It's All About Heat
Capt William Sanders, USAF, Nashville, TN; First off, I hate you for your Buick GS400 score and even more for the fact that I'll likely never see it again unless that promised Edelbrock head buildup comes in the form of swapping the narrow-bore 400 for a widemouth 455 into that car. OK, now my tech question.
I'm having a bit of a discussion with my friends about underhood heat and ways to vent it. Considering my friends are largely classic Buick and Turbo V-6 people, it comes as a topic of import with all the heat that turbos generate under the hood. The GNX has fender vents letting underhood air escape generally about one-third of the way down the fender between the wheelwell and the door. The Viper, Corvette, and various other supercars have the vents located either full length or down behind the trailing edge of the wheelwell. A lot of my friends are saying, though, that this is a styling thing and that while it lets heat out, it would be best vented from the base of the windshield since heat rises and the air all slams into the firewall before going out under the car. I think the high-pressure area off of the windshield, the very thing that generates a cowl induction, would hamper this. Lots of cars pull the A/C or vent air from this area, and it would lead to more heat in the cooling system if you run A/C. What really is the most efficient place in general to put vents for underhood heat?
Jeff Smith: This is an interesting question, William. The first thing we should be concerned with is when you want to vent the heat. As you say, heat rises, but as you also have accurately mentioned, venting heat from the base of the windshield is not such a good idea since that is indeed where most cars pull in fresh air for vents and A/C. The reason for this is that at highway speed, the second highest location for aerodynamic air pressure is at the base of the windshield, with the highest being the leading edge of the hood. With this high pressure at the base of the windshield, it doesn't make sense to attempt to "vent" underhood heat at that spot, because the pressure will push air into the engine compartment rather than allow it to escape. This is also why racers use cowl induction hoods to feed fresh air to the engine.
'CC Quickies
Do you know this guy? Amazing who you will meet hanging out at the dragstrip.
Hint: His initials are W.J.>
It's possible that side vents could be beneficial, although at the cost of cutting holes in the fenders. At speed, these are generally low-pressure areas, so they are good for venting. It seems that directing air out from underneath the engine compartment at speed is the most logical direction to take. Preventing excessive amounts of air from entering the engine compartment is generally helpful if you are attempting to run a vehicle at high speeds. This is why the NASCAR racers use tape to block off air entering the front of the car. This air tends to increase the pressure under the hood, causing lift. Even a small amount of pressure under the large expanse of a hood is more than enough to lift the front end of a normal passenger car several inches, especially at speeds of over 130 mph.