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When traveling at about 60 or above I get so much air pressure under the hood that I feel like it might blow the panel right off. the only areas where the under hood air can escape are the opening directly below the engine, the header opening, and a cut-out in the hood for the engine. As well as a narrow driveshaft tunnel (BEC). None of the openings are oversized. They are all just large enough to get the job done. The surface area of the inlet in the nose is no greater than the surface areas for exit, but still... the air seems to build under the hood, like a baloon. When driving at speed, I can see the rear of the hood lift up where it meets the scuttle so the air can escape. I'd like to equalize this pressure, so the hood dosent want to fly off. how have others delt with this issue. I'd like to avoid cutting up the hood, but if i do, I'd like advice on the most efficent opening that will draw air out.

videos of the car can be seen here;

http://www.youtube.com/watch?v=-NRtYtHgikA&feature=plcp

http://www.youtube.com/watch?v=vUbuKx00IBA&feature=plcp

I have a similar action. At highway speeds the rear edge of the bonnet will sometimes pop up and almost as quickly, be sucked right back down again. You can actually see the length of the bonnet go slightly convex and concave as the relative pressures swap positions. The variations seem to have something to do with wind gusts/direction in addition to the car's speed. I have no folded edges nor reinforcements to strengthen the bonnet across the front and back, just the single layer od 0.040" thick aluminum. My bonnet is held down with spring hooks so I'm not worried about it coming loose. I have over 3,000 miles tested so far. Most at highway speed.

DaveW (I'm pretty sure it was Dave) actually had his bonnet fly off so make sure you are Ok with how your hood is latched.

Good to know i'm not alone here. my hood is attached with 4 DZUS fastners, I bought them from Jack, who supplies a lot of people with parts, so I'm feeling reasonably good that the hood won't actually fly off, but i'd like to get this high pressure area under control. It seems like restricting the amount of air coming in is part of the solution, and providing an appropriate exit is another part, and a third may be looking into wasy to better manage the flow of air once it is inside the engine bay. looking for clever solutions that have worked.

BTW- I think the current generation of Caterhams have a simple curved baffle behind the radiator that directs the air down and out under the engine. But I cant be sure that is where i saw this.

It's not necessarily "high pressure"--it's more the pressure differential between the low-pressure, high-velocity air above the hood combined with the static air under the hood. It's kind of like roofs of houses in a hurricane. If you don't open up the windows, then the roof is liable to get sucked right off the walls!

Louvers are probably the most direct way of getting the air to equalize pressure above and below the hood, but it sounds like you'd like to avoid putting holes in the hood if possible. Would it be possible to design a vent at the backside of the hood or on the leading edge of the scuttle? It sounds like the hood naturally wants to vent right there, so perhaps giving it a way to vent would let it settle back down during higher speeds.

What Matt said. I don't think it's the ram air in the nose as much as the negative pressure over the hood. Unless you're willing to add stiffeners to the rear of the hood, there's not much to keep it in shape...

Hey, wait a minute. The scuttle is plenty stiff there in the middle, maybe a Dzus plate attached to the front of the scuttle, and a Dzus fastener in the center-rear of the hood. (Note: this is not a rhetorical question. If there's a reason not to do this, I'd like to hear about it).

good ideas. thanks.
what about exploring ways to create negitive pressure under the hood by directing/managing the airflow in the engine bay/cavity in deliberate ways. I'm thinking of an under-hood diffuser, deflector/splitter, etc. ? and if i were able to successfully strike the right balence in pressure - could it be made to be variable with speed, or would the bonnet always want to sucked down or blown upward depending on speed?

related- if i managed the airflow under the hood and directed it out the opening under the engine bay, would I create the unintended consequence of air managment problems under the car? my car has an entirely flat bottom. This is a road car, not a racer, so i'm not anticimating any meaningful problem, but thought i'd throw it out there. the car will rarely go above 70-80 mph.

seperately, anone know the science behind louvers? they are an old idea, and i tend to think of them as classy looking exit holes, but is there more to them than that? do they nutralize air pressure above and below the surface? how do they really work? Do they really work?

seperately, anone know the science behind louvers? they are an old idea, and i tend to think of them as classy looking exit holes, but is there more to them than that? do they nutralize air pressure above and below the surface? how do they really work? Do they really work?

??? anyone got some insight?

I don't remember who it was but someone had posted pictures of a single air vent built into each side of the engine compartment paneling [vertically] just in front of the scuttle. They appeared to be about the height between chassis horizontal tubes and about 5" wide. Sort of like one big louver on either side of the car.

I'd check with cloth tufts in that area first to get an idea of the air flow before I'd cut up the side of my car. It could be a high pressure area.

Jack, have you done any testing in that area in your streamlining efforts?

On my car the air cleaner sticks up through the bonnet so I made the hole about 1" to 2" more than the max size of the air cleaner. The air scoop covers the hole and allows the air to escape since both ends of the scoop are open. The scoop is also wider at the rear than the front to help pull the air out.