Cadillacin wrote:I haven't found a whole lot on the subject. So I thought I would bring it up for discussion.
I was wondering if anyone had any kind of wind tunnel testing information or if any of you racing guys have done a string test to see where the high/low pressure areas are and where/how the turbulence forms.

In my research(Ive found very few results) I've found the Cavaliers and Sunfires Cd(coefficient drag) testing out at .34-.38 (one site claims .32 on a 95 sunfire) I also came across one site that claims they did a side by side of the 2, then a sunfire/cavalier combo(sunfire front end, cavalier rear/spoiler) which turned out to have the best Cd. rating in the 3 tests. But thats neither here nor there.

I'm curious if anyone knows where to find or has credible information on the Jbodys aerodynamics and Cd.

Just for discussion, any theories(radical or reasonable) on how to improve it? How about a squared rear? What would happen if you deleted the trunk and shave down the quarter panels. Maybe move the bumper in a few inches for more of a 911 fastback kinda shape?(I know its not that easy lol) ...But... what if?

Do you think a Ram Air hood would raise its efficiency, or lower it?(I would think it would lower it due to the air from the engine compartment needing to be directed out under the car) But idk? lol opinions?

I understand for the most part none of this matters under 55MPH or so lol Just shootin the s#!%

Wow...great question...
Thanks for researching and at least coming up with the Cd.
I'm not an aerodynamics, but I do have about 7000 hours as a professional pilot, so I'll just make a few comments.

A yarn-tufted "string test" will reveal the direction the air is flowing, but nothing about how well it is flowing. To do that requires pressure sensors. The "easiest" way (and probably the Least expensive way would be to buy a seat of SolidWorks and the CFD add-on. I used to sell it. Awesome stuff but about 15k. Wind tunnel time is hard to come by and very expensive too.

Your question about a "squared" rear end, if I understand it, is describing a "kammback". http://en.wikipedia.org/wiki/Kammback
They were quite popular in the late 70's when the "oil crisis" made us lose interest in HP and shifted our thinking to MPG. If you look on youtube, you'll see many drag cars that have an extended surface from the trunk to a foot or two straight back. They are probably aiming for some down force, but I think they are also looking for decreased drag and increased stability.
You also mentioned what sounds like cutting off some of the back of the car. I'm only speculating here but I would be willing to bet that would INCREASE drag. I can't remember who (Bob Glidden maybe), but in the 70's he used a Ford Maverick as a pro stock drag car. They actually found that the longer 4 door version had lower drag. IIRC, it was because the longer car had less pronounced changes in shape for a given length. It would be nice if I could recall the source. It was a hot rod type article.

Most drag is found under the car. If you look at the bottom of most cars you'll see a very uneven surface with exhaust, suspension, oil pans, etc. For the most part the underside of a car is an afterthought. My opinion is that if it doesn't scrape, the manufacturers don't see a reason to change it. Google "underside of a prius" and there is a pic of a rolled prius....even it has obvious air flow obstacles.

This may be of interest to you. http://en.wikipedia.org/wiki/Chaparral_Cars especially the 2J.

The second place where drag is found is the radiator. Generally, the amount of air flowing through the radiator and engine compartment is more than what is really needed. Most of the cooling air for the radiator is actually flowing in from under the bumper, not through the grill. So it has to make a series of turns and find it's way through the radiator and into the engine compartment, where it then flows down under the car. Not exactly low drag.

You are somewhat correct about 55mph not having much drag, but remember that drag increase at the square of the speed. 55mph has 4 times the drag of 27mph, and 110mph has 4 times the drag of 55mph.

A ram air scoop is of little benefit until you are approaching 150mph. http://www.wallaceracing.com/ram-air-calc.php
Until then it is just something else in the way of the air smoothly flowing over the car. A GM style cowl induction hood is probably more effective. It is designed to be low profile and draws air from the base of the windshield where there is a natural high pressure area created by the rapid change in direction of flow. The sweet spot for it is IMO, the area of the center 1/3 of the windshield up to about 6-8 " high. Bear in mind I'm trying to remember things I learned quite a while ago.

Hope this is helpful.
dennis

Lots of discussion about aerodynamics at http://ecomodder.com/forum/ . Scroll to Aerodynamics forum. All about aerodynamics for better fuel mileage, but aerodynamics is aerodynamics.

Wow thanks for the info guys, this is all helping me learn a lot.
As far as the Kammback goes, thats alittle more exaggerated that I had though. Its my understanding on our cars we experience more drag from the air coming off of the trunk. Since our trunk is round(and a round extended bumper doesn't help), the air coming off of it is prone to more 'violent' turbulence that would create more drag, as opposed to say a mid-late90s Toyota Corolla. The Cd of that car is .30-.31 and if you look at the side view it isn't 'shaped' better than our car overall. Size doesn't have anything to do with it because(for those who don't know) the Cd is calculated based on the vehicles Frontal area(So a 40" wide car and a 70" wide car if shaped properly can have identical Cds.)

But look at SIDE VIEWS of the trunks. It drops straight off the back of the car. All of the most fuel efficient cars do this. Trunk drops straight down to a bumper that only sicks out a couple inches as opposed to our 4-5 inches. If you look at side by size pictures of the Sunfire/Cavalier vs a 90s Toyota Corolla you'll be confused why their rating is so much better because our car closer resembles the desired reverse teardrop shape(dennis im sure as a pilot, you know this is roughly the shape of a plane wing only upside down to create downforce rather than lift) Other cars you can compare our better shape, yet their better Cd are.
99Corolla, 99Jetta, 99Maxima(bumper sticks out a little more), 99Civic, 99Neon just to name a few.
I know this isn't the only contributing factor, but it might be something addressable.

Another big one is as Z Yaaa said, lowering the car. Our 'stance' or rake I think its called. is angled forward / much more than most cars in our class. Almost all cars come angled 1- a few degrees from the factory but our cars are unnecessarily high in the back. Which if you think about it changes the shape of the object going through the air. Its like when you put your hand out the window horizontally and it cuts through the wind the more you raise the rear part of your hand the more drag it creates. The token old time racer in our town told me that a cars best stance is when the front control arms are level with the ground(equal height in the front and back)

The problem with doing this in our cars(according to him after looking at it from below on his lift) is the underbody where the trunk and rear bumper is, is going to act like a parachute, actually creating drag countering the effects of lowering(fuel economy wise). He said most people won't notice this when they lower their cars because they aren't doing it for fuel economy, and performance people often overlook this until they address the entire underside(whether it be a complete cover, or partial/sectional). He said most of the older cars he worked on had the gas tanks back there, and if I leveled the car, I would have to cover that area for maximum efficiency.

I'm diggin this ecomodder site. im going to bury myself on their now too >.<