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
