theoretically, at speed, air moving across your hood is deflected by and over the windshield. the theory behind 'cowl induction' is that air reflects off the windshield and is sucked into the lower pressure area (into the hood) and provides more air under the hood helping alleviate under hood temperatures, provide cooler/denser air for intake, etc. most people use them for clearance problems, ie running big intake/carb/air cleaner and/or forced induction that require large air boxes and piping over the carburetor.

 

To expand RaSMG's post, a high pressure zone is created at the base of the windshield when the car is moving. This is how your cowl vent gets all that air inside it. I would think that it would work better if the hood cowl induction opening was closer to the windshield instead of the other side of your cowl vent opening for pre 71 cars.

John

 

I'm sure there is some theory stuff that I am missing, but...I once ran my hood with the bolts removed on the hinges. As speed increased the rear of the hood raised up indicating a lower pressure above the hood. A cowl vent that is sealed to the carb and closer to the base of the windshield may have a forced induction effect. It will have a cooler air effect...if it is sealed to the carb.
Cowl hood that is just vented to the engine compartment will vent some of the air that our big mouth Mustangs stuff under the hood. But that ain't a bad thing...I did notice that the high speed front end float was gone when My hood was raised up a bit. Just don't bust a radiator hose because the windshield is the first to get it with a cowl vent. I may get a physics theory slapdown on this, but thats O.K. But this does make sense to me.
Bruce (TX)

 

As 70 cougar said. But, to add to that, Nascar uses cowl induction so it does work. I'd met Rusty Wallace and looked at his car about a year ago and he had a huge fiberglass housing around the carb on the car going up to the cowl vent area used on our cars for heating and ventilation.
-Brian

 

The hood probably raised because the air pressure under the hood was probably higher caused by the air coming through the radiator and the heat of the engine compartment.
-Brian

 

When I was a kid, my brother's SS Chevelle had cowl induction. I don't know if it helped horsepower or not, but when you're like 13 riding in the passenger seat of your 16 year old brother's first car, he pedals the rat motor, and part of the hood springs open...it's pretty cool. ::

My "feel" is it's probably more of a gimmick than anything else...but it's a cool gimmick and does add a few horsepower. Years after my brother had bounced his Chevelle off a combine and into a tree, I bought myself an '83 TransAm. It had cowl induction too. The rear facing scoop opened up via a solinoid when the throttle went past a certain point. That allowed GM to squeeze 145 Horsepower out of only 305 Cubic Inches

Phil

 

They're so ugly they must add HP. Ducking the ensuing flames LOL.

John Harvey

 

The benefits from a cowl induction hood depend on the quality of the design and aerodynamics. NASCAR does use cowl induction, but they have the benefit of running wind tunnel tests. ALso, they spend thousands probably to optimize the duct system that feeds from the cowl into the engine. I doubt most aftermarket cowl hood setups are rigorously tested in wind tunnels, etc. I think there has been some road tests that indicate the cowl hoods give slight gains in performance, but not sure how effective they really are without solid data.