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Old 09-30-2011, 07:59 AM   #1 (permalink)
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how to measure air pressure?

I did search but found nothing.

How do you measure air pressure around a moving car? I have 2 reasons for wanting to do this

*To work out where to put an engine air intake (I want high pressure when the car is moving at 50 - 100kmh)

*To make an underbody tray - aero device to smooth air flow but also to work out how / where to place vents in a low pressure spot under the car to draw air through the radiator grill and out of the engine bay. I live in inland NSW Australia and it is often at 35C in summer, so although I have reduced the size of my grille (partially blocked) I still want to know there is good airflow once I make an underbody tray.



Thanks - any ideas appreciated

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Old 09-30-2011, 10:28 AM   #2 (permalink)
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GOOGLE "manometer"

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Old 10-02-2011, 02:50 AM   #3 (permalink)
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short of spending quite a lot, i can't think of a way to use this & get readings while at 60 - 100kmh. Without crashing that is
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Old 10-02-2011, 11:08 AM   #4 (permalink)
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...use a length of tubing from area to be "measured" to the top of the manometer which you locate inside the car (Duck Tape™ does an excellent job).
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Old 10-03-2011, 08:50 AM   #5 (permalink)
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What you need is called a "Magnehelic gauge"
Get one calibrated for 0-2" or 0-5"WC.
They must be held vertical to work correctly but won't spill or slosh
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Old 11-30-2011, 06:53 PM   #6 (permalink)
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for a *Ram air* intake I have 2 suggestions. 1) Front of the car, Grill area. where the car initially bulldozes through the air, Headlight area of the Radiator support generally has some good sized holes that work great for this purpose. (Very popular with the go-fast croud) 2) base of the windshield, air builds pressure changing from the soft angle of the hood to the more acute angle of the windshield. (Also known as a "Nascar style setup", Dont ask me if this is how its done in Nascar. I dont know.)
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Old 12-04-2011, 10:23 AM   #7 (permalink)
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You have both answers here: pressure is best measured with a manometer or magnahelic gauge inside the car in an easily observable location, with one piece of rubber tubing going to the location you want to measure the air pressure. Don't point the end of the tube directly into the air stream. Manometer is cheaper, magnahelic is easier.

Pressure alone is not a good indicator of airflow. In fact, high pressure often indicates low flow because the path is restricted (pressure is higher in front of a filter because the filter restricts the flow of air). Airflow is better measured with a pitot tube or a hot-wire anemometer. A pitot (pronounced "pee-toe") connects to a manometer or magnahelic inside the car with two rubber tubes. The hot-wire anemometer would need a sensor cable long enough to travel to the location you want to measure. Pitot tubes and anemometers are not cheap, but not terribly expensive either. The anemometer is direct reading for velocity, you have to do some simple math with the pitot tube.


Last edited by instarx; 12-04-2011 at 11:28 AM..
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