Project S2000: Part 18 – DIY Vented Hood

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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