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They both do the same thing, and that is more or less nothing without building an effective duct out for the radiator. And neither is big enough to effectively duct all the radiator airflow out the top. You will get more performance from reducing the mass of the hood. But first and foremost this is for looking cool.
 

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If you think that you are not smart enough to read books about automotive aerodynamics but you want to put carbon fiber things on your car to make it look cool, perhaps just go for "looking cool." Otherwise you can start with "Tune to Win" which has a great introduction to ground effect aerodynamics... more for the mechanic than the engineer.
 

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Discussion Starter #6 (Edited)
didnt border also make a radiator relocation kit?

Im going with FRP for cost reasons, I posted these hoods up for instructional purposes only (good pictures).

A frp border hood is less than half the price of a asksport FRP hood, before shipping, one I can get in the US, the other I have to order from japan (shipping cost nightmare I'm guessing)

Raceonusa has a pretty extreme ducted hood, plenty for venting all the air from the radiator but it limits usage to the fronk, and when im not racing I like having the option of a spare tire (im pretty talented at finding screws and nails).

All in all the asksport hood is probly the best looking, the border the cheapest, both the same performance, asksport is probly alot higher quality

Another question. The border front bumper looks longer, is that extra length needed to relocate the radiator?

Also planning on venting the excess air onto the front brakes instead of pulling it under the body. Need some underbody panels for that other than the stock plastics

Some other info on the two

asksport claims 7.2 kg on the FRP (little over 14 lbs)
Border carbon+frp hood-the standard- is said to weigh 16 lbs, not sure on their exclusive frp hoods
 

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Discussion Starter #8
isnt the border hood a replica off the border race car? they used it for a reason right?

just a non-educated guess
 

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Discussion Starter #10
right, my point is if the border team used it as they did (with the propper radiator relocation) then its likely that its a track tested design with significant improvements.
 

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Eversor said:
right, my point is if the border team used it as they did (with the propper radiator relocation) then its likely that its a track tested design with significant improvements.
"Significant improvements" on a road-car-derived race car are still not very significant compared to the total grip envelope of the car.

When paired with an unstreetably low, flat bottom race car with a gigantic wing and 100+ mph regular cornering speeds, yes it will make a noticeable difference (noticeable, but still much less than mechanical grip).

When put on a car for looks, combined with road car ride height, stock underbody, and no wing, you probably get close to zero benefit. Especially with such a small opening such as those shown above, that aren't anywhere close to big enough to exhaust all of the radiator exit air.

Don't fool yourself, aero modifications will not have any effect on your street car on the street (or at autocross speeds) except for hurting gas mileage.
 

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Discussion Starter #12 (Edited)
well if i do it right it should help the auto x, got a evo out there thats pushing 1400 lbs at 100 mph, and roughly 400 to 500 at 60. The aero mods are almost absurd to look at but hey he takes some serious corners

raceonusa puts out a hood with a pretty massive hole in it. about the same size as the radiator.





something tells me i would need a different brace with that hood, dont think mine would fit right
 

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having the vent the same size as the radiator is a bad thing though. thats the kind of intelligence and logic id imagine your average carbon fibre styling company has.

the radiator is a big chunk of metal with some tiny holes in it. the total flow area of the radiator is substantially less than its actual frontal area, plus the flow disturbance then takes away even more flow. also radiators cant flow air at more than something like 54mph, cant remember the exact number but there have been plenty of studies done into it. there are probably more efficient and less efficient designs, im not sure where exactly the 54mph came from but it came from a small race car manufacturer that did some development.

so youve got fast air enterring the front that needs to be slowed down, because if it isnt then its going to create more drag. then as it exits it needs to get sped up.

use a simple venturi and you find your answer. small radiator inlet opens out to the radiator, the exit vent closes up at the hood, and youve got your venturi to slow the air down into the radiator and then recover the speed as it exits. having the vent the same size as the radiator is going to create lots of drag at the entry and a load of flow disturbance at the exit as slow air meets fast air over the hood.

the exact sizes of the venturis are a question of testing and development. iirc something like 60% inlet, 50% exhaust was a good starting point, talking about the area of the openings relative to the operational frontal area of the radiator. i cant remember if those are the exact numbers now though. again, these numbers came from the same race car manufacturer as the "terminal velocity" of radiator flow. they ended up at something like 64/50 with their final design i think. either way, the inlet is bigger than the exhaust!
 

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So Jim, what you are really saying is that one might be able to get away with taking the border hood and directing all of the exhaust air through it? From my measurements of the stock bumper inlet (and a wild guess at the border vent dimensions) it looks like it may actually not be that much smaller than the inlet on the stock bumper. Maybe in line with the 64%/50% actually?
 

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Alex W said:
So Jim, what you are really saying is that one might be able to get away with taking the border hood and directing all of the exhaust air through it? From my measurements of the stock bumper inlet (and a wild guess at the border vent dimensions) it looks like it may actually not be that much smaller than the inlet on the stock bumper. Maybe in line with the 64%/50% actually?
youll "get away" with pretty much any size on a road car, i doubt its of much significance at lower speeds and the benefits are probably fairly negligible. without knowing dimensions its tough to say, but both of the hoods in the first post look to have a fairly sensibly sized exhaust opening on them.

the Raceonusa on the other hand looks to be the size of the radiator itself!!!

equally as important is gradually tapered ducting between front bumper and radiator, and radiator and exit vent. if you dont properly duct it then the air will just flow wherever it wants to and youll lose the venturi effect.
 

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Discussion Starter #16
id like to see it drafted up by someone with the software/skills to do it so that i can get a visual on exactly what needs to be done to make the ducted hood a viable and fuctional option
 

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even a small outlet will do something. not only does it help with aero, but cooling should be better. might not be huge, but along with the weight savings, i would consider it a win/win situation. not sure why it's getting a bad rap here?

i plan on sticking with an NA radiator for weight savings and using a vented hood to help keep it cool on the track. our cooling systems are overengineered by far, so i figure is best to do it that way.
 

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even a small outlet will do something. not only does it help with aero, but cooling should be better. might not be huge, but along with the weight savings, i would consider it a win/win situation. not sure why it's getting a bad rap here?...
our cooling systems are overengineered by far...
There's no cooling benefit unless you believe that more air is going to flow through the radiator if it's ducted over the hood instead of under... and as you point out, nobody is having cooling problems anyways.

A properly designed and fabricated duct might take a little more air through the radiator than before. Without an accurate CFD or on-car testing it would be pretty hard to say. If you are just going to bolt on a blingy hood with a hole cut in it like above, and cut a hole in the firewall behind the radiator, you're not going to gain anything... even the one with the big duct, like Jim said, does not re-accelerate the air to the car velocity after it exits the radiator and will cause more drag than good. The air duct has to be reasonably smooth and any bends have to be of a reasonably small radius.

ALL of the answers that the OP is looking for are in the book I recommended above, in plain terms that the average person can read.
 

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Alex Jacome said:
I agree with Proj3ct [email protected], ever since I did this to my hood, not only is the cooling better but the front end feels planted to the road at freeway speeds!
So what you're saying is, your car now oversteers more the faster you go?? And if we for a moment believed that this was true, how would this be a good thing??
 
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