How Does Air Pressure and CFM Affect Performance in a Racecar Scoop?

[magnethead494]

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 ), we have a scoop of given dimensions (that i don't 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

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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

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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

 

[jack action]

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".

 

[magnethead494]

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).

 

[jack action]

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" . An aerodynamicist might provide a better guess.

 

[PJC01]

I can provide some insights on the relationship between pressure and CFM (cubic feet per minute) through the port in the context of your racecar.

First, it is important to understand that pressure and CFM are two different properties of a fluid, and they are related to each other through the concept of flow rate. Pressure is a measure of the force exerted by the fluid on the walls of the container, while CFM is a measure of the volume of fluid that flows through the port in a given amount of time.

In your racecar, the scoop and air pan create a pressurized area that forces air into the carburetor. This pressurized air is then used to help the engine intake strokes, which ultimately affects the engine performance.

Based on your calculations, the CFM through the scoop can be estimated by multiplying the number of intake strokes per minute by the engine displacement and dividing by the number of seconds in a minute. However, it is important to note that this is just an estimation and the actual CFM may vary depending on various factors such as engine efficiency, air density, and air resistance.

To find the pressure inside the scoop, you would need to consider the Bernoulli's principle, which states that the total energy of a fluid (which includes pressure, velocity, and elevation) remains constant along a streamline. This means that as the air flows through the scoop, its velocity will increase and its pressure will decrease. However, the exact pressure inside the scoop would depend on the specific geometry of the scoop and the air pan, as well as the air density and flow rate.

In summary, while your calculations provide a rough estimation of the CFM through the scoop, the pressure inside the scoop would require a more detailed analysis taking into account the specific design and operating conditions of your racecar.