On a track car yea.

 

there was a guy that has done the same thing with his ls1 nb over on m.net it looks really good.

 

IF you track your car, cutting your hood to allow heat to escape from behind the radiator can be beneficial. If you don't go to the racetrack, there is absolutely no need.

While the pattern he chose to cut out (copying the pattern on the underside of the S2K hood) looks cool, it is really not ideal for cooling. The rear half of the hood is seeing positive pressure, which means any holes in the hood on the back half of it will be letting air in, not out, which is not the goal. Looking at the pic of the underside of his hood, he would have been better off cutting out one of the center sections, right behind the radiator. None of the holes he cut are very close to the back of the radiator, and the back two are not going to be helping vent air out at all.

What I'm talking about can be done on our cars. This is an old pic, but it shows what I did:

Those three trianglular holes mimick the shape of the bracing on the underside of the hood, and sit just behind the back of the radiator.

Also another important thing to understand is that a proper "extractor" hood will work better than just holes cut in the hood. By this I mean something that dips down into the engine bay to facilitate the flow of air up and out of the hole in the hood. But, holes like I did will indeed help.

Agian, there is no point if you aren't tracking the car. Even turbocharged, I had no heat issues on the street or at the autocross with merely a 37mm Koyo radiator. It's not until you are going hard for 30 minutes sessions at the track that this becomes a necessity.

-Ryan

 

There is always the older FM extractor hood.

One benefit "the Pass" did not mention, that both the location of his hood vents and the ones picture below benefit from, is a reduction in lift.

The highest point of drag on 99% of cars is the cooling system and front end openings. Once the air is in the engine bay there are not many smooth paths for it to escape. An extractor is a great way for this air to escape as long as it is in a low pressure zone. If its in a high pressure zone it wont be nearly as effective, and could even increase drag and lower the effectiveness of your cooling system.

One thing I am curious about though. I understand the low pressure/high rpessure zones across the top of the hood, but what about in comparison to the pressure zones under the hood? A lot of air gets sucked into the bay at speed, I wonder where the low and high zones are under the hood?

Could a high pressure zone on the top of the hood be considered low pressure in comparison to what is under the hood in that same zone? Channeled air under the hood could very well be higher pressure due to the areas of low volume it has to escape through. I wonder if there are ideal places to vent based on under hood air flow?

 

Wow a lot of good information. I like that design from "ThePass"

Thanks for informing me on all the benefits and down sides of different designs.

 

Gives you a good idea of where the air flows on this car.

http://www.youtube.com/watch?v=_BXMB8-pe-s&feature=player_embedded
credit goes to 'thesnowboarder' on MT.net

From what I understand, on top of the hood, there is a high pressure zone at the base of the windshield. It forms an "eddy" or a pocket of high pressure... this is why when the rear of the hood is lifted, air gets forced in, so while it may help IATs and underhood temps, it creates more drag. I think you want the opening up close to the front, after the radiator to pull it out asap to get the air on top of the car instead of stagnating wherever it wants to go in the engine bay-- whether it be out the tranny tunnel, or out the front wheel wells.

 

when the swap is done on the miata..I might look into doing this

 

I wouldn't do it on a DD, but on a track car those would be sweet.