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After posting some pictures, I received some requests for a DIY thread.
Rationale for this mod:
I wanted a vented hood for three reasons. 1. To let heat (from header, engine, & radiator) escape from engine bay 2. To get cooler air into intake 3. To use the low pressure area above the hood to help extract air from under the front of the car thereby increasing velocity under the splitter and floor of the nose which should increase downforce.
Why not buy a carbon fiber vented hood? The affordable knock-off carbon hoods are heavier and less stiff than the stock aluminum hood. I believe the stock hood weighs 14 pounds. The high quality "authentic" hoods (cwest, mugen...) cost a bundle and still need to be painted unless you are going for the carbon look. Some if not all composite hoods require hood pins for high speed use. Oh yeah, and I'm cheap. I planned on venting both sides but ran out of time and decided I like the look of just one side. Still debating.
Here's my solution:
How to build your own.
Please use appropriate safety equipment and don't screw up.
Yes, you are about to cut holes in a perfectly good hood so be prepared.
Remove the hood and the hood liner and support it upside down on some padded saw horses.
Make templates by laying paper over each support hole and tracing the edge (use the pencil rubbing technique). Use the templates to cut out some perforated aluminum (clean and paint the both sides of the aluminum). http://www.mcmaster.com/
Use a 3/8" spacer against the hood support edges to draw the hole shape. I left a 3/8" lip for mounting purposes.
Using a UniBit, drill a hole in each corner. UniBit
Using an air saw, cut out the shape from the under side - leave a small gap to finish off with sanding. Air Saw
Use a sanding drum on a die grinder to smooth to shape. I tried using a sandpaper wheel on an angle grinder but it gets too hot and the paints starts to peel back.
Once the shape is close, flip the hood over and apply masking tape on the top side all around the new openings.
Use wet dry sandpaper to smooth the cut and feather the paint. The masking tape is there so if you slip with the sandpaper, you won't scratch up your hood.
Lay 3M double stick tape used for moldings/body work on your 3/8" lip and trim with a razor to fit just inside the cut out (find it at automotive body shops).
Use a fine paint brush and carefully paint the exposed edge with factory touch up paint.
Install perforated aluminum and your done. I used some small sheet metal clips to hold the perforated aluminum in place just in case the double stick tape failed due to engine heat but I ran a full track day without them and everything was fine.
Now I fully admit I know next to nothing about aero...but shouldn't the leading edges of the holes be flared up up a bit? I.e., while at speed, is there enough of a low-pressure area over that region of the hood to effectively suck hot air out? Or will the flat surface in front of the grilles just result in some local turbulence without causing much cooling?
I'm actually quite curious about the mod, since I'm picking up a spare OEM hood tomorrow...
wow very nice job!.....now I'm assuming you would not be able to drive around with this hood in the rain? The job you did on this hood looks so good it looks like you bought this hood as an aftermarket hood
Damn if you just proposed the idea I would have completely blown you off and called you crazy. But this actually looks decent! I do have a spare hood lying around... hmmm. But then again I need to drive in the rain and this would be nighmareish with those kind of holes.
Good stuff. I came to the same conclusions about cf hoods as you did and I decided that venting the stock hood was the best (and also cheapest) way to lower under-bonnet temps. Difference is that I cut out only the portion of the hood above the headers. Doesn't look nearly as bad ass as yours, but since the headers are generating the most heat under there it's probably just as effective. And when it does rain, I slide out the mesh and slide in a piece of CF.
Price of everything, including spare hood was about $125.
09-23-2005, 04:59 PM
DIY: Vented Aluminum Hood
