Dicky Posted February 16, 2017 Share Posted February 16, 2017 Anyone know if there's any performance advantage in these or do they just provide aesthetic value Quote Link to comment Share on other sites More sharing options...
Olly350z Posted February 16, 2017 Share Posted February 16, 2017 (edited) Id be inclined to say they help to a certain degree if you have an open filter cone. Heat soak seems to be an issue and under bonnet temps seem high on these But all of the above is totally useless if you're using the standard air boxes. Id say it would probably be a very small advantage on an NA car Edited February 16, 2017 by Olly350z Quote Link to comment Share on other sites More sharing options...
Strudul Posted February 16, 2017 Share Posted February 16, 2017 (edited) Technically it improves engine cooling as the heat can go straight up, but usually it's just cosmetic. But if you do it properly, it can have a benefit... Mx-5 related rather than Z, but same logic applies: -Hood vents With a good undertray, leaks sealed, reroute and high performance radiator an N/A up to about 170whp can stay cool even drafting in 100° heat. Start going above 200whp and hood vents are needed to increase airflow. Go above 300whp and you need the highest performance radiator available too. $200 Chinese ebay specials need not apply. Hood vent location is critical. There are a few images on this forum mapping the areas of low pressure across the hood. Put the vent in the wrong place and you will actually shove air back in and make the cooling worse. In general, a loaf of bread sized area just in front of the valve cover but behind the fans is good. The other good area is two angle areas starting at the front corners of the valve cover and extending about a third the way back towards the hood latches. Combine them together and you get a sort of flat bottomed “V†shape. Kickers or spoilers help a bit but really all you need is a hole with maybe some mesh on it to keep rocks out. Hood vents make a huge difference in airflow through the radiator as they extract from the engine bay. The OEM aluminum hood is a scant 14lbs and very stiff. It needs to be stiff as there is significant aero lift across its surface. Make and aftermarket hood to flexy and it will lift enough at speed to see a 3†gap along the side. Not good. The only “carbon†hood that are actually lighter and as stiff as the OEM are vacuum bagged dry carbon prepreg and quite expensive. In our shop, we simply hack up OEM hoods as they end up being light and stiff enough while being a fraction of the cost of the sexy dry carbon hoods. Edited February 16, 2017 by Strudul 1 Quote Link to comment Share on other sites More sharing options...
nub Posted February 17, 2017 Share Posted February 17, 2017 (edited) I had this question once. And I think the truth is, or rather as I was told they're mainly cosmetic. And that applies to all vented bonnets. The reason is because at speed, even 20mph - 30 mph the higher pressure air running above the bonnet then over the windshield and roof is too high and although hot air rises, it's being essentially pushed down by the fast flowing high pressure air above the vents and not being allowed to escape. However at very low speed or better, when stationary the air will come straight out the vents so in that scenario yes, it will allow hot air to escape but only until you get moving again and it won't be enough to lower IATs. So in a way, it may prevent the heat building up much - but not if the car is continuously on the move. It wouldbe great for drag racing and just generally reducing heatsoak. Sent from my iPhone using Tapatalk Edited February 17, 2017 by nub Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 17, 2017 Share Posted February 17, 2017 I wouldn't agree as you have air coming in through front bumper over radiators and engine and has a flow. This flow if vents are placed correctly will go out. If you look all serious car have vents already from factory Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
Commander Posted February 17, 2017 Share Posted February 17, 2017 Depends on their position, and the correct answer for 'what position' is different from one car to the next. I do know that on an e46 m3, the point where the bonnet ends and the windscreen starts is particularly high pressure on the outside of the car and so that's where the cabin cold air feed is, with the high pressure helping to push the air inwards. There's a BMW Tech document about it all because it impacts how the aircon works when traveling at different speeds. My XKR has bonnet vents (though I've no idea if they're cosmetic or functional ) Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 17, 2017 Share Posted February 17, 2017 Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 17, 2017 Share Posted February 17, 2017 That's possibly the worst graphic to demonstrate airflow I've ever seen. Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 17, 2017 Share Posted February 17, 2017 That's possibly the worst graphic to demonstrate airflow I've ever seen. Just a quick one plain and simple, first one I stumbled on Google. But just to get idea. Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
-Bradders- Posted February 17, 2017 Share Posted February 17, 2017 Dont think mine do anything other than get the engine covered in water marks when it rains lol 1 Quote Link to comment Share on other sites More sharing options...
ilogikal1 Posted February 19, 2017 Share Posted February 19, 2017 It depends on the car and how the airflow is designed. The airflow is usually carefully considered and designed in the rather expensive R&D process, cutting holes in places will disrupt that. Whether it's a positive or a negative disruption depends on the design. It's not just a case of adding vents and it'll be better, in certain cases adding vents actually increases under bonnet temps as the air no longer flows where it was designed to as efficiently as it needs to. Don't just assume that because x variant of said model has vents therefore copy and paste is fine, it'll depend entirely on what other changes have been made to affect the air flow. Quote Link to comment Share on other sites More sharing options...
Paul K Posted February 19, 2017 Share Posted February 19, 2017 Sent from my SM-G850F using Tapatalk This is a CFD image (computerised Fluid Dynamics). I use this in work to determine airflow around computer centres Sent from my iPhone using Tapatalk 2 Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 19, 2017 Share Posted February 19, 2017 Well I have a similar bumper now on Z and would like to figure out where to cut on HR hood vents in order to have so actual purpose and function Sent from my SM-G850F using Tapatalk 1 Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 19, 2017 Share Posted February 19, 2017 Without some very expensive air flow measuring/CFD stuff, you simply don't know: It'll be guesswork. You might get lucky, you might not. Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 19, 2017 Share Posted February 19, 2017 Yeah I reckon so. Most of the things I was looking/searching/browsing for tend to put it as far as possible close to cabin. Some say just putting spacers at the mounts of bonnet and lifting back part of bonnet will do the trick Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 19, 2017 Share Posted February 19, 2017 Oh god please don't do that lifting bonnet thing, chav central! 2 Quote Link to comment Share on other sites More sharing options...
ilogikal1 Posted February 19, 2017 Share Posted February 19, 2017 Especially as it's been proven not to work. Ever. Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 19, 2017 Share Posted February 19, 2017 How? Logically heat and hit ait tends to rise. I didn't went too much into it this thing with hood lift but shouldn't the hot air escape there Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 19, 2017 Share Posted February 19, 2017 Because the base of the windscreen is a high pressure area: You'd be forcing the hot air to stay in. 1 Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 19, 2017 Share Posted February 19, 2017 Well like that if you look all hood is than a high pressure area. I reckon as soon as you have airflow over bonnet is hard to allow hot air to get out Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 19, 2017 Share Posted February 19, 2017 Exactly right. However, the join where the windscreen meets the bonnet is the worst possible point to try and extract air, as all that flow that goes over the bonnet hits the base of the windscreen and creates a really high pressure zone. 1 Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 19, 2017 Share Posted February 19, 2017 Will need to do bit home work before I start drilling and messing a new hood. I've seen few universal carbon vents from APR 20"×4.5". That might do the trick Sent from my SM-G850F using Tapatalk Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 19, 2017 Share Posted February 19, 2017 Coming from a different angle, why do you think you need more cooling for under bonnet temps? Quote Link to comment Share on other sites More sharing options...
Snjur Posted February 19, 2017 Share Posted February 19, 2017 (edited) Well a stock Vq is generating a lot of heat. I have two quite big turbines inside engine bay. Summer time it's like inferno under. So far I haven't had any problems with overheating in daily usage of car or 1/4 events. Only thing is on track after 3 fast laps ECU cuts me Rev limiter as water Temp starts going up. This was on OEM radiator. Now I have this new bumper design and I have bought Skunk2 Alpha radiator which is bigger so will see. I have bad experience with coolant and engine bay temps from past Sent from my SM-G850F using Tapatalk Edited February 19, 2017 by Snjur Quote Link to comment Share on other sites More sharing options...
Ekona Posted February 19, 2017 Share Posted February 19, 2017 Sounds like you just need a bigger rad to me, which you've now got. I'd be testing that thoroughly before cutting any holes Quote Link to comment Share on other sites More sharing options...
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