Air Flow Through Front Holes, The ones behind the bumper. Options

[McMark]

I just bought myself a little handheld anemometer. I took out one of the plastic plugs behind the bumper and duct taped the meter into place in the front trunk. Went for a little drive. Here's what I found...

At 40 MPH, I was getting a wind speed of 17.5 mph in the hole. Converted to CFM, this is 25.718 CFM. Not bad for one little hole. The bumper was still in place and the LE airdam was still installed as well. Next I'll test a stock valence.

Just sharin. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Oh, BTW, the air pressure was 4 bft. I don't know what that part means, but there it is.

[Brian Mifsud]

McMark,

I do alot of airflow measurement for a living.


Pressure measurements with the anemometers I've worked with are calibrated in inches or feet of water.

1 atmosphere (14.7psi) will raise a column of water 32feet (in a evacuated tube).

Most fans and blowers flow air at pressures WAY under 1 psi (1psi = 26" inches of water), and usually develop pressures rarely over 1" of water. A high quality blower like you have in your furnace at home will push about 2000cfm at about 2" of water to give you an idea of flow.

On you CFM calculation, taking the velocity x the area of the hole is usually on optimistic calculation. The actual flow through that hole is most likely at least 30% lower than what you calculate.

Generally, measuring pressure rather than velocities will give you a much more accurate number to do calculations with across an opening. Velocity measurements are very directionally sensitive. If you don't happen to have the probe at the perfect angle, you may not be getting the "true" average velocity.

Static pressure is a better way to go before and after the opening.