Hood Venting

[Gkuhn41]

Hello gentleman,

 

We have recently added a splitter to our car, sealed off the bottom reasonably well for a bunch of hacks, now were looking at venting the hood. There are 2 ways being discussed.

 

1. Cut a hole in the hood just behind radiator, bend a 1" wicker at the leading edge to force air up and over, should help draw air out (I think). Opening would be about 2" narrower than radiator opening and 8" deep. 

 

2. Simply cutting a hole will not help even with the wicker, and may in fact draw air in from the to so we need louvers made or cut in for the hood to help extract the air. They must be placed appropriately and sized appropriately to work. 

 

Im an operations guy, i want to cut a hole and run it. Others are engineers and want to calculate, test, calculate, test, calculate, approve and install. 

 

What have the masses seen work real world. Does it make a huge difference? I cant see air being forced into the top of the hood at speed....but im not very smart.

[theblue]

you can avoid cutting holes by just putting washers at the back of the hood to raise it up and let air out

[Hi_Im_Will]

9 minutes ago, theblue said:

you can avoid cutting holes by just putting washers at the back of the hood to raise it up and let air out

Wrong direction, lifting the back of the hood forces air in and reduces downforce, as the base of the windshield is a high pressure area. Either seal off the back of the hood or duct it to your intake.  Best places for holes are the high velocity/low pressure areas directly behind the radiator and the path from the grill over the top of the fenders.  Venting the fenders behind the tires also does a good job of getting hot air out from under the hood.

 

See below for example of truncated fenders, center hole, and hood louver position

 

[theblue]

I can't argue with that logic,  thanks for the info

[Originalsterm]

23 minutes ago, Hi_Im_Will said:

Wrong direction, lifting the back of the hood forces air in and reduces downforce, as the base of the windshield is a high pressure area. Either seal off the back of the hood or duct it to your intake.  Best places for holes are the high velocity/low pressure areas directly behind the radiator and the path from the grill over the top of the fenders.  Venting the fenders behind the tires also does a good job of getting hot air out from under the hood.

 

See below for example of truncated fenders, center hole, and hood louver position

 

For your hood, are both the center hole and louvers required in any setup? Or is it a function of the shape of the front of the E30? Or any other frontal work you've done?

[Hi_Im_Will]

Pretty generic for anything with a windshield, but sometimes you can find a pressure distribution for your specific car on Google images.  Put your holes in the low pressure areas.  Ours are adapted a bit to avoid the hood reinforcements.  

 

Pictures of pro level GT cars are also a good reference, you'll see that they generally all have the same strategy.

[pintodave]

1 hour ago, Gkuhn41 said:

Hello gentleman,

 

We have recently added a splitter to our car, sealed off the bottom reasonably well for a bunch of hacks, now were looking at venting the hood. There are 2 ways being discussed.

 

1. Cut a hole in the hood just behind radiator, bend a 1" wicker at the leading edge to force air up and over, should help draw air out (I think). Opening would be about 2" narrower than radiator opening and 8" deep. 

 

2. Simply cutting a hole will not help even with the wicker, and may in fact draw air in from the to so we need louvers made or cut in for the hood to help extract the air. They must be placed appropriately and sized appropriately to work. 

 

Im an operations guy, i want to cut a hole and run it. Others are engineers and want to calculate, test, calculate, test, calculate, approve and install. 

 

What have the masses seen work real world. Does it make a huge difference? I cant see air being forced into the top of the hood at speed....but im not very smart.

 

Option #1 has worked well for us. Made a tremendous improvement in cooling. 

 

As @Hi_Im_Will said, as a generalization you do not want the back of the hood lifted - that's why it's called cowl induction and not cowl exhaust    I've had many-a-debates about that. Most likely as you get away from the center and taper out towards the a-pillars it would probably exhaust, but w/o wind tunnel testing it's just a guess.  

[Hi_Im_Will]

1 hour ago, pintodave said:

 

Option #1 has worked well for us. Made a tremendous improvement in cooling. 

 

As @Hi_Im_Will said, as a generalization you do not want the back of the hood lifted - that's why it's called cowl induction and not cowl exhaust    I've had many-a-debates about that. Most likely as you get away from the center and taper out towards the a-pillars it would probably exhaust, but w/o wind tunnel testing it's just a guess.  

 

You can also tape little tufts of yarn to your hood to see which way they point while you're driving.  Easy way to verify you got the hole right.

[NigelStu]

^or get fancy with data acquisition and some MAP/pressure sensors - find the biggest delta from under/over the hood and put the holes there.