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  • Hood Vents

    I want to do something totally not stupid soon.

    I recently drove the car to Huntington Beach on the 5 and 405 and got the opportunity to not only watch my hood bounce up and down like mad, which I'll fix soon, but also got to watch my temp gauge go nuts whenever traffic slowed down.

    I don't have a clutch fan nor do I have an e-fan installed yet, so it wasn't as troubling as it could have been had I retained the stock cooling system, but since all the track days I do are at hot tracks, I'd like to get me some vents. The aerodynamic advantage of releasing engine compartment pressure is an added bonus.

    I was thinking one 20x9.5 (17x6.5 louver opening, 5 slots) on each side of the hood bulge (Silvia front) and two 20x11.5 (17x8.5 louver opening, 5 slots) in the middle, or two 13.5x10.25 in the middle and a NACA to the boxed cold air intake. I don't want to overdo it, mostly because the vents are a little on the pricey side.

    It looks like the DSG Maxi runs something like a 24x10 in the front and a smaller one over the turbo. Bill, where did you get that size vent?

    Anyway, my question is where should I locate my hood vent/vents, and how big should they be? I have no turbo to cool.
    Last edited by SOneThreeCoupe; 04-05-2008, 12:44 PM.
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  • #2
    im sorry dude but i think the first step would be some good electric fans... and then go from there...

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    • #3
      ^ agreed

      Most PM answers: F 17X9 +22, R 17X9 +35

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      • #4
        I bolded my actual problem. If I was stupid enough to think my radiator would work sans any sort of assisted airflow, I would be posting on NiCO and ask about how to get the stretched look on the stock rimz.
        Last edited by SOneThreeCoupe; 04-05-2008, 12:50 PM.
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        • #5
          i think the best thing would be to raise the back of the hood with spacers to allow the hot air to escape i have yet to see a hood scoop or vented hood that didn't look like ass
          my car domain

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          • #6
            You want a vent at least 60-75% the size of the radiator swept area(probably 30x18"ish) located directly behind it.

            If you really want to do things right, you would duct your radiator completely out to this vent to greatly improve its efficiency, but just having the vent there with a good underpanel and ducting around the front of the radiator will help lower the pressure behind the radiator.

            A NACA duct would work over the intake, but I think it'd be need to be larger than what most people put on their cars if you're going to completely seal the airbox to it. I'd want the actual NACA inlet(at the end of the duct) to be at least 120% the area of your intake piping to get any benefit out of it. That said, I think you hit about 0.25 psi of BOOST! if you go about 150 mph with a correctly sized duct.


            Don't angle the hood up, the base of the windshield is a high pressure area at speed.
            '18 Chevrolet Volt - Electric fun hatch for DD duty!


            DefSport Koni Sleeve and Spring Perch Buy!!!
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            • #7
              Alright, Def to the rescue. Now to figure out which vents to use. I'm going to make a sheet aluminum duct system because I'm sick of half-assing things.

              I'll leave intake without NACA and just make a box for it to lower the temp.

              Looking like ass is my specialty.
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              • #8
                So far with my track experiences just raising the back of the hood is the cheapest way of reducing engine bay temps (not coolant temps necessarily). I have never had my engine bay heat up so much to cause melting issues of anything on track, yet a good friend of mine melted his brake reservoir and burned part of his hood in on etrack day, he had not raised his hood.

                Def i know you are saying its high pressure area, yada yada, but statistically speaking this small amount of effort actully seems to be helping, its either sucking in cold air or releasing hot air.

                also dude if youa re having issues with cooling, a large hood vent shouldnt be what is needed at all, some type of fan on the radaitor would be ideal. I know you seem to want hood vents and some wild ass ducting but think cheap and functional.


                sorry for the rant, and i know some of my methods contradict what the technical information says, but i know whats been legitimatly been working for me on track, and thats all i really know.
                NASA HPDE Instructor

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                • #9
                  koguchipower hood.. but price $$$

                  [img]http://www.studio-mix.net/IMG_79141.JPG[/img]
                  ^His s13 hatch when it was pure sex black

                  ^Latest iteration

                  Plus has the rear of the hood shaped upwards for air to escape
                  http://www.koguchipower.com/
                  "hexa-dodecahedron-triple-threaded-super-eleventy-way-adjustment-spec" dampers. -Def

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                  • #10
                    I have a cheap, Japanese, beat-up, extremely light FG hood that I'm not going to replace until I can afford to have Mike at Kognition make me a dry CF copy. It's the lightest aftermarket hood I've seen apart from one-offs.

                    I have an e-fan, and it's not installed yet. However, I'd like to be proactive and build myself a vent and duct setup before I have any heat issues at the track. I drive my car to and from track days, so I absolutely NEED to have it survive through both the track day and the traffic I'll encounter on my way there and back.

                    Most of you are missing the point. It's not "how do I get my car to stay cool in traffic even though I'm not running a fan?" If it was, I would have asked that, and I would be someone with the IQ of a carrot.

                    I just wanted to know where to put a vent and how big it should be, and I think Def answered that question.
                    Last edited by SOneThreeCoupe; 04-06-2008, 10:43 AM.
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                    • #11
                      Originally posted by sr20goofus View Post
                      So far with my track experiences just raising the back of the hood is the cheapest way of reducing engine bay temps (not coolant temps necessarily). I have never had my engine bay heat up so much to cause melting issues of anything on track, yet a good friend of mine melted his brake reservoir and burned part of his hood in on etrack day, he had not raised his hood.

                      Def i know you are saying its high pressure area, yada yada, but statistically speaking this small amount of effort actully seems to be helping, its either sucking in cold air or releasing hot air.

                      also dude if youa re having issues with cooling, a large hood vent shouldnt be what is needed at all, some type of fan on the radaitor would be ideal. I know you seem to want hood vents and some wild ass ducting but think cheap and functional.


                      sorry for the rant, and i know some of my methods contradict what the technical information says, but i know whats been legitimatly been working for me on track, and thats all i really know.
                      It is a high pressure area at the base of the windshield - that's why you get air going through your vents in the cabin at speed even with the fan off. You're stagnating streamlines at the base of your windshield, there is no way the pressure CAN'T be higher than the surrounding air.


                      I don't have the back of my hood lifted, and still have the factory weatherstripping back there and I haven't melted anything at the track.


                      A hood vent has a variety of benefits. If I didn't hate my heavy ass stock hood I'd probably do something interesting to it like the OP is doing.

                      BTW, what I posted was just a general rule of thumb, but it works well since the 60-75% takes into account the tubes and fins taking up some frontal area of the radiator. If you duct to it, it also helps to try to keep a ~7 deg "funnel" going to the radiator(very hard to do in our cars), and roll the edges of the forward face of the duct so as to not cause flow separation there. Think of it as a gradual bellmouth. It's good for a few percent less drag AND more flow through the radiator.
                      '18 Chevrolet Volt - Electric fun hatch for DD duty!


                      DefSport Koni Sleeve and Spring Perch Buy!!!
                      http://www.nissanroadracing.com/showthread.php?t=5902

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                      • #12
                        Jesus, I need to read more on fluid dynamics. A lot more.

                        I'm probably going to add two more vents to the sides of the hood to release air from the engine bay, and use the primary to vent air from the rad.

                        Unfortunately, I only have an area about 23.5" wide by 5.5" or so deep with which to work with ducting between my engine block and intake. I'll have to work on that. I'm still trying to figure out what kind of vent I'm going to have to use. I may be cutting an available vent in half or something similar.

                        This may take a back burner to ponying up the serious cash for some new rotor hats.
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                        • #13
                          You need to get a differntial pressure gauge. Put one tube on top of the hood where you think you want a vent, and the other tube underneath the hood. The location with the largest differential is where you want a vent.

                          I bought a set of 3 off eGay that are non-electronic, but a friend of mine working in electronics calibration said you can get one pretty cheaply.

                          Some scoff at the suggestion, but this is the only true way of telling whether a vent will be effective.

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                          • #14
                            Or you could just build a manometer out of a piece of clear tubing and some water. Works just as well on a smooth patch of road.
                            '18 Chevrolet Volt - Electric fun hatch for DD duty!


                            DefSport Koni Sleeve and Spring Perch Buy!!!
                            http://www.nissanroadracing.com/showthread.php?t=5902

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                            • #15
                              Originally posted by 2Fass240us View Post
                              You need to get a differntial pressure gauge. Put one tube on top of the hood where you think you want a vent, and the other tube underneath the hood. The location with the largest differential is where you want a vent.

                              I bought a set of 3 off eGay that are non-electronic, but a friend of mine working in electronics calibration said you can get one pretty cheaply.

                              Some scoff at the suggestion, but this is the only true way of telling whether a vent will be effective.
                              Crap. I read about that on Autospeed and forgot about it.

                              There are a few on eGay going for anywhere from 120-190% of the new purchase price from Dwyer.

                              Instead of doing that, I just bought a 2" model for $50 shipped because I'm impatient and hate waiting for auctions to end. If it's new and cheaper than a new one from Dwyer, I'm ok with it. I'm hoping it will have use in other areas than aerodynamics as well.

                              The electronic ones are around $150 before shipping, and I'm not willing to shell out that kind of cash yet.

                              I considered a manometer, but "smooth patch of road" and "living near Indian casinos and a rock quarry" are mutually exclusive.
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