5 Amazing Things About the Corvette C5.R

At the recent HSR Classic 24 Hour at Daytona, we got the chance to take a very close look at a bona fide Chevrolet Corvette C5.R, just sitting there on jack stands in all its glory. That’s the cool thing about historic racing: It’s not factory teams guarding cars full of secrets, it’s privateers (rich privateers, to be sure) keeping their cars on full display for all to see. Since we’re in the middle of a C5 Corvette project, we were more than a little excited to take a look at the ultimate factory expression of the chassis.

The C5.R was built during a transition time in international sports car racing—especially in America. While much of the rest of the world was moving to premier sports car series based around production chassis, the fastest door slammers in IMSA were still tube-framed silhouettes. Built by Pratt & Miller, the C5.R was one of the first wave of American sports cars to usher in the “factory tub” era of domestic sports car competition.

But as we learned from our tour of this C5.R at Daytona, the term “production-based” can be stretched perilously far. Here are five cool things we learned looking over this awesome machine:

1. There’s a LOT of production Corvette here, but also there isn’t.

The C5.R is production based in that it uses a standard C5 frame, but there’s a lot of “George Washington’s Axe” stuff going on here. Yes, the standard frame is corporeally present within the C5.R, much effort has been made to ensure that its effect is minimized. In the case of the C5.R, the hydroformed frame rails are more of an extension of the elaborate roll cage than the other way around. It’s more like a tube-frame car with giant box section tubes (the stock frame) making up the largest part of the structure.

Still, lots of cues abound that make a regular C5 driver feel right at home. The dash is a plastic mold version of a stock C5, and if you don’t look too close, the interior would look right at home in a Corvette at a NASA or SCCA weekend. 

2. It got a BIG engine, breathing through TINY restrictors.

The original C5.Rs used a 366c.i. LS1-based engine, which was quickly dropped in later seasons for a 427c.i. LS-based powerplant, and finally a 427c.i. bespoke powerplant from Katech based on LS architecture. So anyone with any LS experience at all would immediately recognize the engine, but they’d also be horrified at the two tiny inlet restrictors that were mandated by IMSA to equalize performance back in the day. Also, kudos to this particular historic racing team for leaving them in place when they could have easily uncorked the engine for vintage competition without anyone batting an eye. Somehow even with these restrictors in place, C5.Rs were rumored to produce north of 600 horsepower, or as much as 750 or more without the restrictors in place. (And that’s just what Chevy was admitting to.)

Notable with this particular LS derivative, though, is the large plate separating the front of the engine from the accessories drive. That’s actually a piece that ties the engine block directly to the chassis, turning that giant hunk of aluminum into a structural member.

3. This thing has brakes for days.

Stopping a car that’s going in excess of 180 mph lap after lap for 24 hours straight is no small task, and the C5.R has brakes that appear up to the job. The C5.R uses 15-inch front and 14-inch rear rotors—quite similar to our own C5 Z06 project car—but the carbon-ceramic construction and immense, almost comical rotor thickness set these apart from even high-end “consumer” track brakes. And the six-piston front and rear AP calipers squeezing those nearly inch-thick pads look like they’ve seen a heat cycle or two. 
Also visible when taking a good look at the brakes are the bespoke uprights, machined from a single aluminum billet, unlike the cast factory pieces. 

4. It’s got coils!

Coil spring conversions for C5s are nothing new—heck, we have a Ridetech coil-over setup on our project car. But it’s interesting that the same factory that wanted to build a winning race car–but also market a popular street car–abandoned one of that car’s signature pieces of technology. Anyway, coils rule, leaves drool. That’s our scientific analysis.

But the cool thing about the coil implementation on the C5.R is that the positioning of the components is not dependent on factory geometry at all. Remember how we told you that the stock frame was “there,” but it just wasn’t really doing the same stuff it does in the stock car? Well, the suspension components are attached to that stock frame—by way of hardpoints and mounts perched off of that frame and relocated to what we assume are far more optimum locations for performance. The control arm mounts are moved up and pushed out slightly. This effectively lowers the car relative to the suspension, lowering the center of gravity and increasing grip. It looks like the upper and lower A-arm mounts also have a few optional mounting hole locations in the vertical orientation, meaning the entire car can be put in an increased or decreased ride-height configuration without affecting suspension geometry. Or the upper arms can be moved vertically independent of the lower arms, which would affect the C5.Rs camber curve through roll.

The upper shock mounts have been moved up and out relative to the stock upper mounts, which accomplishes two important things: First, as you may recall from our coil-over conversion on our project car, the shocks on a C5 don’t have a ton of travel. Moving the mount up allows the use of a longer shock body, thus increasing the available travel and decreasing the chance of bottoming. Second, you may recall that C5 shocks—especially in the rear—are mounted at a fairly large angle relative to the control arms. This results in a digressive spring rate through compression. Moving the upper shock mount out puts the spring and shock in a far more perpendicular orientation to the control arms, making the effective wheel rate more linear.

5. OMG that rear sway bar is nuts!

We probably spent 30 minutes looking at that rear sway bar, and we still don’t know the full depth of madness going on there. Even one of the techs working on the car basically said: “Yeah, we try not to touch that much.” In response to our query about its many mysteries.

The end links connect to the lever arms in fairly standard—albeit mechanically artistic—ways. But it’s what happens in the middle that still shocks and amazes us. The bar is split in the middle, and the two halves are mechanically joined through a bell crank that actuates on a single pivot. That pivoting crank is adjustable for both rotating tension via a cable connected to a knob near the driver’s door, and… preload? We think it’s some sort of preload? Via a jackscrew directly on top of the rocker. That was the thing that the tech said they never touch. We’re going to keep looking at this things and arranging forks and spears of asparagus and potatoes on our plates in some rough approximation until we figure out the exact mechanical function of all this stuff, and if our families don’t like that, they can eat in the other room.

The best explanation we could come up with with the help of the tech is that the big jack screw controls the amount of force that is transferred from one side to the other. This would be the rough analog to the adjustment on your adjustable rear sway bar, although it does it by increasing leverage. But the adjustment in the hub of the rocker controls the speed at which that force is transferred. It loads or unloads that pivot to create some damping in the force being transferred from side to side, but ultimately controlling the timing, not the amount. Anyway, it’s awesome just to think about how smart/high you have to be to come up with something like that.

 

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