Tom1200
Tom1200 UberDork
1/18/23 2:29 p.m.

So I am playing with "enths" again.

On my Datsun 1200 the cowl vents are completely open. Because I'm not an aerodynamicist I bounce two theories around in my empty head.

1. The vents open relieve the high pressure area at the base of the windscreen but...................the open vents and the grills cause the air to have to make a 90 degree turn and that can't be good. 

2. Sealing the holes may cut drag enough to matter........or not.

My trap speed at most tracks is not more than 95 mph. My corner exit speed onto most straights is about 55-75mph. 

Note I'm also aware that having the vents open inside that hot ass car may help the driver towards the end of a race.

 

matthewmcl
matthewmcl Dork
1/18/23 2:40 p.m.

When qualifying, some race teams will block off regular cooling vents to improve airflow. Even though air is moving through a path of least resistance, it can still have more of an energy change than it would have just going around the car. Convoluted path = more drag than simple smooth path 

That being said, I would expect the increase in driver performance would outweigh the increase in drag.

Now to see if my comments match real expertise that follows

Jesse Ransom
Jesse Ransom UltimaDork
1/18/23 2:44 p.m.

How much air is really coming in those vents, and what happens to it after that?

Unless you can semi-efficiently vent it back out into the low-pressure area behind the car, I doubt it's going to make that much difference.

I am not an aerodynamicist, and don't have the experience to have ingrained patterns and rules of thumb guiding my dumb guess.

I suppose the other question is whether the pile of air on top of the vents at the base of the windshield helps guide the air coming after upward more gently. That is, do you get a rolling pile of air that acts as a fake windshield at a more rakish angle, and would venting that away actually cause more of the oncoming airstream to actually hit the full brick wall of the windshield itself?

I'm probably wrong about everything, but would love to hear a more-educated take.

Tom1200
Tom1200 UberDork
1/18/23 2:54 p.m.

In reply to Jesse Ransom :

The vent is open to the interior...............all of the hoses that would route it through the HVAC are gone so the air spills out into the under dash area.

Jesse Ransom
Jesse Ransom UltimaDork
1/18/23 2:56 p.m.

In reply to Tom1200 :

Yes, but where from there? The interior isn't an infinite "air sink."

sleepyhead the buffalo
sleepyhead the buffalo PowerDork
1/18/23 3:12 p.m.

I suspect closing them off will be of a minor benefit; although, it's probably "in the noise" of lap time data.

however
it's like 5ct worth of blue/black/etc tape to test "sealed" vs. "un-sealed"... plus 1 session of track time.

GameboyRMH
GameboyRMH MegaDork
1/18/23 3:21 p.m.

In reply to Jesse Ransom :

Interior air would mostly come out the side windows if they're rolled down as you'd expect on track, otherwise it would likely escape through hidden body vents in the trunk.

Colin Wood
Colin Wood Associate Editor
1/18/23 3:26 p.m.

I'd think that every car would likely behave differently, but we discovered that adding hood vents alone to our LS-swapped 350Z did reduce drag by 10 pounds, but also reduced front downforce by 41 pounds.

[Placing our virtual Nissan 350Z Inside a virtual wind tunnel | Against the wind: Part 2]

stafford1500
stafford1500 Dork
1/18/23 3:34 p.m.
Colin Wood said:

I'd think that every car would likely behave differently...

This is the real answer.

Try the tape on and off the cowl vent for a session.

Driver cooling will likely be more important and if you find no difference open vs closed, add hoses to blow toward you from under the dash.

Signed by the guy that just spent the entire day in the wind tunnel chasing really small results.

Driven5
Driven5 UberDork
1/18/23 3:36 p.m.
GameboyRMH said:

In reply to Jesse Ransom :

Interior air would mostly come out the side windows if they're rolled down as you'd expect on track...

Which I would also intuitively expect to technically reduce drag.

stafford1500
stafford1500 Dork
1/18/23 3:42 p.m.
Driven5 said:
GameboyRMH said:

In reply to Jesse Ransom :

Interior air would mostly come out the side windows if they're rolled down as you'd expect on track...

Which I would also intuitively expect to technically reduce drag.

You would be very suprised by what the air does and where it goes. Open side windows do not automatically equal outflow. They generally include both inflow and outflow, and not in a simple front to back or top to bottom arrangement. It gets even more complicated if the drivers side and passenger side are not the same or the car is experiencing some yaw, as in a turn (slip angle).

Trust me, the simple answer is to try open vs closed for any car in question, on track. As sleepyhead said, ti is in the noise level for most competition vehicles.

Drag is a function of how much work is done to the air. The more streamlined the better. That includes reducing the amount of air that has to be moved sideways, like in or out of a window.

frenchyd
frenchyd MegaDork
1/18/23 3:53 p.m.

In reply to matthewmcl :

To really know ( everything I read is speculation). You need to do the yarn test.  See where the air is going.  If it's pressurizing the interior enough it's like driving with the side windows closed. If it's not enough and air is turbulent past the windows  tape up the vent ( or close it if it's got a flap to cover it). And then see what is happening to air.  
 If it's still turbulent put a plexiglass ( polycarbonate) window in place but make it flush with the metal.  Most windows aren't flush with the surface.  I use a piece of wood approximately the depth of the surface of the metal minus the thickness of the plexiglass. 
    My goal is a smooth air flow past the window. Not air that  curls into the inside of the car.  
You can achieve pretty close to the same effect in careful placement of your window net.    
     Going back to the cowl vent. If air is still going in, give it a way out that will help reduce drag.   Put a vent in the area directly behind the rear window. ( or a series of holes etc.). What you are doing is back filling the rear window effectively adding slope and thus reducing turbulence and drag. 

Tom1200
Tom1200 UberDork
1/18/23 4:25 p.m.

The car has no side windows because, racecar.

Also note as a 51 year old car the Cd is .42....................not so great.

Next event I will try taping the vents over. I also intend to play with removing the side mirrors.......aero calculator says it's good for nearly 1 mph. 

sleepyhead the buffalo
sleepyhead the buffalo PowerDork
1/18/23 6:03 p.m.
Tom1200 said:

Also note as a 51 year old car the Cd is .42....................not so great.

well, you probably need to drive it backwards, then...

 

L5wolvesf
L5wolvesf Dork
1/18/23 6:37 p.m.
frenchyd said:

In reply to matthewmcl :

 You need to do the yarn test.  See where the air is going.  

THIS - it is simple and inexpensive.

frenchyd
frenchyd MegaDork
1/18/23 10:51 p.m.
Tom1200 said:

The car has no side windows because, racecar.

Also note as a 51 year old car the Cd is .42....................not so great.

Next event I will try taping the vents over. I also intend to play with removing the side mirrors.......aero calculator says it's good for nearly 1 mph. 
 

NASCAR  uses poly on passenger side and window net on drivers side. Pretty sure the last TransAm cars I saw did the same. 
   My Vintage  race car ( Jag XJS ) will have windows and window nets. 
 If it's too hot you need a cool suit.  

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