My '73 ZZ4 3:08 M20 clocked 126 MPH on the chassis dyno @5,000 RPM and I have done at least that on the street with still more RPM left to go.

 

A aero little math to set the record straight. The aerodynamic drag force is D = CD * A *0.5 * rho * V^2. Power is the product of force and speed. Therefore, the power required to overcome aerodynamic drag P = CD * A * 0.5 * rho * V^3. CD = Drag coefficient, A = Frontal Area, rho = air density, V = velocity or speed. The take away is simple. It takes eight times as much horsepower to overcome aerodyamic drag at 200 mph as it does at 100 mph! For most vehicles at about 50 mph the aero drag is equal to all of the other forces acting on the vehicle. Above 50 mph aero forces are predominant. Let me know if you want the English or metric units for these calculations explained in detail.

 

I usually end up hauling *** over the Buckman bridge where I live. 3.1 miles and has a huge hill in the center w/ a solid median about car height in the middle (aka no police)... redline the 80' going downhill and push it till my temp gauge starts climbing like my tach Never had anyone beside me to tell me how fast im going but im assuming im 105+ and it still handles pretty well... I try not to change lanes but hey I gotta pass those slow drivers somehow right

 

One more point. Most production automobiles are lifting bodies. Lifting bodies tend to make lift about the quarter chord point. In technical terms this means there is a positive pitching moment (nose up). In simple terms, as speed increases the front end gets lighter more rapidly than the rear. Earlier there was a post that said add a rear spoiler before addressing the front. The safest approach is to dial the front in first and then balance the car's front and rear lift with the rear wing or spoiler. This is the approach that race car aerodynamicists use. Adding a rear spoiler or wing (reducing rear lift) with out addressing the greater front lift actually increases the pitching moment and makes the front end get light at lower speeds.

 

Are you talking about a Greenwood Daytona like this? (I first saw this car at the '81 Eckler's reunion next to the racer from which it was modeled.)



...or that ghastly knock-off with all the extra cr@p stuck all over it that's been floating around online this past year or so? The dude that screwed up the latter ought to be welded to the floor. As for the real Greenwood Daytonas, while at first glance they may not be all that pleasing to many an eye, IMHO there's a certain beauty about any functional design that is so bad to the bone that rules makers were soon attempting to slow it down (resulting in the "bobbed-tail" GTO version which was quicker still).

BTW, not to get lost here, for pre-'80 C3's the design of the '78 PC style dam/spoiler package is fundamentally sound; reportedly lowering drag coef by ~15%. However, the factory's soft air dam could use some bracing for extended high-speed use. Haven't had a chance to acquire comparative data yet (only recently got my instruments and my shark will be down for a while longer), but from seat-of-the-pants before & after observations it was easy to realize this package made a substantial improvement to the lift/downforce picture, as well.
...and FWIW it looks fairly good, too.

 

While the typical auto body may have the general form of a lifting body, since they operate in close proximity to the ground the pressure differential equation changes, making it possible for shapes that would otherwise create lift in free flowing air to generate downforce while in ground effect. This will be the case as long as average pressures beneath the car are less than those above it, regardless of shape. Inversely, the more air gets under the nose of the body the higher will be pressures there, and should that pressure exceed that above the body then lift results. Just how much and where these pressure differentials are along the length of the body will determine F/R aero balance.

You theoretical point about consideration of the pitching moment during aero design is well taken, but I can attest from having lost both front and rear wings due to contact and/or failures that I'd very much rather loose the front (as long as it doesn't end up under a tire). FWIW, the coaches/engineers I worked with track-side typically suggested my beginning each race weekend with what they estimated to be slightly more rear wing than could likely be gotten away with (drag wise), initially dialed in the front wings so as to yield aero "push", then adjusted the fronts to reach the best aero balance compromise, and finally sought to reduce drag with less overall wing until lap times failed to improve. Of course, this latter process would invariably unmask additional mechanical grip/balance issues.

All that aside, the bottom line remains that one really shouldn't ever ignore either end of the car aerodynamically. My earlier comment was not meant to suggest that adding rear spoiler alone is the best move.

 

UHH....better watch that stuff there, note that I live in OP, and been over that bridge many a time....cops tend to hand on either end of the thing in that turn around aiming up the bridge, and just over the hump you fill find a cruiser waiting on the side, running radar.....also there are bears in the air too....

gas is down in price now, so you gotta be aware....too boot it's holiday season....

 

Hey skunk, curious what you have to say about my observations.....I moved here near 12 years ago from Maryland, up there the ambients were damn rarely ever above 110 on the road, and surely they are lots hotter than that here in Florida, on a summer day in traffic just off the freeway settin at a light, I would say the ambients have to be 140, as my wife's little silver/green Escort hits 165f on paint temp in the sun.....so anyway....I had a problem with the front tags being required in Maryland, so had one on....when noting none needed here in Florida, without any thought, I took my '87 vette tag panel, said Corvette on it, and stuck it on the '72 here, totally stock grill in them days....and had a overheating problem in traffic.....so, one day somehow I took that whole tag mess off there, and the overheating at speed and in traffic was almost cured.....some years later I got a custom aluminum grill made up, and so it's do open in front maybe as well say there is no grill.....no tag crap no lower bumberettes, nothing...not even a lower spoiler section at that time....

the car never had a gasket on the back of the hood, and never had any side metal grill inserts....wide open scoops....

only recently has the center section under the add on aftermarket BB hood bulge, been opened up totally.....

there is no spare tire, which I think would act to allow a air damn in rear under the tail light panel....of course the 'vents' are up top in stock position form the old astro vent setup that never functioned, as it's a a/c car....

so the question is.....overall, that is your opinion of these changes??? do those body mods help or hurt?? the car is stock height, not supposed to be according to VBP with the 460 spring in front, but it is according to frame measurements off the GM literature....

 

Mississippi Highway Patrol says my 69 will do 162 mph on radar.

 

Whilst we're on the topic of fast corvettes here.....
I know it's NOT a C3, but the Calloway Sledgehammer deserves a mention.... as it really was an incredibly fast car.

Keep in mind, that a lot of media attention has been placed on the Bugatti Veyron, as the fastest production car available at present with a top speed of 252mph.

The Calloway sledgehammer, based on a late eighties C4, ran twin turbos to produce a staggering 900 horspower.
It went on to reach a phenomenal 254 mph !!!
And that was in 1988!

Almost 2 decades before the Veyron !!!

Fair bit of good info in link below.... handling the flow of air at high speed was the critical factor in achieving the design goal... not just for aerodynamic drag, but also for cooling purposes, and air intake for engine, and of course, to keep it on the road.

http://www.z06-corvette.com/super/sledgehammer.htm

Interesting to note that John Lingenfelter was the test pilot for the Calloway project back in those days.

 

THAT right there if a beautiful hunk of work,

any car break that record yet today??? I was joking with my wife that she could drive that to the grocery store and then head out over the Buckman at 240 mph or so......her head spun around grinning.....

 

I always thought that it took 4x hp to double the mph?
That is if: 100 hp = 100 mph ........400 hp = 200 mph
I will recalibrate my rule of thumb.

Ricisan

 

my 4:11 geared 68 has gone 117mph in a quarter mile. Theres more there but I would not want to go a whole lot faster in it. Thats one of the things I like about it on the street it feels like i am going faster then I really am.

 

My C3 is a 72 with the base L46 engine. Gears are ~3.32 and it's an auto. The car had a set of headers and a cam. Way back in 1984 with the car in a good state of tune, it hit 135 mph in a street race. There wasn't much left. With a lot of room, maybe the the car would have done 138 to 140. Front end felt ok the car has a small chin spoiler and is about 1 inch lower than the typical C3. BTW vette won the race.

120 mph is about all my nerves are good for these days and that's in the LS1 Trans Am. The Vette saw 120 mph about ten years ago. I wouldn't do that now without going thru the suspension as the springs are shot etc.

LOL! I always wear a seat belt in other cars but never in my 72 :-)

 

i'd put mine in that category. Of course my Olds Intrigue has been to a buck 30 as well. My C3 is by no means a fast car. Its pretty to look at, and has the typical V8 gitty up and go. Nothing out of this world at all, especially stock.

 

Anyone got a cheap set of wings and tailplane available for me - prefferably off a jet fighter (I guess off a Sabre or Phantom would look best on a C3) I am sure with a little bit of work I can graft them to my C3 - hey, I don't have to worry then about that 'airborne feeling' at 120 mph - 'cos I will be airborne'
Anyhow it will look a whole lot more practical than some of those Greenwood conversions

 

you get 4x the drag, it's true, but the power required to produce a force at 200mph is double that of the force at 100mph (assuming constant gear ratios). if you take the drag increasing 4x over the 100-200mph section, and multiply that by the fact that power increases 2x for the same force at twice the speed, 2x4=8.



(note- it has been about 5 years since i last really cracked a physics book. my current job lets me feel all the benefits of physics, but eliminate the math

 

The practical reality is most production cars generate lift and when they do there is almost always more front lift than rear. Automakers go out of their way to ensure that vehicles have under steer since this condition is more forgiving for the average driver. More front lift is directionally correct for this design premise.

From the pressure standpoint the distance along the top of the vehicle is greater than that on the underside and thus the average velocity is greater over the top than the bottom. Faster flow means lower pressure, lower pressure means lift. For pressure to build up on the underside the flow must be obstructed, deflected, diffused or stagnated.

On to race cars... Rear down force bias is generally the fastest aero set up since it allows later and harder braking. This reduces the amount of time the vehicle is transitioning from straight to corner. For optimum race car set up you start with the maximum rules dictated tires and work your way up through the vehicle. When done properly, weight and aero (down force and drag) balanced with the tires, chassis, etc. makes for the optimum set up. Lap simulation tools like Bosch's Lapsim make optimization of these parameters relatively easy.

Contact induced, suboptimal set ups in many cases boil down to driver skill and experience. Under steer is probably the best suboptimal condition since it is the easiest to manage.

I prefer to maximize down force at the hardest end to adjust trackside. Then adjust the easy end to driver preference at the track. Rules generally dictate which end is hardest and easiest to adjust.

Cheers, happy holidays and good luck racing!

 

I felt the subject matter within your query worthy it's own thread, so I started one.


http://forums.corvetteforum.com/c3-t...post1568309199

 

I have no disagreement with the basic theory of lift 101. Was just observing that by placing the body near the ground, things are different than in free flowing air. It is operating close to the ground which facilitates the practicality of creating a sufficient pressure drop beneath a body so as to generate downforce with a shape which would otherwise lift, by obstruction, deflection, diffusion or whatever means.

Having never been fortunate enough to have benefited from having comprehensive setup simulation software (have run with Pi and Stack acquisition, tho), while finding my way around during the first laps on a given track I found it a good bit less expensive to just follow the initial setup advice given me before moving ASAP to optimizing. As for loosing parts off of the car, if not for providence I wouldn't be here from the rear wing mount failure I suffered at IRP. Had nothing to do with my skill, or lack thereof, and was one of those times when it was definitely better to be lucky than good.