Project S2000: DIY Vented Hood

By Khiem Dinh

Khiem Dinh is an engineer for Honeywell Turbo Technologies at the time of this writing.  All statements and opinions expressed by Khiem Dinh are solely those of Khiem Dinh and not reflective of Honeywell Turbo Technologies.

The destroyer of all track vehicles is excessive heat. Whether it is the coolant, various oils, or brakes, too much heat will lead to failures. On Project S2000, we’ve already addressed the engine coolant and oil temperatures with an upgraded Koyo radiator and Earls Temp-A-Cure oil cooler. These heat exchangers depend on airflow to in order to remove heat from the coolant and oil. No airflow means no cooling, hence the need for fans when the car is sitting still. One way to improve the airflow through the heat exchangers is to minimize the resistance to the air exiting from behind the heat exchangers. For the vast majority of street cars, all the air has to dump out the bottom of the engine bay. So how do we minimize the resistance to airflow? By increasing the area the air has available to exit. In this case, we’re venting the hood.

There should be a side benefit to venting the hood beyond improved cooling which is reduced aerodynamic lift on the front of the car. As the air flowing through the engine bay now has an escape path over the top of the car, less air should be flowing beneath the car reducing lift on the front half of the car. Plus, the flaps bent upwards should generate some down force too; double bonus!

For this project, I decided to get a spare OEM hood (thanks Jessica!) to vent. The stock S2000 hood is made of aluminum and very light. In fact, many of the carbon fiber hoods on the market for the S2000 actually weigh more than the stock hood. Aside from weight, the benefit of the OEM hood is perfect fitment and no fear of the latch breaking. Those in the S2000 community are familiar with the DIY venting of the stock hood by making cutouts using the hood skeleton as a template. While this does allow heat to escape while the car is stationary, I do not believe it is very effective while the car is in motion.

So I think this represents the typical S2k DIY hood cutout where people just cut a hole in the hood. Newton’s First Law basically says something will keep on moving in a straight line unless some force acts upon it. In this case, the external air flowing over the hood has a lot of velocity. Air also has mass, so mass x velocity gives you momentum. Now, for the air in the engine bay to escape through the hole, it has to push out against the very fast moving air (and therefore, a lot of momentum). Unfortunately, there is not a lot of ‘push’ or pressure in the engine bay to force the air out. For anyone who has lived in a hot climate, you are probably familiar with the door air curtains. For those of you unfamiliar, these air curtains blow a jet stream of air from the top of the doorway downward; it basically creates a wall of moving air. Their purpose is to basically create a shield (the wall of moving air) between the outside and inside to prevent the hot outside air from going in and the cold inside air from going out when the door is open. Do you know what else is like a wall or jet stream of air? The very high velocity air moving over the hood preventing air from escaping the engine bay is like the wall of air. Some hot air should escape the engine bay due to the phenomenon known as entrainment, but I do not think it is much.

I opted to create louvers using the existing sheet metal. By bending the sheet metal upwards, it pushes the external airflow upwards. This leaves an air pocket, or void, behind the bent up sheet metal. These voids are similar to the big voids left behind cars which we use for drafting. These voids are of low pressure. One of the drivers of fluid flow is pressure differential. Assuming you don’t have another force like gravity to push things around, you have to have a pressure gradient (a zone of higher pressure and a zone of lower pressure) to cause fluid to flow. The high pressure will push the fluid to the area of lower pressure of course. As we have now created these low pressure pockets, it allows the higher pressure air in the engine bay to flow into these low pressure pockets. Why doesn’t the external air flow going over the hood curl inwards? Because that external airflow has a lot of momentum and it would require a lot of force to cause that air to basically make a U-turn. Therefore, less force is required for the air in the engine bay to fill the void as it has relatively little velocity compared to the external air. Plus, the air in the engine bay only has to make about a 45 degree bend requiring much less force than turning completely around. So, assuming my theory is correct, this style of vented hood should be much more effective at extracting hot air from the engine bay.

I started out marking the hood up by taking measurements. I used the duct tape to create a well-defined sharp edge when marking up with the sharpie.
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Monday, August 05, 2013 1:50 AM
shoulda just taken it to a Rod shop with a deep throat louver press
punch & shape louvers in 1 crack
shape of the louvers woulda added strength as well..
cuttin slits and bending is an excellent way make the hood skin flimsy

"downforce" is an aerodynamic sum & working range..
either you enter it, or you don't..
if yer lucky venting the hood might eliminate TinyTiny bits of "lift"
unfortunately claims of adding any "downforce" here or there
is kinda like calling an airplane "Flying" when its only generating some "lift" when still trying to take off.
either you're Flying or you're not..
same with downforce for cars..
Monday, August 05, 2013 7:05 AM
@Lessendz - Downforce is merely negative lift, it can be discussed for the car as a whole, or only related to a single feature, like Khiem has done here. It is still "downforce" because it is simply describing a force and a direction. For the whole car, you are correct, no doubt is this small force simply counteracting the lift generated in other areas.
Discussing aerodynamic downforce for a single feature on a larger body is also problematic due to interaction effects with other features. But I think Khiem has handled the situation perfectly for the scope of this article.
Monday, August 05, 2013 7:15 AM
A deep through louver press probably wouldn't have matched the shapes of the skeleton under the hood.

FYI, the 'skin' is 1.5mm thick aluminum. Also, as I did not cut into the structural skeleton underneath, the hood strength was minimally compromised. Also, I did the rough force calculations acting on the flaps (really, just the front ones). I had to apply waaaaaaaaaay more of a moment force to bend the suckers than the force calculation shows the air can apply. They aren't moving anywhere.

About that force calculation I did, I was debating about saying 'reduced lift' or 'downforce'. The fact is the flaps themselves created a downward force. However, as the calculation shows, it does do something, even a 'tinytiny' bit. But yes, overall on the front end of the vehicle, it most likely acts to reduce lift and not cause the net aerodynamic forces to cross into actual downforce.
Mike Kojima
Mike Kojimalink
Monday, August 05, 2013 7:58 AM
It might work but it's ugly!
Monday, August 05, 2013 8:55 AM
This seems really silly. Were you overheating? What were underhood temps? How are you going to seal the edges of the cuts so you don't get corrosion and paint flaking?

Haha all the engineering speak and math can be applied to what you did I suppose, and there are holes in your hood now, so that's cool I guess.
Monday, August 05, 2013 10:16 AM
3 things that will improve your design:

1/ use a magnehelic gauge to chose the best places to put louvers. The more you make them near the windshield, the less effective they become (to the point the flow will be reversed at higher speed, as air pressure increase there)

2/ Your holes are opened on their sides. This is no good at all; with speed, air will spiral from the sides of the flap. It is like closing the hole the faster you go. Planes have the at the end of wings to prevent air from going from one side of the wing to the other.

3/ duct, duct, duct, and duct again. Channel that air TO the radiator and FROM the radiator. The best design is counter intuitive, it is like "inlet ==== outlet" . Ducts inclination is better to be less than 15° when it enlarges and closes (6-10° is even better but requires room)

4/ Why bother with the hood ? Vent at the wings ! No windshield there to prevent air from escaping ;)
Monday, August 05, 2013 10:35 AM
Dear Khiem,

At first I was very excited to see your progress with your S200 as I am too an S2000 owner. However, this is probably the worst modification you have done to your S2000.

Scientifically, it "looks" like it works, but aesthetically it looks absolutely horrid. If you want to have a vented hood I think you should have it professionally done. I feel that part of the function is the form, fit, and finish of your upgrade. I hope you change it back to the stock hood.
Monday, August 05, 2013 11:16 AM
Mike, hellafunctional on the cheap!

Clay, read previous hot and cold weather track updates for operating temps. The hood is aluminum, so no corrision. Also, I might paint the whole hood later after I hack it up some more. Maybe.... time is a precious commidity and I honestly don't care that much how the hood looks.

1. didn't have a magnehelic gauge. Though, I could have fabbed something up using clear vinyl tubing and colored water, I really didn't feel like it (time, etc. the hood sat in my garage for 6 months as it was before I got around to working on it). My options on placement were primarily limited by the skeleton structure.
2. Correct, holes are open on the sides, but that's what I was stuck with due to the method I chose to make the openings. The original plan was to cut out those openings completly, design proper louvers, and rapid prototype them like I did with the brake ducts. However, that's not cheap.
3. You are correct on the ducting. But as you can see with the transparent overlay pictures, it's no easy task :) And again, this is not a race car, but a fun street/track car. With a race car, I'd ditch the stock airbox, A/C, etc which would free up a lot of space for proper ducting. I'd actually probably just design a new hood.
4. Spare hood was cheap! Scored it for $200.

Super_mut, yeah.. it's not the prettiest thing :) But, it's my 'track' hood. I have my stock hood for street driving use. For 'pretty', see what my original intent was in my reply to Croustibat. The idea in my head was very pretty, but the cost/performance ratio was very bad.

Did I mention the spare hood only cost me $200?
Monday, August 05, 2013 12:06 PM
I actually think it looks cool from a distance. Up close the ragged edges aren't pretty, but not many people will see it from so close or take such a close look at it. And it's your car to make as ugly/pretty as you see fit.

The important thing is that it works. I've thought about doing exactly the same thing on my car's hood. If I had a spare one to hack up I probly would.
Monday, August 05, 2013 1:15 PM
If you cut starting from the painted side, assuming the angle grinder doesn't skip around on you (read: use the damned handle it comes with), it won't be as ragged on the finish side. Also, hitting it with the flap discs I waxed ecstatic about in my Garage Fab article will do wonders for removing general shittiness from your jagged ass cuts Khiem. :)

A useful method to cut from the painted side would be to mark the boundaries of the cut, then drill holes through with a bit about the same size (or smaller) than your angle grinder's wheel. On the other side, simply connect with a piece of painter's tape (for easy removal) for your straight line. The drilled holes will make a nice, rounded corner.

Air-powered angle grinders are also a lot lighter, less jumpy, and overall easier to work with because you can adjust speed.

BTW: rough crowd. This is Khiem's car, not his customer's or yours. He can do whatever the fack he wants! Just wait until you guys see the pink New Beetle flower wheels he's had sitting in his garage with the colored burnout tires mounted... :)
Monday, August 05, 2013 4:17 PM
The cuts with the angle grinder came out really damn clean. It's when I made this mistake of using the end-mill bit (whatever it's called) on the dremel to make the side cuts that I jacked everything up. I should have just used the cutting disk. Oh well, live and learn.

I was going for cheap and effective on this modification. The original idea would be more effective and prettier (custom designed drop-in louvers). My cost estimate for that was: $300-$500 to 3D scan the hood, $1500 for rapid prototypes, and my time in doing the design work and whatnot.

So... if anyone wants to deposit $2500 into my bank account, I'll be more than happy to do the 'pretty' vented hood :) But, for $200, my DIY vented hood is hard to beat. It's about as light as you can get (dry carbon would probably be lighter, but also a few thousand dollars), perfect fitment, and no fears of the latch breaking because it's OEM. If anyone can come up with a similar performance vented hood for $200 (light, fitment, safe, and reasonabley effective), let me know!

BTW, that's a lie about the beetle flower wheels; we all know the S2k doesn't have enough torque to do a burnout. That's not to say those aren't my new time attack wheels. They have a fuller disk surface for reduced aero drag right?
Mike Kojima
Mike Kojimalink
Monday, August 05, 2013 5:30 PM
I think you need more venting.
Monday, August 05, 2013 7:37 PM
I could cut a few more openings in the middle. The more forward 'holes' in the skeleton could also be cut open. But the middle opening between those isn't very good. Plus... I'm going to be cutting into the more forward part of the hood, so I didn't want it to get too busy up there.

I'm a little bummed no one offered me the $2500 to do the pretty vents.
Tuesday, August 06, 2013 2:03 AM
Khiem, look into Road Race Motorsports carbon vents.

The link below shows what I got for my daily EG6. It cost me $180 shipped. Cut the hood myself. Some rivets and 30 minutes job got my a vented hood using the OE hood..

The Road Race vent is functional, well done, and it doesn't cost a gripe. After I installed it, I was surprise to see the temp. gauge dropping 10-12 degree!!!).

Anyway, just a thought.
Tuesday, August 06, 2013 7:58 AM
JDM, yup, I know those products. I actually met those guys are Cars & Coffee once. But they didn't have anything that ideally suited the S2k hood. I didn't want to cut into the skeleton structure so as to not compromise strength.

The idea of designing my own louvers and rapid prototyping them was that the RPs could be used as a buck for making carbon fiber versions to sell. But it just doesn't seem worth my time right now. But, I did leave the option to do it later as all I have to do is cut out the holes completely.
Tuesday, August 06, 2013 9:44 AM
I'm not sure how far up into the airstream you would have to go to create that low pressure area, but what if you simply used a "hole" in the hood with some vacuum tubing or something to create a small lip? You could get the benefits of the vented opening without having to bend metal.

I would have studied OEM vents on vehicles like the Evo to see what they did. I think the trailing end of the opening is turned down with little to no lift at the leading edge of the vent.

In any case, it seems functional and hopefully it will show some benefits in cooling. If not, then you can go back to your stock hood and lift the rear at the hinges like the other Honduh guys do. Cowl induction S2k ;-)
Tuesday, August 06, 2013 10:59 AM
Yup, I studied a bunch of OEM venting. Vipers, Mustangs, Aston Martins, Camaros, new Vette and my old Evo. Keep in mind though, they have to make theirs' pretty :) The Viper SRT hoods have the trailing edge bent downwards and I had considered doing that too, but I didn't feel it would be as effective though certainly prettier. Also, I didn't think I could bend the central vent downwards as I thought it might hit the airbox.

Shelby GT500 and the ZL1 Camaro have pretty nicely done factory venting. I looked at the vents used on the Focus RS sold in Europe, and those are similar to the ones on the Aston Martins. They do something, which is better than nothing, but I don't think their effectiveness is that good.
Tuesday, August 06, 2013 6:09 PM
YAY an article that wasn't about fitting wheels and tires!
Wednesday, August 07, 2013 12:59 AM
Sorry if it sounded agressive, it was not intended.

You would not need 2500$ to make pretty louvers, just get some aluminium louvered vents from home depot and rivet it. that is a 30$ expense, at most.

I am trying to source a cheap hood to put them on, which is quite harder than i thought. (i do need more cooling on small tracks)

I bought some similar to these, just bigger:

I dont do pretty either on my track-ish s13, just efficient. If you ever need more cooling, try just making some sides on these (or switch to the 30$ home depot louver vents, some even have filters to prevent debris or bugs from entering the bay). A simple ducting around the radiator can do wonders too, something that just extends 2" in front and prevents air from entering under the hood without going throught the radiatior is very good.

Wednesday, August 07, 2013 6:18 AM
Croustibat, you didn't :) You are spot on with your comments.

My #1 requirement for this project was to not cut into the skeleton structure in order to not compromise strength. Therefore, there is basically no bolt-on solution.

I debated about the 'sides' to the vents for a while. Unfortunately, without adding metal, there's no way to do it really. I debated about making cuts to make the trianguler sides to the flaps, but there was going to be a compromise either way: side attached to the flap would leave a 'hole' at the hood surface, side bent up from the hood surface would leave a slit between the side and the flap. As all options seemed compromised whether going with the side flaps or not, I went with the easiest solution which was to not do them at all. On the central opening, I doubt it's much of an issue due to the width vs. height ratio. On the sides where the width is much shorter making the width:height ratio worse, it is for sure more of an issue of air curling inwards from the sides.

I debated about adding some skin, like R/C airplane material stuff, to create the sides to the flaps, but it just wasn't worth the effort. Those screen shots of the tufts were taken at about 75mph and it does not airflow thru the sides is much of an issue at these speeds at least. I should note, the sides didn't seem to get flowing until 50-55mph whereas the central vent seemed to be rocking by 45mph.
The original plan was the $2500 route. Make nice drop-in louvers from rapid prototyped ABS like the brake ducts. Then use the RPs as a buck or whatever for creating carbon fiber versions and sell them. That is still an option, but just doesn’t seem worth it right now.
There’s a cost:benefit to everything, and in my analysis, it wasn’t worth trying to make the sides due to the ratio of performance gain vs. effort put in. And it also isn’t worth it to make the application specific drop-in louvers. Of course, the cost:benefit ratio changes depending on conditions, but right now, I value time more than anything and while there is a good potential market for the drop-in louver solution, the benefit side of the ratio doesn’t feel big enough right now.
As for ducting the radiator, already done ;)
Wednesday, August 07, 2013 3:01 PM
If you're going to make drop-in louvers, a fab shop with a louver punch and a bead roller to step/flange the edge (to make it look like it belongs there) will be able to make drop in panels very easily. I can't see a need to do a RP part for something which is so simply created by hand with readily available methods and existing tooling. That even works if you're trying to make a buck to create a mold to make carbon parts. You can fill punched louvers from the rear with whatever (clay, Bondo, fillet wax, whatever) and then take a mold from the front.

I know this was supposed to be low cost, but you could get much better, more aesthetically pleasing results for relatively low cost. I like the fact that you're hacking away at your car in the name of performance improvement, but I really have to question your methods........
Wednesday, August 07, 2013 8:34 PM
Fabrik8, heheh.... I looked up the different methods of louver punches. The thing with the 'openings' in the S2k hood is that they are a non-symmetrical trapezoidal shape. So... I suppose you could use one length being the shortest, but it wouldn't take advantage of the full width available.

So my criteria for this was: don't cut the skeleton to maintain strength, effective, and low cost. I figure with my method vs. a proper louver like Croustibat mentioned, is probably 75% effective. So not too bad. Cost? it's time and money. I spent 2 hours from start to finish while snapping pics and spent ~$30 for the hand seamer. Let's say I found some shop to do it. It takes about 45 minutes to get anywhere in LA, so 1.5 hours round trip. Figure 2 hours for them to do a good job at $100 an hour. So 3.5 hours and $200 is more than 2 hours and $30. Notice, appearance was not a criteria :)

Sure, I care about how some things look, but this is my track hood. I'll put it on when I go to track and take it off when I get home. There aren't any bonus points for looking good at the track, in my book at least.
Friday, August 09, 2013 1:38 PM
"Super_mut, yeah.. it's not the prettiest thing :) But, it's my 'track' hood. I have my stock hood for street driving use."

It's a track hood? In that case who gives a f*ck cut it up. I would too. I thought this was a street setup.

More holes please. Some creative ducting would be cool too.
Saturday, August 10, 2013 10:23 AM
Heck no I wouldn't do this to the stock hood! That's why I bought the spare :) Plus, I wouldn't drive with this hood on the street as I prefer to keep a profile. Oh yes, there will be more hacking... stay tuned.
Wednesday, September 18, 2013 12:48 PM
Hi, computational fluid dynamicist here. Looking at your hood venting theory, it has a few holes/misconceptions in it, although it gets really close along several points. First a big caveat: the system you're designing for here is very complex. You have thermal differences between the inside and outside air, as well as velocity differences. Your velocity differences in turn create pressure differences (high velocity air at the same temperature and ambient pressure has a lower total pressure than low velocity or stagnant air at the same thermodynamic conditions).

So, if we assume away any buoyant forces (hot air rises, and really hot air rises quickly!), saying they're a minor part of what's going on, AND we assume that the airflow going over the S2000 hood at the point where your vent holes are cut is following the surface well, then you have a stream of low pressure air already flowing over your relatively high pressure stagnant air. Assuming this, your lowest drag option is to make the lip on the leading edge *just* enough higher than the trailing edge of the vent hole to keep the fast air heading over the hole and not into it. Pressure is a decent motive force for air (since it's a gas), and this pressure differential will cause your high pressure hood air to be drawn out. With a severe enough leading edge to cause eddies (labeled as air voids in your page 1 diagram), you'll actually just be adding a layer of slow, high pressure air between your hot air and your fast, low pressure air. This is related to the Bernoulli principle, if you're interested in reading more about it.

The reason you see severe vent rakes like this on LMP cars and the like around the fenders is because of the effect a rotating wheel has on pressure. Viscous fluids actually have what's called a "no slip" condition along solid walls, meaning that when they meet up with a solid surface, the first layer of air molecules has no motion relative to the solid surface. This means all solids, so when that tire is spinning, that first layer of air molecules is stuck spinning with it (until it gets squished out of the way by the ground). At the top of the wheel's spin, the tire surface is actually going twice as fast as the car relative to stationary air. as we stem away from those fast moving air molecules, friction drags the next layers of molecules along for the ride (not at 100% speed, but at some high fraction of the speed of the tire), creating a low pressure area at the top of the tire. They can vent in this area to actually draw air out of the void area near the top and behind the fender into the wheel well to provide general ventilation as well as to keep air from charging into the wheel well (a very draggy place for the air to be). For that to work though, they need to draw from a high pressure source, hence the high, multiple rakes causing separation of the streamlined airflow and void spaces.

Also scanning through the comments, one I noticed was an explanation of winglets (the up, or down, or both-turned tips on wings) that wasn't quite complete. They're right about the low and high pressure air on the wing meeting. When you stick a solid body in the side of this meet up, you end up with a resultant spin of the air flow, creating a vortex. Vortices are high energy phenomena and thus require a lot of energy to keep going, and guess what, the only thing creating them is the kinetic energy of that wing going through the air, so that's what it taxes. Case in point: vortices create drag. Winglets are there to make the transition from a low and high pressure side of the wing more gradual to reduce the overall drag of the wing.

Thanks for your updates on this car, it's a fun thing to watch, and I'm learning a ton!
Wednesday, September 18, 2013 8:23 PM
I love having intelligent readers. I actually had two semesters of Fluids in undergrad and a semester in grad school, but I do more thermo stuff nowadays.
You are absolutely correct that it’s a complex system, and quite honestly, I didn’t feel like doing a thorough analysis or researching it. Airflow around wheels and tires? Yeah, I definitely didn’t want to think about that :) Considering you do CFD for a living, you’re certainly more qualified.
Wait until need week to see what other hood hacking I’ve been up to…
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