[shirtpants_]

So I was tinkering around with the idea of adding a vent in the hood in order to let the air passing through the radiator somewhere to go besides under the car. It's my understanding that venting the radiator air out from the hood should reduce drag and lift at the front end.
I doubt I'll be cutting into my hood anytime soon, but this is just a fun little test I did on Sunday to see where the best place to put this vent would be.

There's no Aero section, so I figured you track guys would get the most use out of this.

I used a Digital magnehelic pressure gauge I bought from Dweyer, along with some 3/16" flexible pvc hose I bought from Lowes.
I measured 7 different points along the center of the hood, to see the pressure differential between these points, and the interior of the car.
I drove the same direction on the same road, with no traffic in front of me for each run.
For each run, I had the hose taped on top of each number on the hood. The speed was 51 MPH on cruise control, and the pressure was measured once every second.
The hose was taped to each point on the hood, forming a 90 degree angle to the direction the air was travelling.
The gauge was inside the car, with the AC off, and the window cracked to equalize the inside to outside car pressures. The gauge was zeroed so that when stopped, the gauge read zero pressure differential between the inside and outside of the car. The pressure went back to zero every time the car was stopped, so the results should be fairly accurate.

I have an e90 335i, and my car is just about 25 or 26(I cant remember) inches from ground to fender on both the front and rear.

I was not able to get a steady reading for any of the tests, so I put together a range of the most common numbers I saw on the gauge.
I used a digital gauge so there could be no misinterpretation of what the gauges reading was.

Points MIN Max
1.___ -.3, -.25
2.___ -.3, -.25
3.___ -.2, -.05
4.___ .05, .2
5.___ .15, .3
6.___ .34, .6
7.___ .45, .6

I rounded some of these numbers based on how steady the readings were.

The units the gauge measured in were Inches of Water Column or INWC.
INWC to PSI conversion is as follows.
1 PSI = 27.7076 INWC
.5 INWC = 0.0180456 PSI
.1 INWC = 0.00360912 PSI

This means the the pressures measured are extremely small amounts. This test was done at a low speed of 51 mph, so there are probably much bigger differences at 100 or 150 mph. I didn't have the time or road to cruise around at 100 to see how big of a difference it makes.
This data is collected from a low speed test, but I don't see any reason why the trends would not continue at higher speeds.

Any area with a pressure above 0 would be a higher pressure zone. If we had a vent there, air would most likely by trying to force itself inside to some extent. The higher pressure air under the hood should be able to overcome this, but the vent placement on a high pressure hood area wouldn't be doing us any favors.
Any area with a pressure below 0 would be a lower pressure zone. If we put a vent here, air flowing over the hood would be aiding in pulling the engine bay air out from the vent.

We can look at the hood of the e90 m3 and f82 m4 GTS to reassure my findings. The vent that leads to the e90 m3's intake is right about at point number 6 on my hood. This would be one of the higher pressure zones, and should force some amount of air into the intake. This would also be the worst place to put a vent whose purpose is to lower under hood temps, or to extract "dirty" air coming from the radiator.

Any place we see a negative pressure area on the hood, would be a better place to put a vent. The m4 GTS has a vent towards the front of the car, which should help extract the "dirty" radiator air.

Obviously we cannot put a vent on point #1, but I believe we can put one one on point # 3.

It looks like the best placement for a vent would be as far forwards as possible, while still being behind the radiator. Ducting the radiator up towards the vent would also be useful, but there isn't much room between the radiator fan and the engine as is.

This would be a great way for some of you guy's to cut down on drag and lift on the front end. It would also allow you to put together a more effective under-tray/flat floor, since you wouldn't have to worry about giving the air coming through the front of the car a place to evacuate underneath.

I really like the center vent placement on the space gray 335i with the m3 bumper. Too bad it has the other two side vents, and is only for the e92




UPDATE: 9/29/16

Ok, I got to do a little more testing. This time I measured the pressures of the engine bay, and the wheel well.
All testing was done on the same road as before, and at the same speed (51 MPH).
All other measures were identical, except the placement of the end of the tube.

I put the tip on the edge of the engine cover, right between points 3 and 4 on my original pictures.

All measurements were in INWC(Inches of Water Column) just like before.
Engine Bay
MIN Max
.61 .75
Most common reading: .63

The most common number I saw was .63, so I think It's fair to assume that that is the average.
Since I have an open intake, I think it's possible that that may have lowered the pressure in the engine bay. I didn't feel like swapping it for the stock box, but all testing was done with cruise control on, at low rpm in 6th gear, so it shouldn't matter too much.

Now the wheel well testing was done by feeding the end of the hose through one of the few holes between the engine bay and the driver's side wheel well. I believe the wheel well pressure builds up in the back, closest to the driver. I could not get that far with the hose without it just flopping around, and I think I'd have to get the car up on jackstands to secure the hose where I want it, since there's no room to do so when the car is on the ground.

This reading was more stable than the engine bay reading as well.
Wheel well
Min Max
.28 .34
Most common reading .31

So,
Engine bay: .63 INWC PSI=0.0227 (rounded to 4 decimal places)
Wheel well: .31 INWC PSI=0.0112 (rounded to 4 decimal places)

It's weird that the wheel well pressure is just about half of the engine bay pressure

It was a little damp when I did the the testing, and my rear tires are pretty much bald, so I didn't have the nerve to try any testing at higher speeds. I'll see about re-doing all of these tests at higher speeds after I get new rubber(and new wheels!) next Friday.

UPDATE: 10/19
Got a chance to do some testing of the underhood pressure again. This time at just over 100 MPH.

I used the same location for the end of the tube for measuring.

I ended up with a much more stable reading than any of the other tests.

Hovering right around 2.23 INWC the entire time. This is over triple what the pressure was at 50 mph.
This equates to 0.08048 PSI. I would have done it at higher speeds, but even at just under 110 I was getting an overflow message, which basically means that the readings were going over what my unit is capable of measuring.

I tried to get more data for the over hood pressures as well, but I was getting an underflow message at anything over about 60 mph. So the pressures were below (as in negative pressure) what my unit could read.

This is pretty much the end of any aero testing I'll be doing. My unit cant read anything higher, so this is it.

[spxxx]

Originally Posted by shirtpants_

So I was tinkering around with the idea of adding a vent in the hood in order to let the air passing through the radiator somewhere to go besides under the car. It's my understanding that venting the radiator air out from the hood should reduce drag and lift at the front end.
I doubt I'll be cutting into my hood anytime soon, but this is just a fun little test I did on Sunday to see where the best place to put this vent would be.

There's no Aero section, so I figured you track guys would get the most use out of this.

I used a Digital magnehelic pressure gauge I bought from Dweyer, along with some 3/16" flexible pvc hose I bought from Lowes.
I measured 7 different points along the center of the hood, to see the pressure differential between these points, and the interior of the car.
I drove the same direction on the same road, with no traffic in front of me for each run.
For each run, I had the hose taped on top of each number on the hood. The speed was 51 MPH on cruise control, and the pressure was measured once every second.
The hose was taped to each point on the hood, forming a 90 degree angle to the direction the air was travelling.
The gauge was inside the car, with the AC off, and the window cracked to equalize the inside to outside car pressures. The gauge was zeroed so that when stopped, the gauge read zero pressure differential between the inside and outside of the car. The pressure went back to zero every time the car was stopped, so the results should be fairly accurate.

I have an e90 335i, and my car is just about 25 or 26(I cant remember) inches from ground to fender on both the front and rear.

I was not able to get a steady reading for any of the tests, so I put together a range of the most common numbers I saw on the gauge.
I used a digital gauge so there could be no misinterpretation of what the gauges reading was.

Points MIN Max
1.___ -.3, -.25
2.___ -.3, -.25
3.___ -.2, -.05
4.___ .05, .2
5.___ .15, .3
6.___ .34, .6
7.___ .45, .6

I rounded some of these numbers based on how steady the readings were.

The units the gauge measured in were Inches of Water Column or INWC.
INWC to PSI conversion is as follows.
1 PSI = 27.7076 INWC
.5 INWC = 0.0180456 PSI
.1 INWC = 0.00360912 PSI

This means the the pressures measured are extremely small amounts. This test was done at a low speed of 51 mph, so there are probably much bigger differences at 100 or 150 mph. I didn't have the time or road to cruise around at 100 to see how big of a difference it makes.
This data is collected from a low speed test, but I don't see any reason why the trends would not continue at higher speeds.

Any area with a pressure above 0 would be a higher pressure zone. If we had a vent there, air would most likely by trying to force itself inside to some extent. The higher pressure air under the hood should be able to overcome this, but the vent placement on a high pressure hood area wouldn't be doing us any favors.
Any area with a pressure below 0 would be a lower pressure zone. If we put a vent here, air flowing over the hood would be aiding in pulling the engine bay air out from the vent.

We can look at the hood of the e90 m3 and f82 m4 GTS to reassure my findings. The vent that leads to the e90 m3's intake is right about at point number 6 on my hood. This would be one of the higher pressure zones, and should force some amount of air into the intake. This would also be the worst place to put a vent whose purpose is to lower under hood temps, or to extract "dirty" air coming from the radiator.

Any place we see a negative pressure area on the hood, would be a better place to put a vent. The m4 GTS has a vent towards the front of the car, which should help extract the "dirty" radiator air.

Obviously we cannot put a vent on point #1, but I believe we can put one one on point # 3.

It looks like the best placement for a vent would be as far forwards as possible, while still being behind the radiator. Ducting the radiator up towards the vent would also be useful, but there isn't much room between the radiator fan and the engine as is.

This would be a great way for some of you guy's to cut down on drag and lift on the front end. It would also allow you to put together a more effective under-tray/flat floor, since you wouldn't have to worry about giving the air coming through the front of the car a place to evacuate underneath.

PICS:
I really like the center vent placement on the space gray 335i with the m3 bumper. Too bad it has the other two side vents, and is only for the e92

Wow so much detail. I love this even though I drive an E82... We need an aero subfolder.

I'm starting to think a lot more into aero on my car since it has such a bad front-end design. I hate the fact that so much air is stuck under the hood but I can't justify cutting haha - running no cowl will allow some passage of the high pressure too.

Awesome write up !

[shirtpants_]

Originally Posted by spxxx

Wow so much detail. I love this even though I drive an E82... We need an aero subfolder.

I'm starting to think a lot more into aero on my car since it has such a bad front-end design. I hate the fact that so much air is stuck under the hood but I can't justify cutting haha - running no cowl will allow some passage of the high pressure too.

Awesome write up !

Yeah, the e90/e82 has a pretty large frontal area. If I remember right, the e92 gt2 even was allowed an increase in power(or decrease in intake air restriction) compared to the other gt2 cars because it's frontal area was so large. At least we get a good amount of room for extra heat exchangers.

I don't know how effective it would be to delete the cowl. The air pressure increases as it gets closer and closer to the windshield, where it builds up. That's partly why the filters for the HVAC or AC or whatever are there. So you'll be trying to push air out from the area with the highest pressure, where BMW designed for air to be forced into the car's cabin through the filters. You also might get some nasty engine smells into the cabin.
That's just my take on removing the cowl. I don't have any data to support it.
Maybe ill do some more testing later this week for the under hood pressures, and see the effects of removing the cowl. Although I wouldn't feel comfortable leaving it off in the rain since I have an open intake.

[Cloud9blue]

Appreciate the diligence that went into this. But

1. Unless you are running full aero (front splitter, full flat undertray, diffuser, and rear wing), this will not make any difference.

2. You are ruining a perfectly good hood... And you car will look like shit if it is still street driven.

3. There are very few track in US where you can +150mph in our cars (even with FBO or upgraded turbos). Sebring and Daytona are pretty much the only tracks near you where you can hit +120mph consistently.

4. If you want to improve your laptime. Get a data acquisition app or device and see where you can improve. These cars benefit much more from good driving than complicated but ineffective aero mods.

5. Cooling is the only thing I can see being benefit from this. But you are better off with oil cooler and radiator upgrade...

[shirtpants_]

Originally Posted by Cloud9blue

Appreciate the diligence that went into this. But

1. Unless you are running full aero (front splitter, full flat undertray, diffuser, and rear wing), this will not make any difference.

2. You are ruining a perfectly good hood... And you car will look like shit if it is still street driven.

3. There are very few track in US where you can +150mph in our cars (even with FBO or upgraded turbos). Sebring and Daytona are pretty much the only tracks near you where you can hit +120mph consistently.

4. If you want to improve your laptime. Get a data acquisition app or device and see where you can improve. These cars benefit much more from good driving than complicated but ineffective aero mods.

5. Cooling is the only thing I can see being benefit from this. But you are better off with oil cooler and radiator upgrade...

1. For the most part I agree. Without a full system, the advantages from just one piece are minimal. Anything I have to say about JUST a vent doing anything significant for drag or lift is just conjecture.

2.

Originally Posted by shirtpants_

I doubt I'll be cutting into my hood anytime soon, but this is just a fun little test I did on Sunday to see where the best place to put this vent would be.

My car is only street driven right now. This test was just something I did because I was interested in the results. My hood is in bad shape and will need a repaint at some point. I am considering adding a vent because there are pretty much no downsides as far as I can tell. A vent like the m4 GTS has is subtle and looks fine I think.

3. That's true. I don't really know which tracks have long enough straights to see significant use of the vent. It should still be relevant to some extent, even to the guy's doing the 1/2 mile and standing mile events.

4. I don't track right now, so you're probably right. Hopefully I can get into it later this year. Thanks for the advice.

5. I don't know how effective this vent would be, compared to just a better radiator. This might be a good way to increase your cooling efficiency at any speed, whether or not you already have an upgraded radiator. Either way, it should* help out keeping things cool to some degree.

Thanks for bringing some reality into this.

[spxxx]

Originally Posted by Cloud9blue

Appreciate the diligence that went into this. But

1. Unless you are running full aero (front splitter, full flat undertray, diffuser, and rear wing), this will not make any difference.

2. You are ruining a perfectly good hood... And you car will look like shit if it is still street driven.

3. There are very few track in US where you can +150mph in our cars (even with FBO or upgraded turbos). Sebring and Daytona are pretty much the only tracks near you where you can hit +120mph consistently.

4. If you want to improve your laptime. Get a data acquisition app or device and see where you can improve. These cars benefit much more from good driving than complicated but ineffective aero mods.

5. Cooling is the only thing I can see being benefit from this. But you are better off with oil cooler and radiator upgrade...

Thunderhill 3mi and Buttonwillow see speeds of 130 in lightly modded N54s but I see your point. Adding mods is just fun, however effective they are.

[Cloud9blue]

Originally Posted by spxxx

Thunderhill 3mi and Buttonwillow see speeds of 130 in lightly modded N54s but I see your point. Adding mods is just fun, however effective they are.

You should really get an GPS tracker for reference.

[tetsuo111]

Thanks for this.

Very refreshing to read an intelligent, data-driven aero discussion. Strong work!

[spxxx]

Originally Posted by Cloud9blue

You should really get an GPS tracker for reference.

I use Harrys which is only 1hz but I've seen speeds of 125+ so that was simply an assumption.

[davidwarren]

Originally Posted by shirtpants_

Yeah, the e90/e82 has a pretty large frontal area. If I remember right, the e92 gt2 even was allowed an increase in power(or decrease in intake air restriction) compared to the other gt2 cars because it's frontal area was so large. At least we get a good amount of room for extra heat exchangers.

I don't know how effective it would be to delete the cowl. The air pressure increases as it gets closer and closer to the windshield, where it builds up. That's partly why the filters for the HVAC or AC or whatever are there. So you'll be trying to push air out from the area with the highest pressure, where BMW designed for air to be forced into the car's cabin through the filters. You also might get some nasty engine smells into the cabin.
That's just my take on removing the cowl. I don't have any data to support it.
Maybe ill do some more testing later this week for the under hood pressures, and see the effects of removing the cowl. Although I wouldn't feel comfortable leaving it off in the rain since I have an open intake.

I agree. Bottom of the windshield is high pressure and deleting the cowl would force air into the engine bay, not neccessarily provide an additional exit.

[Cloud9blue]

Back to the discussion on the hood vents.

Aero probably won't make a huge difference, but...

Having a vent there should dramatically increase the airflow through the radiator, which in M4 GTS has that, same with all the 911 GT3/2 cars. The engine bay airflow setup is pretty piss poor on our cars. All the high pressure air from the grilles through the radiator doesn't really have a place to go.

But I have to say it just won't look right on the smooth exterior non-M or M-sport e9x. You could have the bodyshop fill up the two vents on the side if you want to go with it though. Just FYI, most aftermarket hood has pretty awful fitment from all the pictures I have seen over the years.

You might want to message the OP on this thread, who went with a modded stock hood similar to what you have in mind: http://www.e90post.com/forums/showthread.php?t=1172039

Btw, it would be interesting to see the data from the other side of the hood. Surely you can find a place to cruise at 80-90mph, no?

[tetsuo111]

I bookmarked that page awhile back.

Agree he's on the right path because it would enhance the delta P between radiator inlet and outlet. I respect his effort and documentation of the project. Not to be too nit-picky, but if I move forward with a hood vent I would try to position it immediately behind the radiator. That last vent in your posted thread appears to be near or in the higher pressure region

[justpete]

Originally Posted by Cloud9blue

Back to the discussion on the hood vents.

Aero probably won't make a huge difference, but...

Having a vent there should dramatically increase the airflow through the radiator, which in M4 GTS has that, same with all the 911 GT3/2 cars. The engine bay airflow setup is pretty piss poor on our cars. All the high pressure air from the grilles through the radiator doesn't really have a place to go.

But I have to say it just won't look right on the smooth exterior non-M or M-sport e9x. You could have the bodyshop fill up the two vents on the side if you want to go with it though. Just FYI, most aftermarket hood has pretty awful fitment from all the pictures I have seen over the years.

You might want to message the OP on this thread, who went with a modded stock hood similar to what you have in mind: http://www.e90post.com/forums/showthread.php?t=1172039

Btw, it would be interesting to see the data from the other side of the hood. Surely you can find a place to cruise at 80-90mph, no?

Probably a stupid idea but there's really not a lot of metal ahead of the strut towers which is really obvious when the fender liners are removed. Kinda wondering if removing a section of the liners wouldn't be helpful. I know they can be removed in STU class but probably not a great idea to totally remove them on the street, at least not where the oil cooler is on my build, I think. Will have to look at this as a possibility. OTOH, it just might be a stupid idea. Any comments, ideas, suggestions? No idea if the pressure built up in the wheel well would complicate things.

[shirtpants_]

Originally Posted by Cloud9blue

Back to the discussion on the hood vents.

Aero probably won't make a huge difference, but...

Having a vent there should dramatically increase the airflow through the radiator, which in M4 GTS has that, same with all the 911 GT3/2 cars. The engine bay airflow setup is pretty piss poor on our cars. All the high pressure air from the grilles through the radiator doesn't really have a place to go.

But I have to say it just won't look right on the smooth exterior non-M or M-sport e9x. You could have the bodyshop fill up the two vents on the side if you want to go with it though. Just FYI, most aftermarket hood has pretty awful fitment from all the pictures I have seen over the years.

You might want to message the OP on this thread, who went with a modded stock hood similar to what you have in mind: http://www.e90post.com/forums/showthread.php?t=1172039

Btw, it would be interesting to see the data from the other side of the hood. Surely you can find a place to cruise at 80-90mph, no?

Yeah, I think the m4 gts is going to be the closest thing we have to draw inspiration from. The 911's ducting has a very steep angle from the point of air leaving the radiator, to the hood vent in front, which leads me to believe that the low pressure area is doing a good job of pulling the air out from behind the radiator.

That aftermarket hood is for an e92, not an e90 . I really like the way the center vent was done, but oh well. I was also worried about the fitment. I think modified stock hood is probably the way to go. I'll definitely talk to SeanS54 when I get closer to and/or more serious about venting the hood.

Weel see about some "high" speed testing. Maybe by the beginning of next week ill have some more data to share.

[shirtpants_]

Originally Posted by tetsuo111

I bookmarked that page awhile back.

Agree he's on the right path because it would enhance the delta P between radiator inlet and outlet. I respect his effort and documentation of the project. Not to be too nit-picky, but if I move forward with a hood vent I would try to position it immediately behind the radiator. That last vent in your posted thread appears to be near or in the higher pressure region

We really don't have alot of options on where the vent is placed. There are structural supports running along the front of the hood, I've got to figure out where those are in relation the the #3 pressure zone.

[Cloud9blue]

Originally Posted by shirtpants_

We really don't have alot of options on where the vent is placed. There are structural supports running along the front of the hood, I've got to figure out where those are in relation the the #3 pressure zone.

I would cut where the stickers are. You probably have to mod the air intake (if you are still running stock intake) and some the radiator shrouding, since they would be in the way.

[shirtpants_]

Originally Posted by justpete

Probably a stupid idea but there's really not a lot of metal ahead of the strut towers which is really obvious when the fender liners are removed. Kinda wondering if removing a section of the liners wouldn't be helpful. I know they can be removed in STU class but probably not a great idea to totally remove them on the street, at least not where the oil cooler is on my build, I think. Will have to look at this as a possibility. OTOH, it just might be a stupid idea. Any comments, ideas, suggestions? No idea if the pressure built up in the wheel well would complicate things.

You're suggesting that you let more of the wheel well pressure/air mix with the engine bay air? No idea how that would go.
There should be a significant amount of pressure built up inside the wheel well, I would think even more if you have extra ducting to the rotors. But I don't recall where in the wheel well it is. There are probably different ways to go about relieving this pressure depending on it's location.

Maybe ill check out what the wheel well pressure looks like.

[shirtpants_]

Originally Posted by Cloud9blue

I would cut where the stickers are. You probably have to mod the air intake (if you are still running stock intake) and some the radiator shrouding, since they would be in the way.

Yep, I'd like to go with that section, but I just checked, and the horizontal center reinforcement piece lines up exactly with the space between the stock intake snorkel, and the engine/oil filter housing.
In the attached pictures(not mine), the red on the hood lines up with the red in the engine bay, and the yellow lines up with the yellow when closed.
Dunno how I would feel about cutting that out. Probably would just have to make do with vents right before the reinforcement. Oh well, weel see.

[feuer]

Originally Posted by shirtpants_

Yep, I'd like to go with that section, but I just checked, and the horizontal center reinforcement piece lines up exactly with the space between the stock intake snorkel, and the engine/oil filter housing.
In the attached pictures(not mine), the red on the hood lines up with the red in the engine bay, and the yellow lines up with the yellow when closed.
Dunno how I would feel about cutting that out. Probably would just have to make do with vents right before the reinforcement. Oh well, weel see.

If you go thru the trouble of cutting the hood to open a vent then relocating of the support further towards the windshield would not be a concern. Besides, further away from the front more clearence from the engine cover.

[feuer]

shirtpants_ Gpower M3 RS hood is modified LCI non M hood. Just for inspiration

[justpete]

Originally Posted by shirtpants_

You're suggesting that you let more of the wheel well pressure/air mix with the engine bay air? No idea how that would go.
There should be a significant amount of pressure built up inside the wheel well, I would think even more if you have extra ducting to the rotors. But I don't recall where in the wheel well it is. There are probably different ways to go about relieving this pressure depending on it's location.

Maybe ill check out what the wheel well pressure looks like.

I was thinking given the frontal area and high pressure in front of the radiator that the pressure in the wheel wells would likely not be as high, seems like it couldn't be. Would be great to find out some real world numbers though.

[davidwarren]

Originally Posted by shirtpants_

Yep, I'd like to go with that section, but I just checked, and the horizontal center reinforcement piece lines up exactly with the space between the stock intake snorkel, and the engine/oil filter housing.
In the attached pictures(not mine), the red on the hood lines up with the red in the engine bay, and the yellow lines up with the yellow when closed.
Dunno how I would feel about cutting that out. Probably would just have to make do with vents right before the reinforcement. Oh well, weel see.

Can confirm that support bar crosses over directly above the oil thermostat. I had to, ahem, adjust mine when I added an oil cooler off the top of the thermostat.