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PostPosted: Wed Jun 29, 2011 4:51 pm 
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Joined: Wed Jul 01, 2009 9:36 pm
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Location: Christchurch, New Zealand
I want to mount my intercooler at the back of the engine bay, right in front of the scuttle, and vent it out at the lower rear sides of the bonnet. The question is... will the pressure at the base of the screen be high or low... so will the air flow down or up through the cooler?

I don't have room to put it in front of the radiator. The motor is a gen 4 3sgte with a 6 speed box.


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PostPosted: Wed Jun 29, 2011 5:04 pm 
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Right in front of the windshield is high pressure.

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PostPosted: Wed Jun 29, 2011 11:01 pm 
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Location: central Arkansas
You can guess... or you can you can find out for sure.

1/4" clear vinyl tubing is 10 cents a foot at my local hardware store. Google "homemade manometer" and read up on how they work; you basically need clear tubing, something reasonably rigid to tape the tubes to, a ruler, and a few drops of food coloring so you can see the water. A row of manometers is called a "rake."

On my 1972 Capri there was a high pressure area ahead of the windshield, but pressure under the hood was even higher, so opening up the back of the hood bulge blew a lot of air out through the "high pressure" zone.


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PostPosted: Thu Jun 30, 2011 7:11 am 
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TRX
That is spot on, you can not just punch a hole in the hood any place. That is exactly why the NASCAR guys pick up the air intake right a the base of the windshield.
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PostPosted: Thu Jun 30, 2011 9:19 am 
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Hmmm, I saw an article recently (probably linked from here) about downforce on vehicles. It showed where the various areas of a regular car procide lift. I'm pretty sure that it said that as the air gets pushed over the front of the bonnet, it causes a low-pressure zone above the bonnet. I also remember driving my old Mini without the bonnet hinges attached and as soon as I got over about 40mph, the the rear of it lifted up, pivoting on the bonnet catch at the front. This also indicates a low pressure zone above the bonnet.

Now that said, the locost has a much longer bonnet and the dead, turbulent low pressure zone mentioned above probably only occurs towards the front rather than at the base of the window.
Also, these low pressure zones could well just be relative to the very high pressure zones near them - i.e. it might actully still be high pressure, just not *as* high.

Or I imagined all the above in a dream... I`ll try to find the link :)


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PostPosted: Thu Jun 30, 2011 9:46 am 
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My recent rip to The Gathering may add something to this discussion. My bonnet is made of .040" 3003-H14 aluminum with no reinforcing ribs or hems. So it is rather flexible. At highway speeds, the bonnet was stuck like glue to the support on the scuttle. Occasionally, it would "snap" up, only to be subsequently "sucked" back down into place. I suspect the snapping was due to a momentary change in wind direction or a localized gust. So either the trailing edge of the bonnet is normally under high pressure, or there is a low pressure in the engine compartment below. This would suggest that air would flow into the intercooler.

As all Locosts are different, YMMV.

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PostPosted: Sat Jul 02, 2011 7:16 pm 
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TRX wrote:
You can guess... or you can you can find out for sure.

+1 TRX

A water manometer will work fine, or you can buy a low cost Magnehelic differential pressure gauge from e-bay.

These come in a huge range of pressure ranges and scales, so be very sure of what you are buying.

Something that measures up to a "few" inches of water gauge pressure will be ideal for aerodynamic testing.
These all have two pressure ports, and measure the difference in pressure between the two ports.
The scales and pointers are quite large and easy to see while you drive.
With a couple of lengths of small bore hose, you can measure the air pressure difference between any two places on your car.
Where you can find the highest pressure difference either side of a panel, that is where you cut the hole to get maximum airflow.

Once you get one of these, and start actually measuring things, it will open up a whole new world of discovery.
About $70.00 brand new, or a fraction of that secondhand......
http://www.dwyer-direct.com/shop/search ... HAodTQ-AXQ


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PostPosted: Sun Jul 03, 2011 12:28 pm 
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Good suggestion, Warpspeed, I think I'll invest in a Meglaheegla... Manamega... air pressure differential measuring thingie myself. Question is: what range best suits our needs?

Let's see, one PSI equals 27.7 inches of water (thank you, Google), which is close enough to 28.8 inches to suit my tastes, and q (dynamic pressure in PSF) equals 1 at 20 mph (close enough--it's a memorized back-of-the-napkin rough figure I came up with when I was designing aircraft on the backs of napkins...waaay before Google) then ram air pressure at 20 mph is dang close to 5 inches of water, 50 inches at 63 mph. Decisions, decisions.

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PostPosted: Sun Jul 03, 2011 5:11 pm 
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Location: Melbourne, Australia.
I bought five of these Magna-helic-thingies plus a big box of rubber hoses and barbed fittings off e-bay for fifty bux.
All five gauges were the same range, 0 to 10 inches of water.

They are certainly cheap enough to buy a couple in different ranges.

I can do all my testing at sane road speeds that don't attract attention, and measure several things at once.

As pressure differentials everywhere rise very steeply with speed, all you need are good repeatable pressure and a speed readings for comparative testing purposes, and both can cover a wide range and still be very useful.


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PostPosted: Tue Jul 05, 2011 4:57 pm 
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Whoa! MORON ALERT MORON ALERT

Ignore my previous post. q (dynamic pressure in pounds per square foot) at 20 mph is roughly 1, that's 1/144 pound per square inch, and that is about 1/5 of an inch of water, not 5 inches of water. What was I thinking? Sheesh.

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