I don't want to hijack the current Huge Rear Spoilers thread, so here's another question regarding rear spoilers. I suppose the '68-'73 rear bumper covers provide more down force than the '74-'79 sloping bumpers. Correct?
How much of a difference is there? And at which speed does it start to make a difference?
I'm considering replacing my '77 bumper with the earlier years' one, a la brent319... Thanks for your input!

 

Hi RB,
I think your confusing which cars have "bumper covers" and which cars have small spoilers as part of their rear clip.
It seems you're suggesting taking the fiberglass spoiler from a rear clip of a 68-73 and bonding it in front of the bumper cap or on the cap of a later car.
That seems like it would be quite an undertaking!
Regards,
Alan

 

Hi Alan, I'm talking about using CIC's rear conversion, like Brent's '76 Red Rum.
Wondering if there's a benefit from a down force perspective? Starting at which speed?
Thanks all!

 

Hi RB,
Oh!
In looking at the pictures of original road-race cars from the 68-72 era it appears that the small rear 'duck tail' spoiler was enough.
Most of the cars have extensive front spoilers which seem to indicate the problem was keeping the front planted, not the rear.
Regards,
Alan

 

Wind tunnel tests showed 14 lbs of down force at the rear of the early duck-tailed C3. It was the only generation to produce down force (C7 did not exist when they tested).

A C3 certainly doesn't need anything more than the factory duck tail at the rear at any speed. The front is another matter completely (very high lift).

 

Thanks Alan and zwede. So ignoring the front lift issue, are the '74 - '77 C3s dangerous on a track considering they don't produce any rear down force? I have not seen many '74 - '77 pictures of road-race cars out there.

 

No, it's the front lift that makes it hairy at high speeds.

 

That's good news. Thanks zwede.

 

Chevrolet said the 78 & 79 with the pace car spoilers and the 80 and up c3's were the most aero.

 

IMHO few of us will ever need to worry about doing very much more than to install a basic, functional front dam and rear spoiler. Still fewer have legitimate need to develop radical aero kit such as splitters, diffusers, pans, oversized spoilers or wings. True, aerodynamics come into play well before one reaches high-speeds, but IMCO most enthusiasts would be better off optimizing mechanical grip and leaving development of any special aero kit strictly for moderate to fast corners. In any event, as this thread isn't the place to get too caught up in theory or specialized bits, I'll refrain from going there in as much as is possible...

FWIW, the '78 PC aero package is apparently worth ~15% reduction in drag vs. the standard '78 body w/o the dam and spoiler. And, tho I haven't ever seen any specific wind tunnel data pertaining to the PC package, seems to do a decent job of countering lift in a sufficiently balanced manner. ...at least according to my own before-and-after observations in moderately high-speed corners (90-120'ish MPH entry). So, I would submit that the General put a high priority on function when designing the PC aero kit, despite that the front dam could use some additional support/bracing to avoid oscillation and/or eventually folding under at speed (BTDT).

Alternatively, there are sound arguments that the '80-82 is more slippery from a drag coefficient perspective, and that its nose likely does a better job than the PC dam at countering lift. Some believe the PC rear spoiler carries an unnecessary drag penalty vs. the '80-82 tail, and that a shark so equipped is more likely to have aero push with either front dam or nose. However, IMOE aero oversteer is definitely NOT a demon you want find yourself wrestling when you turn into a fast sweeper. (I'll leave it to others to test the late C3 tail in that situation.) So, between the two my bet would be on the PC rear spoiler for generating more rear downforce, particularly on pre fastback C3s where turbulence likely exists behind the rear window.

Unfortunately, the only C3 drag/lift/downforce numbers I've seen published were apparently of an early standard model, and thus do not account for these later developments.

In any event, I'd strongly suggest those genuinely concerned with their shark's aerodynamics lower ride heights, perhaps with a slight forward rake, but NEVER so far as to compromise suspension geometry. (We all spend more time at lower speeds where mechanical grip is king.) And, at speed, whatever might be one's personal tastes, form should follow function first. So, IMCO it is somewhat shortsighted to only work on one end without making like improvements to the other. My $.02


TSW

 

Lots of good info there TSW. Form should follow function, yikes. Personally I'm not a fan of the PC or 80-82 spoilers. IMHO the early C3 spoilers provide the best compromise between form and function. I have been struggling with this decision for a while. The fact that CIC does not sell 4" flares for the '77 bumper style may force me to do the full conversion like Brent's car.

 

Not as bad as the C2 though...

 

Regis, a very common mod made to many of the '73 - '79 Corvettes soon after the '80 debuted was to add the '80 rear bumper cover to their car. I have seen quite a few over the years and find them to be an excellent match. I have always felt that there was something missing on the '73 - '79 Corvettes and the '80 rear cover seems to fill that need well. In my personal opinion even better than the Pace Car spoiler, which simply looks too much like an afterthought, regardless of how effective it may have proven to be.

In any event it might be something to consider if you're looking for some additional rear downforce (whether it's needed or not) or just to clean up the look.

Good luck... GUSTO

 

Yeah. Duntov called the C2 "a very bad airplane".

 

He wasn't wrong...

 

RBrid
If that conversion is what you desire then go for it but the duck tail is not a copy of a stock 68-73 but rather an artistic version of it that has the sharp cuts and angles of a stealth fighter jet, sorry, I just do not care for it at all, doesn't flow in my eyes.
Now if you wanted to save a huge pile opf toad pelts AND still mod your car and add something that does function why not the 78 pace car front and rear spoilers?

 

I molded in an 80-82 style rear on my 75



I also molded in the Indy style spoiler and 80-82 front, and used Sport Mirrors





I wanted the 80-82 looks, but had to have a flat rear window because I retro-fitted a removable window from a 69. I just had to have that window.
I

 

Great graphs, thanks for sharing. I'm not real smart with these sort of things so I'll assume the lower the lift number the better...the C5 seems to fare pretty well, makes me feel good as this is the car I've pushed to triple digits...not so much for the '82. She's fast in a special sort of way

 

How I miss the days of mechanical grip racing. Been somewhere around 160 mph with the 82 IMSA body on the car and the 69 stock body with the cut-down windshield body. But what I remember was the hood bowing in the middle. Lightweight hood. And hoping the four hood pins were going to hold. Don't remember the car being unstable enough to worry about it or making any changes. Don't think anyone has anything to worry about.

Greenwood's corvette was doing 215 mph at Le Mans in 1973 which was a new GT record with just a front spoiler. In the #49 C3. Guldstrand was taking the C2 coupe to 171 mph at Le Mans in 1967. No body modifications. Just the headlights open.

Here's a picture of about 125 mph at a hill climb I ran. Nose is up. But was not a problem or unstable. For you adrenalin junkies, go run a hill climb. At the speeds we ran, a misbehaving car would kill you. So all the time was spent trying to improve mechanical grip with suspension improvements. Look at the objects you can hit. Trees, rocks and drop offs. Always worried about hitting a tree or rock. Think as stiff as the car was, with its full cage. The impact and energy would have been transferred inside the car. Which happened to the owner of Cool Shirt, in his GT1 Camaro at a GA hill climb. Sad, sad day, DOA.

 

my question is...what year C3? the aero and downforce is a lot different on a 68 compared to an 82. The hatchback rear window and the rear spoiler is considerably better than the "sugar scoop" and slight kick up on a 68.
Top speed should not be the deciding factor on "aero".

NASCAR is a good place to se the differences. For example at Daytona, the cars are narrow, the nose pointed, frontal area as minimal as possible (fenders tucked in), lower rear spoiler, they are looking for every mph. However somewhere like Texas (considered a high downfoce track) the front is more upright, the front of the fenders flared out, rear spoiler taller, they need the car to grip.


an interesting read -
http://www.germancarforum.com/thread...ray-etc.29409/