Ralphbf

I know this has been covered a hundred times, but here we are again.

There is no way I can put on a large spoiler on the front of my 75. The way it is now I tear up my front license plate frames.

Is there anything to be gained by smoothing out the under side of the frame? I was thinking about hanging sheet metal under the frame.

I know the stock speedo is off. I have a high performance 700r4 transmission and at 6000rpm in 3rd gear the speedo reads 135mph.
Everything is new and tight and it does feel a little squirrelly at that speed. I'm doing 1800 rpm at 70mph.

How much does blocking off the front grill help?
I have Bilstien shocks, 17" Michelin tires rated at 166mph.

I know it's kind of crazy going that fast, but I just like to know I can.

The 75 has a slight rake to it. 1 1/2 to 2" lower in the front and a new set of steel leaf springs in the rear.

I looked and I don't have a good picture showing the side profile

Any suggestions?

Thanks Every Body

Ralph

gerry72

The C3 aero fundamentals are largely incorrect for high speed stability. This is something that has been known since the first one rolled off the assembly line. The C3 was a design milestone, not a performance one. It looks swoopy and appears to have lines that would cut through the air but it's probably more aerodynamic going in reverse. Look to the track-modified L-88 cars of the era, like the cars that ran on the high-speed circuits such as Le Mans, for some aero clues. You're also working against some odds stacked against you that would require some significant redesign. The flying buttress backlight on the coupe is really bad for aero as it causes the rear to lift. The Kamm tail does nothing to help this. The side windows sit in a recess that disrupt the boundary layer around the side. This is why the racers were using the convertible body with the hardtop.

Belly pans and other aero tricks like blocking the front grill may improve high speed stability (or may make things worse), but you would largely be experimenting if you don't look at things as a package. If you're not willing to control air flow from the front of the car, like a large air dam and Le Mans headlight conversion, and it's not likely you'll see the improvements you seek.

But you have nothing to lose from giving it a go.

MelWff

did you follow the recommendations on this link when you had the car aligned?

https://www.vbandp.com/index.php?opt...=43&Itemid=131

gkull

Look for posts from 69427. He did the belly pan and some other mods.

I go out to Bonneville Salt Flats and year after year there are older c-3's running over 200 mph.

The nose of you car doesn't have to be on the ground, but the chin spoiler under the radiator does to keep air out from under the car

Easy Mike

You don't say whether your car is unstable at 135. Is it?

CiCiC3

Just wondering what type of steering box you are using and also the rear suspension anything special back there?

F22

The general consensus is that while your C3 may go 100 mph+, you don't want to stay there very long, unless you have major modifications to the steering system, suspension and aerodynamics of these cars. They are well known, for being 'darty' at speeds, with a very light steering feel, that doesn't exactly inspire confidence. Just my take.

mrvette

Get and believe your GPS on speeds, the instrumentation on C3 is designed to be very optimistic, and forget the tire diameter/ratio/rpm/speed sites, they are optimistic also....

for instance, after changing my speedo driven gear in the trans, by a factor of ~12% or so, my speedo went to dead nutz on at <70 at 80 it was really only 78mph....at 100 indicated it was really only 92mph....I ran out of room, enough to prove the point, tach was same way, I fed signals to it and it added several hundred RPM starting around 3000 or so, and at 6000 rpm, tach read way high....closer to 7000 rpm.....

but at idle to 2500 rpm tach was dead on.....

SO when my car sez I doing 80, I know it's cool....and totally right at anything slower........

TheSkunkWorks

Unless you're also experiencing steering/alignment related instability at lower speeds, high-speed instability is invariably a function of aerodynamic issues, however accurate or inaccurate may be your speedo.

The typical C3 body tends to produce lift at speed due to higher air pressure building up beneath the body than above (aka, pressure differentials). To combat this build up, beyond simply making sure that ride heights aren't presenting a nose high attitude at whatever speed may be in question, the number one thing you can do is to install a proper air dam and rear spoiler. (Running only one or the other is a sure recipe for aero imbalance at high speeds.) Sorry if that isn't quite what you want to hear, but it is what it is. Besides, as gkull alluded to, your car doesn't have to be completely slammed...

FWIW, the Pace Car design aero kit actually does a pretty good job, but be aware that you will likely need to brace the flexible factory PC air dam so as to avoid it oscillating rapidly and/or folding under at high speeds (BTDT, still looking into workable alternatives to a full-fledged, supporting splitter, but that's another topic).

Panning the belly will help what air is already beneath the body more readily escape, resulting in less drag and higher terminal velocity. But, although this would improve underbody airflow management, doing so will not in and of itself limit the volume of air with which to deal in the first place. Without damming air from entering beneath the body's nose, I wouldn't expect any miracles from simply panning alone, stability wise.

A well thought out air extractor in the hood (or enlarged fender vents aft of the front wheels) would serve to reduce pressure beneath the engine compartment, but best to stay well ahead of the windshield where pressures increase at the cowl.

In any event, the primary focus here should be and remain on reducing/eliminating lift (if not generating downforce), and doing so in a way which achieves/maintains sound front to rear aero balance. (You do want to be able to steer at speed with confidence, no?) So, I wouldn't speed too much time worrying about absolute drag coefficients just yet. To quote Pirelli, "Power is nothing without control." My $.02


TSW

F22

Stumbled across this pic of a very heavily modified C3 Corvette in a wind tunnel. Anybody recognize it? Whoever did this car, had the dough to put it through wind tunnel testing.

gkull

I entered the open road races in Nevada in the class that could go sub 175 mph to avg 150 mph through the slow stuff.

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gkull

you need aerodynamics to go fast
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terrys6t8roadster

Coming up the main stretch at RA.@145+ WOT & transitioning to full brake for station 1, powering through @50+ under mild acceleration to full accel requires some aero work. Top down, roll bar required. My hood on the 6t8 is ported 3-way [custom built]. Also is the front splitter [Ecklers], it is still 8" up and I plan to add more to bring it down to 2 1/2" from the ground [just for the track]. In the rear where the spare is, that cavity needs to be filled in. My thought is to build a rear scoup underneath that would create down force and would exit where the licseinse plate is [track only]. Enclosing that area for your 135 to keep the back end from getting light would be wise. The height on the 6t8 to the top of wheelhouse opening is 27.5 with 25.5" tires. I think my car measured 45&3/4"@ the windscreen with just the roll bar behind it. I guess that's why the racers of yore always never wore their tops, no air drag, and that's so nice when you catch up to a C6, and he realizes that you're faster and he gives you the pass!. T

pauldana

https://www.corvetteforum.com/forums...ve-120mph.html

Belgian1979vette

on my car at 125-130 mph it's getting light and feels unstable.

TheSkunkWorks

Yes, that was (is) a good thread on the topic.


Mods, please don't lock it down should it become active again due to revived interest. Still plenty relevant...

Ralphbf

Does the word squishy mean anything to you?

It doesn't feel solid but it doesn't feel bad.

The only other word I can think of is scary.
At that speed I don't even change lanes.

Ralph

Ralphbf

Hey CICIC3

Stock steering box. With power steering.

Ralph

Belgian1979vette

Something along these lines here as well.

Steering box is thight, that's not it.

I don't think the car was designed to go much over that.

Lakeside49

Let me put this straight that you are certified crazy to drive at such high speeds in a C3 Nut job. Period. End of story. But, my kind of nut job/s because you can, and it all adds to the mystique of the C3 legend. That said, a couple of the posts prescribe shooting the undercarriage. Be careful because you'll generate more lift if you don't do it correctly. I work for a car company. Our engineers worked tirelessly with a clean sheet of paper when they designed the Ford GT super car. Their goal was to properly control the forces sweeping through the undercarriage at high speeds for the sake of performance an safety. If you want a lesson in undercarriage aerodynamics take a good look. It is an expensive design, including the all important rear diverters. But, since I can't contribute a point of view in the same league as my crazy 'need for speed' C3 brethren's, I thought I'd point to another source of American ingenuity on the topic of the undercarriage in event it serves to make one less widow. I need too much help from this site to start losing good folks that post to the land of Darwin