.9 to 1, 11.9 pounds of air per pound of fuel, and you're good to go.


  5. #5
    My new toys at work bruinsrme's Avatar
    Join Date
    Jul 2007
    Worcester MA
    I need a motrin

    Good stuff thank you

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