Pressure & CFM through port

  • #1

Main Question or Discussion Point

I'm generally pretty good with mechanical physics, but I haven't taken a fluidics class yet to know how to answer my own question-

On our racecar (http://www.mcdermottfamilyracing.com [Broken]), we have a scoop of given dimensions (that i dont have yet, but will get soon). It's of a trapezoidal shape, but I'll get what I can.

We have an air pan under the scoop, so that the area within the scoop becomes pressurized, and "rams" the air into the carburetor.

It's a 4 cycle engine, so here's my best theory on how things work-

6800 RPM at half track (330 feet) @ 97 MPH

6800 RPM * (2 RPM per intake stroke per cylinder, for a 4 cycle engine) = 3400 intake strokes per minute

3400 intake strokes per minute * (arbitary time period, say 1 second) = ~57 intake strokes in that one second

57 intake strokes/sec * 505 cubic inches = 28,785 CI/sec = 16.7 CF/sec

So at half track @ 97 MPH, we're drawing about 16.7 cubic feet per second OUT of the scoop

------------

7300 RPM through the lights (660 ft) @ 119 MPH

7300 RPM * (2 RPM per intake stroke per cylinder, for a 4 cycle engine) = 3650 intake strokes per minute

3650 intake strokes per minute * (arbitary time period, say 1 second) = ~61 intake strokes in that one second

61 intake strokes/sec * 505 cubic inches = 30,805 CI/sec = 17.8 CF/sec

So at half track @ 119 MPH, we're drawing about 17.8 cubic feet per second OUT of the scoop

------------

Since I know how much volume I'm drawing out of the scoop, the air velocity coming into the scoop (assuming same as car speed), and if I find the area of the scoop opening, is there a way to find the pressure inside the scoop?

Edit- The opening is 12" x 14" x 3.5". (B1+B2)/2 * h = 45.5 square inches.

The Scoop's internal dimensions are roughly 23" deep, 16" wide, 5" tall. The back is blocked off vertically, so it represents somewhat of a rectangular prism. The air pan blocks off the bottom so it's uniform flat from the front of the opening to the back plate.

[PLAIN]http://magnethead794.com/coppermine/albums/McD_Fam_Racing/camaro/normal_line_00070.jpg [Broken]
 
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Answers and Replies

  • #2
jack action
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If I understand you correctly, you want to know how much boost ram air gives you. It depends only on the vehicle speed:

boost pressure = V² / 56818

Where the boost pressure is in psi and the vehicle speed V is in MPH.

More info about ram air http://www.sportrider.com/tech/146_9910_ram/index.html".
 
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  • #3
not necessarily- though helpful- I'm trying to figure out the amount of pressure that builds up in the scoop area. Our air pan has been deflecting, so i'm trying to figure out how much pressure is building up inside to be bending the pan (1/8" sheet aluminum).
 
  • #4
jack action
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The pressure inside your scoop shouldn't be greater than the boost pressure presented above.

On the outside of your scoop, that is another story. The air stream will change velocity according to the shape of your scoop and the higher the velocity, the lower the pressure around your scoop. The increase inside minus the decrease outside will give you the pressure differential that deflects your scoop. My guess is that it is no more than a 1-2 psi difference. I base my guess by playing around with http://www.grc.nasa.gov/WWW/K-12/airplane/foil3.html" [Broken]. An aerodynamicist might provide a better guess.
 
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