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914World.com _ 914World Garage _ Air Flow Through Front Holes

Posted by: McMark Aug 17 2005, 10:19 AM

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. Oh, BTW, the air pressure was 4 bft. I don't know what that part means, but there it is.

Posted by: Joe Ricard Aug 17 2005, 10:24 AM

So where was the air escaping?
Funny you should mention that, I was just contemplating moving my 2 row cheapy oil cooler up there.
Wonder if you pulled the two plugs in the bottom of the trunk and put screen over it or louvers from the bottom side pointing back.

Posted by: MW 914 Aug 17 2005, 10:25 AM

Are you trying to get enough airflow up there without cutting it up for an oil cooler or suh-n?

Posted by: SirAndy Aug 17 2005, 10:34 AM

if you don't want to cut the tub and depending on how much extra cooling you need, you can just mount the cooler in front/under the bumper.

you'll get less air-flow as on a "through" setup, but for most cars, it should be plenty ...
Andy

Posted by: trekkor Aug 17 2005, 10:53 AM

Is this on the green car?

I'd like to install my oil cooler before the Sept 23rd track day.

I'd like to remove ALL the front plugs and make a shroud to vent the air out the bottom.

Wanna test that?

KT

Posted by: Brian Mifsud Aug 17 2005, 11:16 AM

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.

Posted by: Joe Ricard Aug 17 2005, 11:25 AM

Damn! now my head hurts.

Posted by: McMark Aug 17 2005, 12:02 PM

Damn, I'm really starting to hate air flow. Nothing is as it seems.

Isn't the 4 bft a pressure reading?

Posted by: McMark Aug 17 2005, 12:04 PM

QUOTE (trekkor @ Aug 17 2005, 08:53 AM)
Is this on the green car?

I'd like to install my oil cooler before the Sept 23rd track day.

I'd like to remove ALL the front plugs and make a shroud to vent the air out the bottom.

Wanna test that?

KT

Yeah, it was on the green car.

I'd like to do some testing of your shroud, just to have the numbers in the future for comparison.

Posted by: Aaron Cox Aug 17 2005, 02:15 PM

im about to install a cooler and fab a shroud.....

trek - what are the dimensions of your cooler? maybe we can buy/make 2 shrouds

Posted by: trekkor Aug 17 2005, 04:57 PM

BIG... really BIG

23.5x5.5

KT

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Posted by: SpecialK Aug 17 2005, 08:20 PM

QUOTE (McMark @ Aug 17 2005, 12:02 PM)
Damn, I'm really starting to hate air flow. Nothing is as it seems.

Isn't the 4 bft a pressure reading?

Nope, it's an airspeed measurement. Bft stands for "Beaufort", some old fart sailor from the 1800's.

Posted by: Aaron Cox Aug 17 2005, 10:12 PM

QUOTE (trekkor @ Aug 17 2005, 03:57 PM)
BIG... really BIG

23.5x5.5

KT

im a good ~13" x 5.5
hmmmm