Undercar Aerodynamics Question!
Thread Starter
Safety Car
Joined: Nov 2005
Posts: 3,961
Likes: 573
From: Lompoc, CA. Santa Barbara County
Undercar Aerodynamics Question!
A friend of mine said that the inside of my car looks like a goat exploded in there. So I'm ripping out the old carpet up front, and like the back I did a couple of years ago, I'm covering it with fatmat and felt before I lay the new carpet in. Then I discovered this.
About 20+ years ago, when I installed the 700R4, the driveshaft u-joint was hitting the trans tunnel side, so I had to cut out a little hole. I covered it with duct tape and carpet and forgot about it. Of course yesterday I covered it with an aluminum plate and rivited it in, and glued over it with heat barrior and felt.
But the other morning I drove the car to work with the hole uncovered. Holy Moly, the air flow from that hole was tremendous. At 60-70 mph, putting your hand over this hole was like sticking your hand out the window. It was a cool morning but the air comming thru the hole was very warm. So obviously it had come thru the radiator. So I'm wondering, how does air come thru the radiator, around the engine, and into the trans tunnel, with so much force and velocity. The tunnel does start wide and get narower, but man, it was like an air dryer on high. Maybe the rotating u-joint acts like a propeller? Any ideas?
Bee Jay
About 20+ years ago, when I installed the 700R4, the driveshaft u-joint was hitting the trans tunnel side, so I had to cut out a little hole. I covered it with duct tape and carpet and forgot about it. Of course yesterday I covered it with an aluminum plate and rivited it in, and glued over it with heat barrior and felt.
But the other morning I drove the car to work with the hole uncovered. Holy Moly, the air flow from that hole was tremendous. At 60-70 mph, putting your hand over this hole was like sticking your hand out the window. It was a cool morning but the air comming thru the hole was very warm. So obviously it had come thru the radiator. So I'm wondering, how does air come thru the radiator, around the engine, and into the trans tunnel, with so much force and velocity. The tunnel does start wide and get narower, but man, it was like an air dryer on high. Maybe the rotating u-joint acts like a propeller? Any ideas?
Bee Jay
Safety Car
Joined: Aug 2001
Posts: 4,883
Likes: 11
From: North Easton Mass
C3's were designed in a styling studio rather than in a wind tunnel so there propably wasn't much thought given to airflow under the car. The trans tunnel is sort of shaped like a backwards NACA duct so maybe that has something to do with the high velocity air. You were probably getting some fan effect from the rotating U-joint also.
Rick B.
Rick B.
Thread Starter
Safety Car
Joined: Nov 2005
Posts: 3,961
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From: Lompoc, CA. Santa Barbara County
Bee Jay
Drifting
Joined: Dec 1999
Posts: 1,462
Likes: 1
From: Reno nevada
Lots of air flow through the rad which creates pressure under the car. I had a torn shifting boot and it would feel like a blast furnace in the summer. The stock hole throught the tunnel was way too big. Like your self made hole!
Race Director
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From: Florida
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From: Saginaw Mi
Thread Starter
Safety Car
Joined: Nov 2005
Posts: 3,961
Likes: 573
From: Lompoc, CA. Santa Barbara County
Bee Jay
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Haven't opened an AIM in years, but I remember some of the early cars having them. My 75 still has the foam on the frame rails, but no tunnel foam.
I'm sure these were one of the first pieces to disappear, like the starter heat shields, rear braces etc etc.
I'm sure these were one of the first pieces to disappear, like the starter heat shields, rear braces etc etc.
Race Director
Joined: Aug 2007
Posts: 10,977
Likes: 19
From: Canboro Ontario
My 73 had the tunnel foam, it is in the AIM, sometimes because of wind and wear, they just blow away.
Burning Brakes
Joined: Oct 2008
Posts: 955
Likes: 50
From: Jersey Shore
Bee Jay- Check out these links here... Some interesting reading-
http://www.autospeed.com/A_2159/cms/article.html
http://www.autospeed.com/A_2160/cms/article.html
http://www.autospeed.com/cms/A_2162/article.html
And more aero links here:
http://www.pro-touring.com/forum/showthread.php?t=61936
PS- did you happen to get the weight of all the fatmat you put down? Ive been hesitant to put some under my carpet because of the weight. I must have put over 20lbs of it in my fathers car (55 chevy= lots of floor pan to cover). It sure makes a huge difference in road noise, but Im trying to stay in the sub-3000lb club.
http://www.autospeed.com/A_2159/cms/article.html
http://www.autospeed.com/A_2160/cms/article.html
http://www.autospeed.com/cms/A_2162/article.html
And more aero links here:
http://www.pro-touring.com/forum/showthread.php?t=61936
PS- did you happen to get the weight of all the fatmat you put down? Ive been hesitant to put some under my carpet because of the weight. I must have put over 20lbs of it in my fathers car (55 chevy= lots of floor pan to cover). It sure makes a huge difference in road noise, but Im trying to stay in the sub-3000lb club.
Last edited by vette427-sbc; May 8, 2010 at 05:18 PM.
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I just stuffed some pipe foam / insulation in there when I swapped my tranny, my lower shifter boot was ripped and I never noticed anything, I just recently put a new shifter boot in there and there is no noticeable difference. 
Why not rivet some sheetmetal under the car and make it flat under there like racecars ?
Why not rivet some sheetmetal under the car and make it flat under there like racecars ?
Last edited by MotorHead; May 9, 2010 at 11:32 AM.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Agree with what 72LS1Vette is saying that the tunnel's profile is probably causing somewhat of a venturi effect.
IIRC that foam piece was deleted circa '76, but one is going in my '78 when I get it put back together.
Ran for a while without the lower shifter boot after converting from auto to manual, and IMHO there was a lot of hot air draft into the cockpit.
Got a few mods planned for addressing undercar air, including my own version of a splitter similar to yours (Bee Jay), functional fender well vents above the front tires, very possibly enlarging the engine compartment side vents, and more, but really want to take pressure readings first (have the instruments
) so that I'll have before and after data. Not so sure I wish to cut up my hood to open up vents, but shy of hard data on the C3 I've seen indications from other vehicles that just aft of the rad, rather than near the base of the windshield, is probably the best location for such.
IIRC that foam piece was deleted circa '76, but one is going in my '78 when I get it put back together.
Ran for a while without the lower shifter boot after converting from auto to manual, and IMHO there was a lot of hot air draft into the cockpit.
Got a few mods planned for addressing undercar air, including my own version of a splitter similar to yours (Bee Jay), functional fender well vents above the front tires, very possibly enlarging the engine compartment side vents, and more, but really want to take pressure readings first (have the instruments
) so that I'll have before and after data. Not so sure I wish to cut up my hood to open up vents, but shy of hard data on the C3 I've seen indications from other vehicles that just aft of the rad, rather than near the base of the windshield, is probably the best location for such.
Last edited by TheSkunkWorks; May 8, 2010 at 09:56 PM.
Racer
Joined: Sep 2006
Posts: 251
Likes: 0
From: Victoria Australia
I guess the Trans Tunnel takes the shape of a Convergent Duct and according to bernoulli's theorem...Air Velocity increases as it goes through a convergent duct...maybe thats why the force felt so great....it probably was !!!
Thread Starter
Safety Car
Joined: Nov 2005
Posts: 3,961
Likes: 573
From: Lompoc, CA. Santa Barbara County
Bee Jay- Check out these links here... Some interesting reading-
http://www.autospeed.com/A_2159/cms/article.html
http://www.autospeed.com/A_2160/cms/article.html
http://www.autospeed.com/cms/A_2162/article.html
And more aero links here:
http://www.pro-touring.com/forum/showthread.php?t=61936
PS- did you happen to get the weight of all the fatmat you put down? Ive been hesitant to put some under my carpet because of the weight. I must have put over 20lbs of it in my fathers car (55 chevy= lots of floor pan to cover). It sure makes a huge difference in road noise, but Im trying to stay in the sub-3000lb club.
http://www.autospeed.com/A_2159/cms/article.html
http://www.autospeed.com/A_2160/cms/article.html
http://www.autospeed.com/cms/A_2162/article.html
And more aero links here:
http://www.pro-touring.com/forum/showthread.php?t=61936
PS- did you happen to get the weight of all the fatmat you put down? Ive been hesitant to put some under my carpet because of the weight. I must have put over 20lbs of it in my fathers car (55 chevy= lots of floor pan to cover). It sure makes a huge difference in road noise, but Im trying to stay in the sub-3000lb club.
Bee Jay
