Originally Posted by
Cobra4B
You guys do realize this works in reverse at speed right.... why do you think muscle cars have "cowl" induction. Having the hood lifted at the rear actually creates a low pressure area at the back. As air flows over th ehood and hits the windshield it turns over on itself and is sucked back into the engine bay area. It ends up keeping heat in. It works for feeding the air intake on a carburated motor, but will not cool anything.
On my Corvette I tried removing the rear weatherstripping from the cowl area thinking the same tihng. On th eback straight at VIR is had violent hood shuttering at higher speeds as the air was actually going back and under the hood. Spoke to a few people more knowledgable than I and it was explained in detail. Back in went the weatherstripping and no more bad hood vibrations.
If you want to vent the hood the holes need to be farther forward and your radiator/intercooler need to be angled forward.
Just to add to what Brian said, if you make a big change in slope, like a cowl or at the end of a car, you'll get a big eddy there usually and a lot of turbulence. Holes for cooling don't necessarily need to be farther forward, they just need to be fairly flat with the surrounding bodywork. You want that air coming out to merge with the air going over the hood, not to have the air make a turbulent pocket where it comes out.
Cowl hoods, like most things that deal with aerodynamics, are speed dependent, same with ram air hoods. The other reason for cowl hoods is that carburated engines could pick up a bit of power with a high rise intake manifold so they usually couldn't fit under a stock hood, and a cowl hood is much better for drag than a scoop or something like that.
Hoods with (flat) cooling cutouts in them work like a venturi, and very differently than a cowl. In this case you're using the pressure of the air flowing over the hood to help draw air out of the vents.
Raising the back edge of the hood to help cooling may work at some speeds (probably low speeds), and not at others. Most speeds aren't going to give good results though, because the recessed area where the windshield wipers are is specifically designed to create a dead spot so the wipers don't create much drag. If you raise the hood up, this area will get deeper and much more prone to forming a turbulent pocket right there. This is probably fine at low speeds because there isn't enough air flow (velocity) to create a problem, and for stuff like drift cars that don't go very fast, any extra cooling at low speeds is a bonus. Usually the more power you put out, the faster you go, so you get an increase in cooling, instead of just putting all that power into going much slower than you normally would (by wasting it all as wheelspin).
There are better ways to do that though, but arguably not as cheap or easy as putting some spacers on the hinges. I'm not even going to comment on that hood mod and it's rice-ness by the way. Or maybe I just did.
I guess what I'm saying (and I'm going to speak for Brian too) is that if you don't have cooling problems, don't make any cooling modifications. You can't just do something and expect it to work better without knowing how or why. Also, if you want to angle the radiator forward, you might as well get a bigger radiator while you're at it, so at least you're still using the available frontal area for cooling.