Here's an animation available to see the effects of the Evo's aerodynamics in action to which the heat extractor is also apart of.

http://www.evoclub.net/special/evo8/body_work.html

 

I've had mine out for quite some time now. I drive in the rain and the snow and I have no problems with dirt or anything. Stays rather clean actually. Plus, how much of the rain does the that heat extractor sheild keep out anyways?

 

some older race evo's have them on.

click here

 

If you look at the website given by n00dle, you'll see what the fins were meant to do. They are basically directing the hot air to go along with the air stream that's going over the hood. Now if you take off the fins you'll have hot air that's basically rising straight up out of the vent. This will disturb the stream of air going over the hood and basically reduce aerodynamic efficiency. I bet that you'll end up with having higher drag coefficient w/ the fins off than with them on.

Now it probably won't make much of a difference for normal driving but once you start modding your car and achieve high speeds, that fin will make a difference since the air flow over the car's hood end up going over the rear wing.

But I'd also think the fin would help suck the hot air out since a nice undisrupted airflow will create a better(?) lower pressure area over the hood, allowing more air to be drawn out.

 

It's Simple Physics, folks!
Louvered part on reduces cross-sectional area of airstream = less flow & more turbulent too. It's a simple pressure flow phenomena, whether it be fluid or gas. You could almost go so far as to calculate the relative flow between the open cross-sectional area versus that of the reduced area at the many small louver openings.
Yes, the louvers are angled to make less turbulence and restriction than if you turned the part backwards, but it is still restriction. There is positive pressure driving the air into the front of the car/engine bay and relative negative pressure over the hood cut-out, thus drawing the air up and out of the engine bay!
The older models had side vents and thus had flow going other areas as well--so maybe it didn't make as much relative difference, but I posted the pix of the Evo8 rally cars clearly not wearing their vent bling-bling.
Interesting how so many people get it when talking about intake airstreams, etc, but miss the point for this. If you put a scaled down louvered part into your airbox, do you think it would make the unit more efficient?

 

I think I get what you're saying...So basically those "fins" create restrictions and the flow will be less with the fins on? I would think the fins would be beneficial without the worry of having restricted airflow.

While the louvered part (is this the proper term for these things?) lowers the cross sectional area of the vent, that doesn't necessairly mean the flow will be restricted. The air can(probably is) be exiting from the vent with the louvered part at a higher rate than w/ the part not on. If the louvered part actually creates higher vented airspeeds then it'd be interesting to find out how the air flow (cm^3/min) compares to the flow without the louvered part.

One benefit I can see w/ this is that the vented air better matches the speed of the air flowing over the hood, creating less turbulence.

I think the the louvered part's primary function is to generate better aerodynamics, not to increase heat dissipation, which could very well be one of its benefits which I do not have any concrete evidence for.

On a side note, not everything as it seems. No offense directed at you but it bugs me when people says "simple physics" or "simply this and that". There's always a minute detail that we're missing from the big picture. And by this I'm not saying in any way that I see the big picture. I don't, and all I'm merely trying to do is to get to the bottom of this crazy contrapation and help others determine if this thing is actually worth the its weight in your car.

And as for the rally cars not having the part in...Should be in an episode of Unsolved Mysteries. I honestly don't know why they wouldn't.
But just because the rally guys aren't putting it in doesn't mean we shouldn't or that there isn't a viable use for it. But I do have a guess though. hehe

I think on short courses with lots of turns, where top speed isn't essential, it'd be more beneficial to take these parts out since aerodynamics of the car isn't as vital at lower speeds and having an unrestricted vent would offer better cooling since they'll be putting alot more stress on the engine (accelerating and decelerating frequently). On tracks with lots of straightaways, I would think it'd be better to put the louvered part in to obtain better aerodynamics and stability at high speeds. But then again all this is just speculation (but seems to make sense to me!). And if you got this far...I congratulate you for reading all this. lol. You deserve a cookie. But I don't have any, sorry. But here, have a beer.

 

Just a note for everyone ..

With the vent hood on, the temps reads slightly lower than norm on the temp gauge while travelling.. At the lights, it climbs to the normal temp range after about 3-5 mins of idling..

With the hood off, the temp is 2 ticks lower on the temp gauge (yep.. it looks like it's permanently on cold start cycle )
At the lights .. it takes alot longer to warm up to norm temp.. and it cools alot faster once you start moving again..

FYI, the hood is not there for aerodynamic reasons or heat sink .. it's just a splash shield for rain or crud or snow ..

 

You're probably right. heh, but I still think that it has quite a bit to do w/ aerodynamics. But since pretty much no one can prove it, who knows. We can all just assume our own speculations are the correct ones