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Old 05-06-2020, 03:48 PM   #1 (permalink)
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Hood vent orientation?

So, one of the projects with my Saturn involves hood vents, partly to reduce drag by evacuating hot engine bay air onto the hood, and partly to reduce engine temperatures during hard driving. I finally received my order of four pairs of a roughly 4x8Ē vent, and am working on deciding on where to place them, other than remembering seeing the CFD analysis of the Ion coupe and seeing where people that race the Ion Redline put their hood vents.

My first question regards the vents themselves. They could be mounted either totally flush with the hood surface, with the vent portion inside the hood, or with the vent portion sticking above the hood. Any thoughts on this conundrum?

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Old 05-06-2020, 04:07 PM   #2 (permalink)
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I would mount them with the top surface flush, so as to not disrupt surface airflow so much.

As to where to put them: by far the best way is to use a Magnehelic gauge to measure the pressures on top of the hood, and on the bottom of the hood (ie in the engine bay) and find where the greatest pressure difference is.

No rules of thumb or guesswork needed!
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Old 05-06-2020, 04:33 PM   #3 (permalink)
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As far as placement, I could copy the known good placement used by the racers. They say the cars in stock form generate wicked lift above ~150mph, which the vents alleviate some of. Iíve come up with five different mock ups, loosely placed on the hood. The ones with the vents only above the headlights are basically whatís being run on the race cars. The other ones are basically trying to get more vents closer to the hood break over point
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Old 05-06-2020, 04:59 PM   #4 (permalink)
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Honestly, why copy what other people have done when you can easily find the best locations for yourself?

(And note: you can also measure downforce/lift, not go by word of mouth as to what supposedly happens! For example, if the car develops 'wicked lift' at 150 mph, it must be developing lots of lift at any speed. )

But if you just want people to guess, #4 is probably going to be best, with #5 a consideration if you live in a really windy area.
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Old 05-06-2020, 07:39 PM   #5 (permalink)
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Quote:
My first question regards the vents themselves. They could be mounted either totally flush with the hood surface[,] with the vent portion inside the hood, or with the vent portion sticking above the hood. Any thoughts on this conundrum?
Is this three choices or two? I interpret as 'innies' vs 'outies'.
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Old 05-06-2020, 07:47 PM   #6 (permalink)
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I don’t have a magnahelic gauge to measure the pressure differential. Mostly guessing and going from the known data I’ve gotten from others
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Old 05-06-2020, 07:47 PM   #7 (permalink)
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Originally Posted by freebeard View Post
Is this three choices or two? I interpret as 'innies' vs 'outies'.
Innies vs outies basically
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Old 05-06-2020, 08:38 PM   #8 (permalink)
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Originally Posted by 19bonestock88 View Post
I donít have a magnahelic gauge to measure the pressure differential. Mostly guessing and going from the known data Iíve gotten from others
From about US$25 to buy one, then no guesswork (or blindly following what others have done). To me it's like putting windows in a house without knowing where the sun moves in the sky.
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Old 05-06-2020, 09:53 PM   #9 (permalink)
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^This. Magnahelics are relatively cheap, and you'll know where the greatest pressure differentials are. There might be details you can't see on the Ion race car, i.e. the hood vents over the wheels might be open to the wheel housing below, as is common on racers. I would be hesitant to copy it without more information.
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Old 05-07-2020, 02:40 AM   #10 (permalink)
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the fiesta in nearly all rallycross/wrc/s2000 versions uses the same spot for the hood heat extractors along with some metal (?) around the inside half Im guessing in order to create vortices for even more heat extraction (??)



anyway, since EVERYONE in the motorsport industry uses the same spot I am pretty sure its measured and proven correct, however water ingestion in a daily driver would be a pitta since in both places there are sensitive parts like ecu loom , battery etc so a proper water trap must be used somehow

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