So as I indicated with my teaser pic the other day I have gotten more interested in hood vents for the cooling and potential handling benefits. I searched extensively on here and pt.com for info and found a lot of people talking about "how" to do the testing but nobody actually sharing their results. I also searched a lot online and found some cool writeups by autospeed.com on tuft testing and pressure testing with a magnehilic gauge.
Tuft testing.
http://www.autospeed.com/cms/article...art-1&A=108656
Pressure testing.
http://www.autospeed.com/cms/article.html?&A=111111
So I tackled this myself and am sharing the results with you guys. This is my attempt at comprehensive hood vent testing on a first gen Camaro cowl hood. Doubt my findings would change between a 68 and 69.
First. I did the wool tuft testing shown in this pic. The taped outlines correspond to the holes in the under-structure of the cowl hood.
Here is the video with speeds up to 70 mph.
Based on the wool tuft testing the pressure wave in the cowl area was not as big as I thought it would be. You can see the last row of tufts on the hood itself are starting to deflect. I also taped two extra tufts on the very edge of the cowl portion of the hood and predictably they plus any of the tufts taped to the cowl panel itself are being sucked right into the cowl opening on the hood which is exactly opposite of what you want to happen for cooling or handling performance on a road course.
You can also see that the raise hump on the cowl hood has the smoothest laminar airflow. Based on this test alone it was clear to me that the best location for a hood vent is the cowl hump immediately after the radiator. The problem is that an OEM styled cowl hood doesn't have much of an opening in this area in the under-structure of the hood. The taped outline of the opening in that area is about half in front of the radiator fan opening and half behind. So you can only use half that space which is a tiny area.
I was feeling inspired after reading the autospeed article on using a magnehelic gauge so I ordered one that measured between 0 and 1.0 inch of water.
In hindsight I should of gotten a gauge that went to 2" or more of water. I pegged the gauge in some of my tests. There are areas of the hood with what appears to be significantly greater than 1" of water pressure differential between under the hood and above the hood on a first gen Camaro cowl hood.
I did two types of tests.
Test 1) the pressure difference between the top of the hood and the interior of the car with the windows cracked an 1" on either side.
Test 2) The pressure difference between the top of the hood and directly opposite underneath the hood.
Here are the readings from
test 1 (hood is lower pressure compared to cabin except for position 3).
Position 1.
Reading at 50mph - fluctuated .3 to .4 inches of water
Reading at 70mph - fluctuated .8 to 1+ inches of water. Average of 1.0" I haven't even started measuring underhood pressure differential and am already maxing the 1" gauge.
The little piece of tape sticking in from the left below the black tubing is where my radiator fan shroud ends.
Position 2.
Reading at 50mph - fluctuated .2 to .4 inches of water. Average 0.3"
Reading at 70mph - fluctuated .4 to .8 inches of water. Average 0.6"
Position 3 (pressures differential reversed and is now higher on hood than cabin).
Reading at 50mph - held fairly steady at .1" I wasn't that interested in this location so I didn't measure at 70mph.
Test 2 with tubing to both sides of the hood.
This is what it looks like in Position 2 (note the openings I've done a tape outline of on the top of the hood).
And the Test 2 measurements.
Position 1.
Reading at 50mph - fluctuated .6 to 1.0 inches of water. Average of 1.0"
Reading at 70mph - Pegged the guage. Didn't even wiggle. Because the gauge was pegged I compared to the underhood pressure to the cabin. The hood was fluctuating between 0.4 - 0.8 stayed mostly between 0.6 and 0.7. Adding the above hood pressure in test 1 to this test would be a differential of 1.6-1.7".
Position 2.
Reading at 50mph - fluctuated .4 to .6 with momentary spikes to .8 inches of water. Average of .5+"
Reading at 70mph - fluctuated .9 to 1+ inches of water. Average of 1.0"
The Magnehelic confirmed the instincts I had from the tuft testing. The center of the hood immediately behind the radiator fan shroud (Position 1) is the best place for a vent and significantly better than vents to the side of the cowl hump farther back. BUT this location benefits best from either cutting the understructure of the hood or a high dollar hood with no understructure. However, the side location (Position 2) would still provide excellent venting with 0.5" pressure differential at 50mph which is significant and requires no cutting of the understructure!
For random comparison I took the pressure readings from the front of the A/C condenser and back of the radiator with the car off and fan on (12v instead of 14v) and the radiator fan was pulling 0.36" of water.
Conclusion TL;DR
Position 1 is best but benefits from modified or no understructure.
Position 2 is a great alternative and doesn't require modifying the understructure.
Regardless of which position block off the cowl hood opening and get a super thick cowl to hood seal!