what do you guys think of this idea. i thinkg that im going to purchase teh iroc turbo hood for my camaro and use all of the holes as vents fot underhood heat. would this be a feasable solution to that problem. id rather not remove the strip for the back of my engine bay. do you guys think that this would cause a decent decrease in underhood temp? thanks in advance.

Shadow

 

the daytona hood is a cowl hood correct? if it is , the cowl should significaly drop the temp.

 

Cowl hoods typically raise underhood temps and create cooling problems, rather than alleviating them... unless they're ducted to the engine's air intake itself, and otherwise sealed off.

All the air that gets forced into the engine compartment has to find a way out. The cowl is a high pressure area. Air from there will be forced in. The front of the radiator is also a high pressure area. The only exit from the engine room ordinarily is under the car, also a high pressure area; that's part of what the air dam does, is to help create a low pressure behind it, so that the air won't just stack up and accumulate, but rather has some helo in being drawn out.

The best cooling add-on for any of these cars is the "shark gill" vents in the fenders. That's a low-pressure area, and will help the necessary flow of air, rather than opposing it.

 

You could find a hood with vents or holes just after the radiator, like the Shelby GT350 has. This location is a low pressure are and will draw air thorugh the radiator and out the vents.

 

the cowl induction hood will draw the heat out. This is so because the high velocity air passing over the cowl at teh rear of the hood is at a lower pressure than the realativly still air in the engine compartment (remember bernulli's law?). This creates a pressure differential between the engine compartment and the open area of the cowl and the air is drawn out. Like RB said, all the ductwork for the radiator has to be in place otherwise the air in front of the radiator will be drawn up over the rad. support and into the engine compartment, therby rederecting the air past the radiator.

 

The open area of a cowl hood is at (duh) the cowl, the base of the windshield, which is one of the highest pressure points on the whole car. That's why people use them: it's ram air that actually works, unlike things like nostrils in the front of the car, which don't do anything until ridiculously high speeds. Look at a NASCAR racer someday, that's where they take their air in, because it's dense there.

The top of the hood near the front (but not right at the front) is a low pressure area, because of Bernoulli's law. The same is not true of a cowl hood.

 

If you really want to scavenge air from under the hood (and the rest of the vehicle) try lowering the front air dam and side air skirts. Aerodynamic turbulence will be reduced, and as long as the rear of the body is left "open", air will be drawn through the radiator and out of the engine compartment with fierce velocity.



I'm with RB on the high pressure at the cowl area.

 

not out to start an argument but since the cowl area of a car isnt a stagnation point , its at a lower pressure then still air. It is a higher pressure area realative to the other areas of the car that have flow moving over them since there is a change in velocity as the air flows over the windshild, but, the flow isnt stalled so it is still at a lower pressure then the realativly stationary air in the engine compartment. What your saying is that the air would make a nearly 180 degree turn to enter the engine compartment through the cowl if what your saying was true. The air's momentom prevents this since the path of least resistance is on to and over the windshild. Although i cant prove this with text alone, the air there should be at a lower pessure then the stationary air in the engine compartment. If this was false, then airplanes woudlnt fly and ground effects wouldnt help generate downforce. Not saying the cowl will generate large ammounts of airflow but at the very least, it should provide an additional way for air to escape the engine compartment.

 

Your getting confused with momentum or flow and pressure differential.

 

i was illustrating the momentom of the air would forbid this since it would have to make a sharp turn in order to flow back into the engine compartment rather then continue to flow over the windshild. One of the best examples of the cowl at work is an engine falure down the 1320 i saw a while back. The smoke from all the oil flying around emmerged from the cowl and underneath the car. After seeing this, id have a hard time believing the cowl doesnt allow air to flow out of the engine complartment.

 

ok im with RB and Vader on this one.... think about it... picture a stream of air going over a car and over the cowl.... it's going to skip over the cowl onto a higher point on the windshield than if it was a normal hood.... the hot air is in the engine compartment.... well that stream of air is acting like a barrier for the heat to get out, it traps it in , it cant escape through the cowl.... at least that is how i understand it.

 

thats jsut it... there is a lower pressure within the air stream that will draw the air out, thereby creating a net flow out of the engine compartment and into the surrounding airstream. I think your thingking of a "streamline", which defines boundries in lamenar flows where no net mass transfer occures.

 

A vented cowl hood looks like a cowl induction hood from the 1960's but doesn't do the same thing. Cowl hoods like those on late 1960's Z28s were used to make more horsepower. Vented cowl hoods are designed to allow cooling air an easy escape path, allowing the engine to run cooler and possibly increasing stability by reducing the high pressure area under the nose of a car. The old cowl induction hoods used raised center section that ran back to the base of the windshield. Instead of having the opening on the front of the hood scoop, it was on the back. This allows the carb to pull its air from the relatively high pressure area at the base of the windshied, providing a very mild passive supercharging effect and possibly a few more horsepower. When a moving gas like air is brought to a halt, there is an attendant rise in pressure (the kinetic energy is converted to static pressure). Bernoulli's equation illustrates this:


P + (rho*V**2)/2 = constant

where:


P = air pressure

rho = air density

V = air velocity


When you decrease the air velocity, pressure must increase to keep the quantity a constant.

The vented hood allows air that enters the engine compartment a route to escape. It has to go SOMEWHERE after it's cooled your radiator, and without an explicit exit, it will go under the car, creating lift and more drag.

 

Air traveling over a
convex surface increases speed and reduces pressure. The converse is true:
air moving over a concave surface will decrease speed and therefore
increase pressure. Some of you might be thinking that the pressure over the
top of the car is lower than the pressure below it, and in general you
would be correct. The problem is that no car I've seen is entirely convex.
There is always a concave portion at the base of the windshield. Yup, you
guessed it, pressure higher at this point. While overall pressure over the
top of the car is lower, the pressure along most of the base of the
windshield is lower. Someone argued that the air at the base of the
windshield is sucked up by air over the car due to the motion. Sorry, not
true. The air at the base of the windshield will tend to stagnate (i.e.
slow down, but not completely due to the geometry).

 

Cowl hoods worked better on the older cars since the angle of the windshield was higher. There was a high pressure area at the base of the glass/cowl area. Bodies with a lower windshield angle will exhibit less of this tendency. Theory is great, but reality has to prove it out.

To truly answer the question, one needs to study the aerodynamic traits of a real vehicle in a wind tunnel. Observing the smoke trails over the hood can reveal a lot of information.

If you want to simulate the wind tunnel in your own experiments, disable your HVAC blower by unplugging the power wire, turn the controls to VENT, lay a chunk of a camphor block or Limburger cheese on the cowl air intake below the rear of the hood line, then drive down the road at about 65 MPH. That odor should tell you that there is a high pressure are at the base of the windshield, and that the factory was right in placing the air intake at the top of the cowl.

 

vader u crack me up, limburger cheese, lol. thats some stanky stuff

 

Lets not forget that many vehicles are designed with vents toward the rear of the vehicle that create a pressure difference to purposefully draw in air to venilate the veicle.

One thing i hate about these damn theory discussion threads is that they can be debated endlessly untill hell freezes over

"For that matter, the bulge on the hood protrudes into the air stream and makes a low pressure wake behind itself and there for......."

 

Yeah, I know. Theory is great, and there can generally be a formula, theory, axiom, or computation found to support almost any position. Like I said, reality has to match the theory, since, in the end, reality generally wins.

As someone here once most eloquently stated, and I paraphrase, Not sure who that was, but it wasn't me.

At any rate, you can easily find out if it works through a little trial and some minor experimentation. I know that creating a vacuum under the car with dams and skirts draws a substantial volume of air through the radiator (just like the factory theory behind the lower air deflector in Firebirds with NO grille openings), and also helps keep the car on the road better at "airspeeds" of 95 MPH+.

 

If they don't work why does everybody use them? All show and no go?

Why are they called, cowl induction, cold-air induction, etc; not engine bay vent.

 

I've seen enough leaves plastered to the screen in my cowl to believe that air is blown into the cowl without even thinking about the physics of it.

 

after driving around in my honduh im never going to get a cowl wahtever you want to call it hood. The a/c compressor would take too many gerbles to run so i rarly used it. On a cool day whenever id turn on the vent or open the rear windows, all id get is mad hot air from the engine compartment being taken in from the HVAC intake at teh cowl. Man i hated that. It was like driving an incinerator. The only options i had for fresh air was so hot it makes your shorts stick to your *** and you leave a sweat mark on the seat air.

Vader, I love that quote, ill have to remember that.

 

I wish I could remember who posted it first, so I could give the appropriate props.