On the subject of hood vents

Many cars, mine included, have issues with air management under the hood. As speeds increase, the ability of air to pass through the radiator opening and then out of the engine compartment becomes somewhat troubled. In almost every instance, the high pressure that builds up inside the engine bay can actually stall airflow through the radiator, and cause overheating.

In my case, the high pressure built up also will exert a force on the hood from the bottom, which can actually lift the front end of the car off the ground at speed. I have experienced this first hand. It’s not cool.

So, with the track activities this past year exposing not just the front end lift problem, I also experienced issues with the car getting too hot at high speeds. A sure sign of airflow stalling as it moves through the radiator. This also leads not just to higher-than desired water temperatures, but the lack of airflow out of the engine bay leads to extremely high underhood temperatures, too. Stuff starts melting.

So, the answer? Vents. The air needs a place to go. The question is where to put them. Super-serious race cars vent it out the sides, through the fenders. This gets the heat out without affecting the dynamics of air flowing over the car, which could mess with a race car’s carefully designed and adjusted aero bits.

ButĀ  that’s hard for a street car. The easiest for me is a vented hood. Nobody makes one, though. So I’m going to have to buy a fiberglass hood, and cut holes in it.

But where to put the holes?

I needed to run an experiment. I needed to measure the air pressure on the top of the hood at speed in various locations. The spot with the lowest pressure would provide the greatest differential between the already-known-to-be-high pressures under the hood. Maximizing the pressure differential will maximize airflow.

First thing we need is something to measure with. You can use a fancy expensive magnahelic gauge, or go cheaper. I bought a manometer from Amazon for $35.


It has two ports. You leave one open to atmosphere, and run a hose from the other to the spot where you want to check pressures. It compares the different in pressure between the two ports and tells you what it is. The plan is to tape the end of the probe hose to various spots on the hood, drive around at a couple of speeds, and see what the results were.

So, since I’ll have to drive the car, I enlisted my daughter to operate the manometer and record the results.

First, the hood grid:

Gridded and labelled

Each spot corresponds to an area roughly the size of the vent that would end up being there. The plan?

Drive at 35mph and 65mph with the probe in the center of each section, record the pressure. We also measured, for novelty, the pressure on the fender where GM and ASC/MClaren put the side vents on the GNX.

Side vent!

And, to settle a debate that’s bounced around on the interwebz for awhile about removing the weather strip at the cowl end of the hood we also checked at the base of the windshield in the center:

Cowl test

So, fully instrumented, we set off. After about an hour of driving around and moving the probe, we had our results!

Speed/Location 35 65
1 -0.05 -0.06
2 -0.02 -0.06
3 -0.04 -0.02
4 -0.02 -0.39
5 -0.08 -0.38
6 -0.04 -0.33
7 -0.07 -0.27
Fender 0.7 -0.08
Cowl 0.18 0.42

The measurements are in KPA/100, and represent theĀ difference in pressure between the reference port inside the car and the pressure on the hood. So -0.04 means 4kpa less than the reference port.

So, interesting!

First off, this proved a high pressure area forms at the cowl. So, do NOT remove the weatherstrip at the base of the windshield for cooling. It will actually force are INTO the engine bad from the top and stall out the radiator. That’s not what we want.

The other interesting data point was the fender location. That is where GM put the vents on the GNX. Part of the fender location was tradition: Harley Earl first used three vender vent holes as a styling cue for Buick back in the 1960s, so GM put them there. But that’s not the best place. At 35mph, we recorded a positive pressure there. At 65mph, the pressure differential was no greater than several spots on the hood were at 35.

So, no need to invest in a GNX vent kit and cut my fenders. It’s a bad spot!

The best spots were areas 5, 6, and 7 on the hood:

The best was 5. So, a vent just in front of the bulge is going to provide the greatest pressure differential. Side vents at position 2, like you commonly see on Corvette hoods, won’t be optimal on this car. A forward location with respect to the engine will also help a lot. Air passing through the radiator slams right into the intercooler shroud and the front of the engine. With louvers in a forward position, that air will get sucked right out. It will also pull air past the turbocharger and wastegate complex, which should cut down on underhood temps overall.

So, something like this installed forward of the bulge:


Or a smaller pair on either side of the hood centerline in the area of zone 5, like these:


Either should dramatically improve airflow through the radiator, reduce pressurization of the engine bay and allow more are to move through it, and reduce front end lift at speed.

All I have to do now is measure again against some templates made for various louver offerings from Trackspec, and get myself a glass hood. And now that I know that placing vents up closer to the window in zones 1 or 2 won’t work as well, I don’t have to waste money on vents for those locations.

Isn’t science great?




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