JPhil

I'd like to present a topic for discussion concerning aerodynamic airflow and use of pressure differentials. The three main areas of thought are concerning engine induction, engine bay ventilation, and high speed uplift/downforces.

The C3 is not actually very aerodynamically clean. As it passes thru the air, the pressure wave it creates is shaped like a drop of water, with the blunt end at the front. Inside the perimeter of this area is low pressure, outside of it higher or atmospheric pressure, and where it intersects a body is a much higher pressure with surrounding turbulence.
The front end of the car is the point of highest pressure, the base of the windshield (a vertical surface as far as we are concerned here) would be an area of high pressure, and the underside of the car is an area of both high pressure and high turbulance, due to the initial pressure wave being forced under the car between the underbody and the road combined with all the obstructions and interferences under there. (That's why high-speed cars use smooth belly pans)
The points of lowest pressure are within the widest part of the teardrop, typically the first few feet of the car both on the hood and the sides of the fenders (For our flat window coupes, the area directly behind the cabin is also--that's why the cabin vents are there).
You can see why cowl induction, or front grill induction, works well--we're taking advantage of the high pressure to force more air into the carb.
The side gills use the low pressure area on the side to draw air from the engine bay.
The front air dam reduces the amount of air forced under the car to reduce the high pressure there and thus reduce uplift and turbulence, which equals drag.
OK, so now let's put this to use...
A) Cowl induction thru a plenum to the carb--'nuff said on that.
B) Vents in the front section of the hood as far forward as possible, ducted back to the engine bay, plus increased size of side gill vents, to utilize the low pressure areas to draw pressured up air out. This reduces the amount of air being forced under the car to escape the engine bay thus reducing the high pressure underneath, and the top vents would be introducing air into that low pressure area thus also reducing the uplift on the top surface of the car.
Of course, ground effects body mods would enhance all this, but I'm not going to get into any of that. What I'm driving at here is this: My side gills ('73 style) have about 12 sq. inches each of area. I could open that up to easily get 48" of area. Using an L-88 or a higher rise cowl induction hood with ducted louvers or vents (with a drain tray built in) I could get another 20" or 30" or more of vent area for the engine bay.
I know the speeds of reality on the street do not worry about the uplifts I was talking about, but it could help on that 150 MPH UK run or the occaisional bout of "spirited driving." The main benefits would be in heat reduction of the engine bay, which is no small complaint for many of us.......

Thoughts?

John

xcelciador

I am thinking about extending the stock front spoiler and reinforcing it. Right now I have some rubber riveted on it. Does this help what your trying to say.

lvrpool32

My front spoiler is a copy of the ones they used on the Heinz and Johnson #4 L88 race car.........it extends underneath to the cross member........I have noticed a more nose down stance at speed (no front end lift) and better cooling...since this picture I have added aluminum ducts left and right for the brakes...I guess you could also use them as some type of ram air system.

Nick

CGGorman

I think you are right on target. Sounds like you have it well thought out, and I can't add anything productive at this point.

Bee Jay

Look closely at the L-88/big block hood. It was styled to simulate a radiatior extractor at the front, and an air induction scoop at the rear. Chevy only used the air induction. Also, the 1963 Corvette, even more notorius than our C3s for front end lift, had stlyish non-functional vents on the hood. The split window made it to the real car, but not functional radiator vents. I plan on putting some type of vent on my hood. I'm looking for Z-28 or Trans-Am side fender vents. I think they would work great on top of my hood. I'm also looking for a spare hood to experiment with, preferably an L88 style. Check out the new Ferrari 599, it has a functional hood vent that looks really good. Also, I think I can make my side vents functional by moving the charcoal canister and AC Evaporator out of the way. Isn't the front wheel well another high pressure area. The side vents should help vent that also, if I remove or vent the fender liner. Anyone have a wind tunnel we can borrow. If not, I'll tape yarn tufts all over the car, and video them at speed.

Bee Jay

I just saw this on another thread posted by 1969Vette383. Great minds think alike.

I have a question regarding the L88 hood, but it's not about the rear cowl opening. My question is about the first "valley" in the hood.

From the front, the hood starts rising. After what, one and a half, maybe two feet of travel, it has a vertical drop and starts rising again all the way to the cowl, right?

What would the positives and negatives be in regard to cutting out the portion of the hood that makes up the vertical drop?

One negative I can think of immediately would be that the hood's structural integrity would weaken. What about just opening up the area immediately to the right and left of center, leaving the sides alone?

The valley looks to be far enough forward that driving in the rain should be possible, and it looks as if it could be used as an extraction point for air under the hood.

If I am correct in this aerodynamic principle, it is true that air passing over an opening draws air out from under that opening, yes? Is this opening in such a location that air passing through the radiator can be drawn out from under the hood?

Did the Le Mans cars punch this piece out? I know the Cobra Daytona Coupe had a huge "hole" in the front of its hood, in much the same design, meant for air extraction.

I'm more curious than anything as I am looking into buying a long style L88 hood sometime in the future. Thanks to all who chime in.

JPhil

Actually the air dam and related ground effects body work is a bit seperate from what I'm talking about. They reduce the drag, turbulence and uplift forces by redirecting the air around the car, a very important aspect of good high speed handling. What I'm getting at is evacuating the built up air in the engine bay.
In the world I live in, a sexy fiberglas air dam 2" off the ground would be ripped off before I got to work....I do have an idea for an extension of the OEM one using truck mudflap rubber so it could fold....

Bee Jay-
Yes! "Air Extractor Vents" is what I'm talking about. I've seen some really interesting one on race cars which got me to thinking on this.
Yeah, the rear of the wheel wells is another area which could use venting, but I would think you would want a dedicated vent, ala a high mounted gill vent, to evacuate that, rather than blowing all that turbulence--and dirt, etc.--into the engine bay.....
Just thinking out loud here.
Ha ha, yeah I've thought about the yarn tuft idea, too! Maybe we oughta get together on that....

John

my72vette454

I looked at a few hoods and decided to use this daytona hood because it gave me room to run the intake tubing to the carb and also because it had vent holes to get rid of hot air from under the hood. I did have to cut the center section apart and raise it one inch to clear the carb hat. I built a cowl induction setup for the twin air filters to get fresh air instead of hot air from in the engine compartment. I have heard how there is a high pressure area at the windshield but it was proved to me one day, it started raining lightly and the rain drops on either side of the windshield were running up the glass (had rainex on the glass) and in the center area the drops were running down the windshield. Looked kinda cool to see the drops moving like that!

Summerfun

First off, I don't know anything about this topic.....

Let's use some logic and stuff we've read everywhere.

As I understand it airplanes fly because air flows a longer distance over the top and rounded part of the wing than the bottom and straight part of the wing......thus creating lift as more air passes under the wing to reach the point at the back of the wing where the top and bottom air meets. More pressure from the bottom or vacuum from the top, depends on which way you look at it.

Now take a C3. First off I don't know at what height off the ground you will start creating airflow underneath the car as the car moves forward.
Logic says that the lower you are to the ground the less airflow underneath the car you will have when the car moves forward.

Why would there be positive pressure inside the engine bay if it is a "static" environment that moves with the car.....unless the belts and heat of the engine increases it a lot. I don't know.

Will a supercharger work better than all your vents to "suck" the air out of the engine bay into the engine to be blown out the exhaust, thus reducing the engine compartment pressure?

What about a ram air hood (stinger style). Someone said that as air flows over a open area it will suck air out of the opening. Does this effect make a ram air hood useless at speed?

If you have a spoiler that redirects whatever air is close to the ground in front of the car around the sides of the car, would that increase turbulence around the sides of the car? On the other hand reducing airflow under the car and thus reducing lift. What is the effect of this turbulence around the sides of the car.

I know one thing, no two in fact.......
1)My spoiler made my car more stable at high speeds
2)I ripped out the rear window and installed a removable rear window.
Taking that window out when cruising reduces turbulence inside the cockpit significantly.

There is a lot to say about this subject.....I wish the gurus will chime in and chat away.

....just trying to torture someone into saying something good

ML67

This is an interesting topic. I know a clever engineer and friend who used tufts of yarn to test hood scoop design and placement effectiveness. Somewhat surprisingly, he found the tufts of yarn placed at the base of the windshield stood proud at speed, suggesting stagnant, low pressure air. Now to be fair, his testing was performed on a 57 Chevy which obviously has very different aerodynamics than either a C2 or C3 Corvette. He settled on a forward facing hood scoop to feed his healthy 540.

I run an L88 cold air hood on my C2, and although have never performed any testing on mine, I’ve read w/ interest the (temperature) testing posted in these forums. I also recall reading some early GM test results of the L88 induction system where 0 – 150 mph runs were done w/ and w/o cowl induction. Although I no longer have access to this report, the L88 cold air system was reported to be better than 10 sec quicker to 150.

Has anyone performed similar acceleration runs comparing a standard BB hood compared to a true L88 hood? What were your results?

Mark

BBShark

This is a topic that I am interested in but know very little about. Most of the air flow at speed information I have seen is for racecars at high speed. A street car probably averages 25mph. I think what would be great for a C3 is a low speed scavenging effect similar to the side gills but on top of the hood.

I have a stock BB hood on my car with the fake vents. First of all, I hate fake ornamentation on cars but I think there may be some opportunity to vent the underhood right behind the radiator using these "vents". These vents would be much larger in order to be effective but they take advantage of the low pressure envelope at the front of the car and the high pressure under hood. Perhaps supplemented by an air dam or barge board at the front of the crossmember?

A large scale version of this would be the C5R racecar. There is a tunnel on the hood that dives down in front of the engine that exposes the back of the radiator.

noonie

Concerning the OP's idea of venting the front of the hood
The original ford gt40's had a unique hood design that allowed airflow for cooling, helping keep the nose down and creating low pressure under the car. Their radiator's leaned forward instead of rearward. You can't dispute their top speed or winnings at Lemans in the 60's
Wonder if it would work on a shark?

From a side view, it doen't look any more areodynamic than a shark.

Another interesting note. I had a 86 325 bmw back then and even though it had a very boxy design with only a small chin spoiler, the faster you went the lower the front sat and better it hugged the road. I followed a Mercedes once across alligator alley with the cruise set at 135.
Never did understand why such a boxy car handled so well.

C3's should have a lot of potential.

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mod81

holy crap it's the summer before my SR. year of college...somebody give me the cliffs notes when this is all over with, my brain is turned off untill august 18th, when i have to go back

rihwoods

Check out the early 63's Grand Sport Corvettes(Racers)...hood vents were added forward on the hood to reduce engine compartment air pressure...in fact,there were two vents..side by side....

Edit: and read thru this:

http://forums.corvetteforum.com/prin...=571930&pp=100

Rich

Graemeinvette

Creating a duct behind the radiator as per the GT40 photos could be done with an L88 hood (and could be made to look really nice) but it would be a lot of work and I'm not sure if the total gain would be worth it without addressing the other issues like an underbody tray. Certainly the rear end, especially without the spare wheel carrier lends itself to creating a rear diffuser as per the blue C3 racer above.
An undertray could be make for most of the car, say in 3 sections, front to firewall, firewall to diff and diff to bumper. It could be a couple of inches lower than the crossmember etc. and slotted where required. If they were held on with Dzus fasteners or the like they could be easily removed.

Intersting, but a lot of work.

G

1969Vette383

Amen brother I wish I had more time to devote to this stuff. I would also love to have the math background to study fluid dynamics.

Thanks Bee Jay, I don't think I've ever had anyone cross-link a post of mine before. Congratulations, you're the first ... I think And thank you.

In regard to more air extraction locations, I also think of the *cough* *cough* cheating *cough* NASCAR Superbirds and Charger Daytonas of the early '70s. Look closely at their fenders above the front tires. Hood mounted tach units Hah hah, they have the same design as them, but they're actually for scavenging turbulent air out from the tire wells. Mopar said something to the effect of, "Those are for allowing extra suspension travel." Suspension travel ...? Yeah ... and I'm Steve McQueen.

As BBShark brought up also, the factory racing C5-R (and now C6-R) have the radiator air extracting hoods, they also have fender louvers.

Companies such as Vanacor sell fiberglass louvers that one can purchase and install on the top of their fenders. http://www.vanacorvette.com/page23.htm If anyone is getting serious about aerodynamics and air extraction, it may be worth the $95 for you. Since I have a lot of fiberglass work to do already I was actually considering some for my car.

That is a great theory, but it doesn't work in practice. From what little I understand, a supercharger (or turbocharger) without any intake tube, [read: sucking away at underhood air] would create a problem. The problem is that engine bays are not vacuum sealed and must draw air from somewhere. As you suck air from the engine bay, more air will get sucked into the engine bay to fill the void.

Think of it this way- I can grab you by your foot and pull, but eventually I'll start pulling the whole "you." You wouldn't like it very much (and for this example nor is it scientifically possible) for me to pull your foot, and not end up with the rest of you. The blower unit has to suck in all of the surrounding air. The air's foot may be in the engine bay, but the air's legs, torso, arms, shoulders, neck and head are all outside of the car... infinitely outside of the car. We live and breathe in the air that surrounds us.

My dad's Studebaker Avanti that I got to drive to high school, was a factory Paxton supercharged Studebaker 289. Its supercharger has an intake tube running to a location under the front bumper. The Granatelli brothers did some work on what eventually became my dad's Avanti. That car has been to the Bonneville Salt Flats when Studebaker came to the Salt Flats loaded for Bear. It had (maybe still has?) some of the 29 U.S.A.C. records set by Studebaker in the 1960s. I would follow the Granatelli brothers lead and draw my air from elsewhere.

You say, "aerodynamics" and in regards to racing I say, "Chapparel." The Chapparel 2J "vacuum cleaner" race car had two large fans essentially powered by snowmobile engines. What on earth would a car need fans for? Sucking! Of course! The 2J had extensive work done to the rear of the car which made it nearly air tight in relation to the ground. Maybe someone can dig up an Internet photograph? When the fans were switched on, more than 51% of the radiator's air was drawn through by the fans. It was this technicality that the sanctioning body allowed the cars to race.

A majority of the radiator's airflow was not all those fans sucked: rocks, pebbles, tire rubber, coins, garbage, insects, and whatever was on the race course ... The drivers of competing cars complained because the fans were mounted such that every bit of debris was shot straight back from the 2J.

What else did those fans achieve? Oh yeah, they also glued the car to the road because of their sucking power. Having the capability to corner at speeds above which your competitors are able to is a nice benefit. I believe a race team also tried some "fan equipped" open wheel cars if I'm not mistaken ...? Downforce suction cars have been long outlawed.

Summerfun

Interesting thoughts.....keep them coming.

1969Vette383

I could be mistaken, but what I understand is that the air travelling over the wing has a longer distance to cover in the same amount of time.

Air molecule A and air molecule B hit the leading edge of the wing simultaneously. "A" chooses to go over the wing and "B" chooses to go under the wing. However, at the trailing edge of the wing, "A" and "B" are paired up exactly as they were at the front of the wing. This means that air molecule "A" must travel faster than air molecule "B" because it has a greater distance to cover in the same span of time.

Sorry if it sounded confusing, I didn't phrase it completely. Ram air is just as it sounds. The idea is to ram as much air into the orifice as possible. Yes, I said "orifice."

I'm not sure exactly, but I believe this is the case: air will be sucked out of an opening if the opening is oriented perpendicular to the flow of air. Imagine air traveling over that beautiful Ford GT40's hood. For this example's sake, the air flowing over the hood will be undisturbed, but air will get pulled out of that opening because of differences in pressure.

I have heard that the two best locations to put hood air induction are at the very front of the hood (as in ram air), and at the cowl. Putting a ram air unit somewhere close to center, does not net as great a result as setting that puppy right at the leading edge of the hood.

Downforce, no matter how little, is a wonderful thing. Also, the 200 MPH stock bodied Avanti "Due Cento" had a custom muffler fabricated. The humorous thing is, it looks like and functions as an air dam. The rules don't say anything about how and where the exhaust can be routed. LOL.
To those of you with 4-door sedans, have you ever tried lowering the front window, and its accompanying rear window to the same height? When I drive someone else's 4-door car and I want to open my window, I'll open the window behind me to the same level. The wind buffeting, noise, and flow all smooth out beautifully. I'm wondering why?

Someone with a damn degree in this stuff please chime in. In the meanwhile, we'll all throw our cookies on the table and see what we come up with.

1969Vette383

How about wheels that double as fan blades? I must be delirious now that it's 4am PDT. Think of it, you can set the pitch of the blade to suck air out from under the car. Not only that, but the wheels are right next to the brake rotors. They could help suck heat right off the rotors.

Okay, enough... I'm crashing for the morning.

xcelciador

Wait till they come on market and the ricers get some.