Thread: Lowest air pressure/highest air velocity part of the hood?

Would anyone know this information or have a heatmapping style graph for this?

my guess is immediately post radiator, pre valve cover area, but if there's a definitive answer I'd love to know.

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*for a b13

Great question. It got my brain going before the coffee kicked in...

"According to Bernoulli’s principle, for a given volume of air, the higher the velocity the air molecules are travelling, the lower the pressure becomes."

"Cowl Induction"

Base of windshield and rear edge of hood see the highest pressure. For air intake that is the ideal spot.

Center of hood has a generally low pressure area which us good for rear facing vents for extraction purposes.

Thanks, this is what I'm looking for. I'm considering adding ventilation for my rally car (with mud covering the lower half of the radiator I was seeing 250* F coolant temps... my rad fan was also malfunctioning but still).

I was thinking of adding some vents to the center hood area as well to let heat out. One thing though that I've seen some folks do is to add some spacers to the hood hinge by the windshield to lift it up slightly. Based on this diagram, though, it doesn't seem like that would be the most efficient way to accomplish this?

That is the ghetto way to do it and it really doesn't work as intended. Many do it for clearance issues.

When you have air trying to come in through the radiator and at the rear of the hood when shimmed up it winds up stalling the incoming air through the radiator and basically stops air flow. In some instances the pressure buildup is enough to bulge the hood a good amount. Not good.

That is the reason why there is a rubber strip sealing the hood to the cowl.

The "Hood Spacers" are more useful when battling heat soak during non-moving conditions.

There is the correct and incorrect way to do this.

Add vents a few inches right behind the radiator with turn "ups" to generate movement of hot air out of the engine bay without disrupting over hood airflow. The idea is not only to vent heat out, but to actually PULL the air out using the airstream over the hood. The correct way to do this is
1) Use vents with slates angled "up (rear higher than front)"
2) Make sure you have a diffuser in front of the slats to redirect airflow up and over the vent, and this low pressure area produced by the change in direction of air will allow the high pressure air in the engine bay to escape and join the over the hood airstream.

The last thing you want to do is impede air escaping out the hood due to the fast flowing air over the hood not being used to "energize" the stream coming from the engine bay.

In addition

1) I Highly recommend ducting the front end (this may take some thought as well, the vehicle is rally based and now you have obstacles to deal with (trees, rocks, cows, dildos..........etc)
2) I recommend AGAINST raising the rear of the hood if driving at speed. That actually acts as a high pressure zone impeding air escaping through the cowl area and actually traps hot engine bay air. more detrimental for the end goal at hand.

Throwing this out there:



Via this site: https://www.carthrottle.com/post/engineering-explained-10-aerodynamic-features-of-race-cars/




Also @D-Unit121 and @NissanEgg for saying the same thing regarding raising the rear of the hood. Makes perfect sense when you think about it, only will help when you aren't moving, and at speed will hurt. My application will be a track car (endurance), so this idea is definitely out for me and for @wildmane and his rally car.

@Boostlee, what are your thoughts on the size of the diffuser to redirect airflow over the vents? Most of the off the shelf vents I've seen have a fairly small moulding piece around the vent, the actual vents mostly recessed into the hood. I'm wondering if this small bump is really enough to do the job effectively? Example of such a piece would be this: