The Free Mod Sieres: Part VI Acing Aerodynamics

[Toysrme]

I'll keep this very short, as nothing long is needed. userlain you are totally incorrect.

The reason the front of the car lifts, is because of the high speed air flowing across the hood. The higher the velocity, the lower the pressure.

Ask yourself why is this air moving at such a high velocity? Because it achives laminar flow accross the hood... There is no turbulance to slow it down.


What is underneath the car??? Absilutely nothing but large crevases, flat sides sticking into airflow. The few surfaces flat, have extensive texturing... This signifigantly reduces the velocity of the air.



What does this mean? This means the differance in velocity between under car, and above the car grows even LARGER, creating MORE lift. The undertray above gives the airflow a large, smoothe surface area to achive laminar flow on.

[woody]

the idea about the dimples would work if the dimples were the size of the dimples on a golf ball. Golf balls fly so well because of those dimples. They create a zone next to the body where a layer of air sits and reduces drag for the air flowing under the car, Mostly it maintains the laminar flow over the larger area of a curve, reducing the turbulent area on a golf ball.

Whether it improves a flat surface under a car would be a matter of testing.

If you were to provide vents in the hood that were to draw the air out of the engine bay over the roof, you could sheet in the whole engine bay, greatly reducing the lift that you see since none of the air going through the radiator would be going under the car.

[Toysrme]

That always comes up. Wouldn't work in this application. You loose that much more laminar. The second idea is the the (first) part IV cooling. Remove the rubber seal around the abck of the bay, and shim the hood up. You would never be able to seal the entire under engine bay and keep a sleek upper profile. Just not enough path for the air to flow out.

[whocares04]

ok, can wait for more FMS

[woody]

I like that idea of the hood shim, I can see that working ok. The only problem I see is that the base of the window is a high pressure area and most muscle cars used this, and shimming their hoods, to draw air in from this area into the engine bay.

This is why most race cars run laminar vents out of the engine bay about the middle of the hood.

edit: The muscle cars never had the bottom of the engine cover though and they had lots of ground clearance too, so...

[Typrus]

I know its a long-not-discussed topic.
BUT!
Consider that a gentleman did a full bellypan on his T100 pickup. In combination with his other mods, he dropped the CD to something like .029? Right around a Subaru SVX. In a truck.

What you CAN do to aid in extraction of cooling air for the engine is create a full-length pan with louvres in it in key locations. Make sure there are enough as well.
How is this helpful?
You will create a venturi effect. For those who do not know what this is...
When you pass high speed air over an opening it will create a suction. This is do to the pressure zone differences discussed earlier. This will pull on the opening. Carburetors operate off this principle.
As the high-flow air passes under the body, it will hit the louvre's, causing it to rise up and gain even more velocity (like in an airplane wing) and create suction on the backside of the louvre, or the opening. Effectively "vacuuming" the air out.
The louvres would need to be in places where air could be pulled through to cool the exhaust (particularly around the cats).
It is possible to run a full bellypan with the escape vent being in the rear of the vehicle. There is also a low-pressure zone here, which would aid in pulling the air through, however if this was your only vent you'd run into the issue of turbulence between the floor, exhaust, axles, and pan. How much? Is it significant? I don't know. Would it be wise to louvre just under and behind the engine, then have an outlet in the rear? Not sure.

Part of the consideration is ease of removal (access to things like suspension, exhaust, etc) and removal of heat. The exhaust will heat this air-cushion. If the flow is not high enough, particularly in the vicinity of the cat, you risk melting your carpet, your undercoating, and maybe even overheating the fuel and brake lines. There is a thought that you could make it such that the exhaust exits down and through the bellypan fairly far forward, but the issue would be that at a stop or low speeds, fumes could still enter the cabin. There is also the thought of a full-exhaust ceramic coating. Cost of that? No idea. Could the cat be safely coated? No idea.

Benefits? Well, obviously aerodynamics. Thus fuel economy increase at a given speed, more noticeable above 50mph. More response at higher speeds, and of course a higher top speed. If engineered well, potential also for creation of downforce. Thus an increase in handling at 60+mph. Road noise could also potentially be reduced inside the cabin.


One thought would be to create in segments. One for under the engine, one for the middle of the body, and one for the rear. Have the section in front of a given be lifted (lowered?) slightly above the next, leaving an air gap. Viola. Louvre. With the added bonus of easier removal for maintenence.

[1slick92]

What all did you have to swap out and how about the lights did you run into any problems there? Thanks