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Old 04-09-2007, 11:21 AM   #1
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Default Heat vent in Firewall?

Is there some reason I have never seen anyone put a vent on the upper part of the firewall to allow heat out of the engine compartment? The area just behind the upper firewall does not go into the cabin of the car it just vents to the windshield area. Why could I not cut a 4 inch by 2 inch vent and allow air to escape that way? With TSI or other front vents there is lots of air coming in but not much way for it to escape this seems like a natural area to use as it is right behind the turbo. I know that for spec racing it is not allowed but for a daily driver has anybody done it?
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Old 04-09-2007, 11:43 AM   #2
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I think Tom (brgracer) had fooled around with it.

the problem is, the pressure at the back of the firewall is higher than in front of the nose. So, while driving air will enter the back of the firewall, not escape....eitherway, if it lowers temps, than so be it.
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Old 04-09-2007, 03:51 PM   #3
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yeah I think braineack is onto the fundamental flaw with venting at the cowl... at high speeds its no longer a vent.

I have my hood pitched for looks and to help cool off while waiting in the grid at drift events. it actually mad a big difference in how fast my bay cooled off at idle. and the top speed at most of the smaller events barely break 3rd gear in my (then) stock miata.

but if I were doing like an open track event I think its the worst way to aid cooling. alot of race cars seem to vent just in front of the door at the rear of the fender or manage the air under the car, so I would guess those are the most efficient ways.

a miata I bet the stock belly pan with the normal hood placement is the most efficient you can get without lots of fab work.
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Old 04-09-2007, 05:32 PM   #4
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I have been running with no belly pan since I got my car, the old one was broke so I chucked it. I am going turbo soon so should I get some sort of belly pan in there to aid in cooling?
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Old 04-09-2007, 05:36 PM   #5
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yeah, i couldn't stop overheating without one on the track. everytime i hit about 100mph(substained) the water temp would skyrocket....as soon as i slowed it down a bit it would go back to normal.

installed the pan, add a cheap *** scooper....went back and could hit substained 120-130mph and not a blip on the OEM gauge.
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Old 04-09-2007, 06:41 PM   #6
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Yeah, put the pan back on. It helped my cooling a lot.
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Old 04-09-2007, 11:24 PM   #7
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from another thread but I think this is the best way.
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Old 04-10-2007, 09:58 AM   #8
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a lot of us right hand drive guys (like here is australia) have a hole cut in the part of the firewall that you are talking about. but yeah we use it as a cold air intake, not a heat vent, for the reasons mentioned by braineack.

this is what mine looks like at the moment (not boosted yet)

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you guys have a lot more stuff on that side (brake cylinder etc yeah? ) so can you guys do this on a left hand drive car ? coz some of the turbo guys here do have a cold air intake similar to mine on their turbo set ups.
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Old 04-10-2007, 10:02 AM   #9
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look at uofacat's setup.
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Old 04-10-2007, 10:06 AM   #10
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Yep, above 35mph the cowl becomes a high pressure area. I have a hole there from my CAI (similar to fatty's) and so does Uncle Arthur. You can see the holes in his pics that he posted recently.
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Old 04-10-2007, 10:13 AM   #11
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Quote:
Originally Posted by t25miata View Post
Yep, above 35mph the cowl becomes a high pressure area. I have a hole there from my CAI (similar to fatty's) and so does Uncle Arthur. You can see the holes in his pics that he posted recently.
Hmmm; cooler air intake at higher pressure. Certainly beats a "Tornado" on an NA Miata. I wonder if anyone has taken actual measurements here to see what kind of "boost" it provides
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Old 04-10-2007, 10:16 AM   #12
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Black cherry on .net did i believe.
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Old 04-10-2007, 11:05 AM   #13
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green sucks, red blows. you can see how air wants to enter by the firewall....
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Old 04-10-2007, 12:03 PM   #14
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wait, you mean the JDM style hood vents are right and the FM hood vents are less effective?
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Old 04-10-2007, 12:14 PM   #15
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no id would say the fm style hoods are correct, cause there is a low pressure zone and the air will not try to enter back into the hood but exhaust from it....the jdm riser hoods allow an opening for the high pressure to enter from behind into the cowl, and not escape
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Old 04-10-2007, 03:04 PM   #16
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Not sure what you mean by JDM hoods but what braineack said is right. The other way to look at it is to say the red areas are low pressure areas, where you will find the most consistant slip stream. The grean areas are high pressure where the air meets some sort of resistance or wall it will build there until it spills out somewhere else.

God convertables mess with your air flow over the car. Wonder if anyone has a flow pattern analysis with hard tops and with any of the rear spoiler types...meaning simple lip v.s. GT wing.
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Old 04-10-2007, 03:34 PM   #17
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i think the above pic is velocity, not pressure, but whatever... it still gives you the right idea.
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Old 04-10-2007, 04:08 PM   #18
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Quote:
Originally Posted by Braineack View Post
i think the above pic is velocity, not pressure, but whatever... it still gives you the right idea.
well now it doesn't!

I was gonna say FM puts their vents under the green part and the JDM ones (garage vary, stage21, etc) put them under the red... if green was pressure it'd push the air INTO the vent and red would pull it out. in through the nose, out through the vent.

but since it's velocity, you can't really tell. though generally higher velocity is lower pressure a la bernoulli. for example the blue area behind the seats has low velocity but it could be a low pressure region too. hmm.
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Old 04-10-2007, 05:04 PM   #19
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also the dangerous thing about graphics and graphs like that is there is no reference for the speed at which that particular event happens...

at a dead stop that graphic is moot, and speeds lower or higher it might be a different gradient... VASTLY different. there is also no reference to the pressure gradient at the edges of the hood or body, while it might be true at the dead center it might be the inverse at the edges...

and even so while it might be true at one speed its not true at all speeds.
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Old 04-10-2007, 05:20 PM   #20
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not though it's a high pressure area it really only works (the velocity image that is) if you look at if in terms of applying what you know to the image. The red areas clearly indicate the fastest velocity areas. if an area has a fast velocity it means it has a drop in pressure. Anywhere that the velocity drops (in the air dam, the windsheild) the pressure increases.
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