Open window vs rain deflector vs venting hatch

[hispanicpanic]

I tried doing a search but couldn't find too much information..... One coast down test posted a long time ago stating that open front windows on a boxy 80's hatch causing 1.5% greater drag at 65 mph, but that surely would change with modern cars.

Car in question is a 91 crx HF. I run with the windows all the way down to keep cool, and am looking to cut the drag as much as possible caused by the parachute effect this creates. The options i have are

1) get rain deflectors, force the air around the window. Probably is not effective in turns or above 50 mph, but probably still better than nothing.

2) remove the rear hatch glass (4" by 24" window above the bumper), and allow any air that comes inside the car to fill the low pressure zone behind the bumper.


I don't think coast down tests would be able to provide the resolution necessary to test some of these items. Perhaps i can fit an engine management system and monitor injector duty cycles and back calculate the fuel required to maintain 80mph with each..... Hmmmm....

Please chime in with any experience or suggestions

[freebeard]

The result, as you say is probably buried in the noise.

I would be weary of opening the cabin into the wake. Carbon Monoxide.

I can imagine a duct on the A-pillar might effectuate an air curtain over the opening. The only thing I've seen like that would be the Mercedes-Benz Air Cap:



coolhunting.com/tech/mercedes-benz-a/

Those plastic deflectors might be a good base, cut slots and cover them with mesh.

[hayden55]

Careful with that rear window. Its fine if its a roll down window like the Sequoia, but it sucks sometimes. Sucks everything into the cabin so dirt, dust, smog, all inside and the dash will have a layer of dirt on it if you don't roll it up sometimes.

[hispanicpanic]

Originally Posted by hayden55

Careful with that rear window. Its fine if its a roll down window like the Sequoia, but it sucks sometimes. Sucks everything into the cabin so dirt, dust, smog, all inside and the dash will have a layer of dirt on it if you don't roll it up sometimes.

Even with both front windows open all the time?? I figured the airflow from the windows would prevent backwash from the low pressure zone in the rear of the car.

[hayden55]

Yup its swirly. The only way to get laminar flow through was with the sun roof cracked and rear window cracked. Need more air from the roof coming in than the rear window can push in and swirl around i think.

[Blacktree]

Not to be rude, but I think you're operating under a false premise. There is no "parachute effect" with the side windows open. Actually, quite the opposite. The air flowing across the window openings causes a pressure drop in the cabin. If you then open the cabin to the rear wake, it will suck in air (and dust, and exhaust fumes) from the rear wake. It will also increase aero drag, because the car is pulling air forward, into the cabin (which means the air is pulling backward on the car).

Edit: Rain deflectors also increase drag. They increase the frontal area of the car. Plus they disturb the airflow down the sides of the car.

[Vman455]

Originally Posted by hispanicpanic

I don't think coast down tests would be able to provide the resolution necessary to test some of these items. Perhaps i can fit an engine management system and monitor injector duty cycles and back calculate the fuel required to maintain 80mph with each..... Hmmmm....

I don't think injector duty cycles are going to give you reliable data for drag comparisons. Why not try something like throttle-stop testing? Your car is old enough that it has a cable-operated rather than electronic throttle. Details in this thread.

Originally Posted by Blacktree

Not to be rude, but I think you're operating under a false premise. There is no "parachute effect" with the side windows open. Actually, quite the opposite. The air flowing across the window openings causes a pressure drop in the cabin. If you then open the cabin to the rear wake, it will suck in air (and dust, and exhaust fumes) from the rear wake. It will also increase aero drag, because the car is pulling air forward, into the cabin (which means the air is pulling backward on the car).

You may very well be right. The original post is an example of intuitive prediction that does not work with fluid flows.

A simple tuft test on my truck showed the wake behind the cab is the same pressure as the air inside the cab with the windows closed:



With windows open, air flows into the cab through the rear window and out the sides:



OP should put tufts on his windows and see which way the air flows with them open.

[freebeard]

Originally Posted by Blacktree

Edit: Rain deflectors also increase drag. They increase the frontal area of the car. Plus they disturb the airflow down the sides of the car.

Might increase drag? An increase of ~1x16 times two, but it's in the wind shadow of the A-pillar. Properly designed they might increase the radius of the curve.

Consider the Subaru SVX.

[Blacktree]

Originally Posted by freebeard

Might increase drag?

They will increase drag when the windows are closed. If the windows are cracked open, air from the cabin will fill the gap. And in that situation, the drag could be reduced, due to the larger radius of the curve. But to see a net benefit from the rain guards, you'd have to have the windows open basically all the time.

[freebeard]

I'm a fan of wind wings and pop-outs, myself.