Project S2000: Part 18 – DIY Vented Hood


This was my attempt at getting a temperature profile along the hood starting at the leading edge and moving towards the central vent using a handheld pyrometer. Keep in mind, the car has been sitting for a few minutes with the hood shut. As you can see, the hood gets progressively hotter starting at 95F and approaching 112F near the central vent.

Moving up the driver’s side of the hood, it gets warmer than the central and passenger sides as the Fluke thermal imager also showed. I did measure the passenger side with the pyrometer and it was indeed cooler than the driver’s side EXCEPT at the vent itself. Up to the vent, the hood was a few degrees cooler, but the vent itself was a few degrees hotter compared to the driver’s side. I believe this was due to the heat coming off the exhaust manifold as I could place my hand over the vent and feel the heat coming out. Before the vents, the underside of the hood had the insulation material there which is now removed. So I think the insulation caused the passenger side to look cooler in the original thermal image.

Hey lookie, I learned a Photoshop trick! So why is the driver’s side warmer than the passenger side? I think it’s due to two things on my car. The first reason is the air box which restricts airflow on the passenger side. The second is the oil cooler I have mounted on the passenger side of the radiator which further restricts airflow on that side. Remember, air follows the path of least resistance! This picture gives you a good idea of the placement of the vents relative to the parts in the engine bay along with how big the openings are.

I purposely located the central vent in front of the Whiteline shock tower brace as I did not want the brace to restrict airflow through the vent. Remember, measure twice, cut once.

So it appears as if our hood modifications are functional in extracting hot air from the engine bay. Now let’s do a (super) rough calculation on the downforce generated (Chuck, yeah, I gotz time for dat!). There’s this thing called Newton’s Second Law which basically says the time rate of change in linear momentum of something is equal to the sum of the forces acting on it. In our case, the ‘something’ is air and the time rate of change in the linear momentum of the air is due to us turning the air upwards. Newton was a pretty smart guy and came up with a Third law that basically said for every action, there’s an equal and opposite reaction. In our case, the ‘equal and opposite’ reaction is downforce and drag acting on the hood.

After playing with linear momentum equations, we end up with these two equations to give us downforce (Fz) and drag (Fx) forces.

After making a bunch of assumptions (flap height = 0.025m (which assumes the air affected is of only this height, so double ASSumption), flap width = 0.6m, air density @30C/86F = 1.165 kg/m^3), we get the following downforce and drag curves versus velocity for a flap at 45 degrees. 70 m/s velocity is about 157mph and 60N of force is about 13.5 lbf.

Reducing the flap angle to 30 degrees gives these results.

Keep in mind many assumptions were made in this back-of-the-envelope calculation. We’re not taking into consideration the air exiting the vents that would have otherwise left the engine bay out the bottom of the car thereby increasing lift. Is the ‘height’ of air affected by the flap only equal to the height of the flap? I doubt it. But hey, this simple calculation gives us an idea of what’s going on along with the trends in the downforce and drag curves versus flap angle.

So there you have it, a DIY hood venting job that should improve our cooling and even give us a little downforce on the front. Yes, the hood is Silverstone Silver whereas my car is Sebring Silver. I’ll get around to that later, but I have another project that involves hacking up the hood some more so I’ll save any possible painting until after then. Oh yeah, math is cool, stay in school. Fluid mechanics is pretty cool too. And so is Mechanical Engineering.


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