Nutin But A Hot Rod C5 Supercar

We test Chevrolet’s 450hp C5 Supercar. Boy–Did We Have Fun!
by Hib Halverson
©1999 by Hib Halverson
No Use Without Permission.

That Arrest-Me Yellow Corvette that was on the cover of the May Vette Magazine? Sure, GM gave it a buzz-word name–"Corvette Supercar"–but to us; it’s simply a hot rod C5. First thing anyone around here does with a hot rod is find straight stretch of road and tromp on the loud pedal. You don’t get far into first gear when the engine starts to rpm, you feel the car trying to stop the earth’s rotation and you think, "Uh-oh. This ain’t no regular C5."

This car is interesting to us for a couple of reasons, besides being a hot rod. First, it’s demonstrative of what traditional performance enhancements can do for the fifth-generation Corvette’s LS1 engine.

Secondly, this factory-tweaked version of America’s Sports Car is a rare case of the smoke, mirrors, rumors, "no comment" responses and GM bureaucratic nonsense that clouds our view of future product briefly clearing away so we can sneak a peek at a few aspects of Corvettes that could be soon to come.

Why a hot rod and just how hot is it.
The "Specialty Vehicles Department" is part of General Motors Communications, the company’s public relations entity and is keeper of the General’s big-boy toys. Working there is, clearly, the best gearhead job at GM. In recent years, the folks at Specialty Vehicles have overseen the construction of virtually all Chevrolet’s performance-enhanced concept vehicles and they get to do that with other people’s money. Our envy of them is way more than green.

In fall of 1997, Specialty Vehicles had a Detroit prototype shop, Wheel-to-Wheel, Incorporated, modify a C5. The intent was to focus media attention on answers to questions like, "Can the C5 be hot rodded?" and "What if it had 450hp?"

An early-build, manual trans./F45 ’97 (ser. no. 00139) was acquired for the project and modified over the winter. The Corvette Supercar debuted last April at Chevrolet’s "Toys Test" an annual, invitation-only, junket for media hot rod freaks. The event, staged at Memphis Motorsport Park, brings together selected members of the automotive press and nearly all of Chevy’s current stable of performance-oriented concept cars and trucks.

Dick Almond, Corvette Brand Manager at that time, said of the Supercar, "The 1998 Corvettes are effectively sold out, so these experiments are definitely not aimed at boosting sales figures.

"We know our customers don’t regard their Corvettes as static sculpture. If they are not enjoying them on some open-road adventure, they’re contemplating modifications aimed at better handling or quicker acceleration. With that in mind, we’ve recently conducted our own research into the most effective ways to improve a current Corvette using aftermarket parts and established procedures.

"The Corvette Supercar demonstrates that there’s plenty of potential to take on true supercars after only a modest investment in performance improvements. Specialty Vehicles’ menu of bolt-on modifications should provide Corvette owners with ample inspiration for tuning their cars."

Almond went on to address the future of the production Corvette.

"Our experiments not only provide enterprising owners with a menu of surefire modifications, they also offer a realistic look at the types of improvements we’re investigating."

Dropping hints, much?

No Corvette magazines were invited to the 1998 Toys Test but, after eight months of squeaking, last December we finally pried the hot rod C5 out of the General’s hands. You’re reading the first, in-depth technical article about it in America’s Favorite Corvette Magazine.

Chevrolet limited Vette’s evaluation of the car to 48 hours forcing us to skip our usual battery of quarter mile, skid pad or and slalom tests but it did supply data from drag strip passes made by different members of the media at the Memphis event. The five quickest passes averaged 12.88 seconds/113.71 miles per hour, however, they were made by three different drivers. Expectedly, there was a variance in skill and technique affecting the average. The quickest driver went 12.69/114.69 with a 2.205 60-foot time. There was a fourth driver who did a 2.209 sixty-foot, but the rest of his run, a 13.17, was trashed, perhaps by a missed, 1-2 shift.

Bottom line? The Corvette Supercar is pretty damn quick.

While most of our work was subjective evaluation or photography, we did sneak it onto the Dynojet chassis dynamometer at K&N Engineering’s research-and-development facility at Riverside, California during a test we’d previously set up for one of our project vehicles. We saw 364hp@6000rpm and 365 lbs/ft torque at 4600 rpm at the rear wheels–mighty strong stuff.

Had Vette been allowed to test the car in the quarter mile, based on Dynojet results and knowing after its debut last April, it underwent additional modifications that added seven horsepower and 22 lbs/ft. torque, we think the Supercar would have bettered the Memphis numbers. Today, it might run high 12.50s and, if it were fitted with sticky rear tires, like BFGoodrich Drag Radials; it just might see 12.40s and sixty-foots in the two-tens.

Gen 3, only more so
Much of the Supercar’s thrills come from an extra-gnarly version of Corvette’s Generation-Three (Gen 3), LS1, V8 engine. We’ve taken to calling it "LS1-Plus" and according to data supplied by Wheel-to-Wheel (or "W-W" as we’ve affectionately come to call them), it puts out 450 horsepower at 6000 rpm and 440 lbs/ft torque at 5000 rpm. W-W’s figures were taken with all accessories operating, a full exhaust system, an air filter assembly and on 92-octane fuel. When you factor parasitic losses, those numbers, except for the torque peak, are about where our chassis dyno data says they should be. No doubt, the LS1, as a kick-ass, hot rod motor, is coming of age.

Were a lot of modifications necessary to make that kind of power? Surprisingly, not. The engine’s shortblock was blueprinted and received the following modifications: an aftermarket camshaft; some modifications to oil galleries to improve oil flow, an external adapter for engine oil cooler connections and a 3/4-in. deeper oil pan with reworked baffling. The rest of the shortblock is stock.

The camshaft profile was designed by Wheel-to-Wheel and machined on an 8620 steel billet. It has .575-in. lift and durations at .050-in. valve lift of 220° for intakes and 226° for exhausts. Lobe centers are at 114° and the cam was installed straight-up. For comparison, GM Powertrain’s LS1 Project Manager, John Juiriga, told Vette the stock, LS1 Corvette camshaft for 1997-’99 has .479/.472 lift and 210°/202° duration.

The other big enabler of the Supercar’s 105 extra horsepower are reworked cylinder heads. W-W began with 1997 LS1 castings then milled them .030-in. This raised the compression ratio almost a full point to 11.0:1.

Next came port/polish work specifically matched to the camshaft’s higher lift. The roof of each intake port around the valve guides (called the "bowl" by head work experts) was substantially streamlined. While this decreased swirl in the intake charge a bit, it increased flow enough that the loss in swirl becomes a good trade-off. The intake rocker-arm-bolt boss’ protrusion into the intake port was reduced and the port downstream of the port entry was widened slightly. The intake ports were raised as much as gaskets and the intake manifold would allow. The "short-side radius," where the port floor curves down to the intake seat, was smoothed and widened.

In the exhaust ports, the short-side radius was smoothed and widened and the small bumps in each exhaust port floor were removed.

The combustion chamber walls were modified to unshroud the valve seats a bit. The valve seats retained their O.E., three-angle finish. The valves, 2.00-in. intakes and 1.55-in. exhausts, are production GM items which come stock with three-angle faces.

The Wheel-to-Wheel folks were kind enough to supply some flow test data for the LS1-Plus heads.

Clearly, W-W’s strategy of improving high lift flow was sound and the results came at no loss to low lift flow.

With the exhausts, we again see significant air flow improvement at high valve lifts, but low-lift flow also increased quite a bit.

Some changes in the valve gear were necessary. First, the valve springs are Pacalloy units from Comp Cams designed to accommodate the .100-in. increase in lift. To improve reliability at high rpm, lighter pushrods and titanium retainers are utilized. The pushrods are, also, shorter because of the .030-in. head milling. The spring seats in the heads are machined .060 deeper to prevent coil-bind at maximum valve lift. The rest of the valve gear is production LS1.

The stock, glass-reinforced, Nylon intake manifold was used as is. Its design allows it to still fit heads that have been milled .030-in. As the port entries in the head are stock no port matching was required. The mass air flow (MAF) sensor is stock as are the injectors, fuel rail and fuel regulator. The production air filter assembly was deep-sixed in favor of a W-W intake duct and a pair of K&N Filters.

Wheel-to-Wheel fabricated a set of tubular, stainless-steel, low-restriction exhaust manifolds. They have 1.75-in., non-equal-length primaries, three-inch collectors and were ceramic-metallic coated by Tofeco Engineering. The rest of the exhaust system consists of production catalytic converters, a production intermediate pipe/cross-over assembly and set of Corsa low-restriction mufflers having polished tips.

The final engine modification was a pair of 12x4.75x2-in. engine oil coolers mounted in the front fascia. These are linked together and to the engine with Aeroquip, AN-10 braided-stainless-steel covered hoses.

Driving the LS1-Plus
The engine feels like some Gen-2 Small-Block-based, 383s we’ve driven in C4s and it runs harder than a production LT5. Low end torque seems to have suffered little or not. Mid-range torque feels comparable to some 383s, probably because of a) longer stroke (3.62-in vs. O.E. of 3.48), b) longer connecting rods (6.1-in. vs. 5.7) and c) the Gen 3s intake tuning being effective in the mid-range whereas that of the Gen 2s is not.

At the top end, it runs like a bat-outa-hell peaking at 6000 and pulling well to the rev limiter. In contrast, the stock LS1 peaks at around 5500 and is maybe a bit weak after 6000. One hundred five extra horsepower and 400 more usable rpm makes the already-swift C5 driving experience downright thrilling. Credit that to basic hot rodding, head work and a camshaft.

Drivability is quite good for an engine with the cam the LS1-Plus has. We observed one quirk: after each start, if you do not disable the traction control system (TCS); it comes on a shortly after you start moving, regardless if there is wheel spin or not. A front-to-rear difference in tire diameters causes the traction control system to "think" there is wheelspin. At present, the "workaround" is to simply disable TCS using the button on the center console.

We queried Jeff Bietzel on calibration and were astonished to learn, other than an increased the desired idle specification, the Supercar’s calibration was stock. When asked how production fuel and spark schedules could work with an engine having 105 more horsepower, Beitzel told us W-W’s tests showed the O.E. spark curve provided best performance. The MAF sensor and the powertrain control module (PCM) software are capable of measuring the increased air flow and the injectors are capable of providing and controlling the extra fuel flow.

The only characteristic we didn’t care much for was the exhaust sound. No doubt, the Corsa exhaust’s lower restriction is required on a car with a 450 horses but, its sound, while appropriately louder, seemed not very refined–"rasty" is a term that comes to mind. Corsa was one of the first in the aftermarket with a C5 exhaust. Some of the other, newer systems, such as the Flowmaster, sound better. Hopefully Corsa’s later interations have improved sound quality.

Ride and Handling
Any car with 105 more horses than stock requires a handling upgrade. In an afternoon of subjective evaulation over Angeles Crest Highway in the mountains north of Los Angeles, we found The Corvette Supercar handled very well during an agressive touring driving cycle. That surprised us because, to date, we’ve not been big fans of the F45 Real-Time Damping (RTD) system in a hard-core, performance driving environment.

As we ran the car hard through the twisties, the car’s substantially increased grip was obvious and yaw response was just a bit more crisp. Threre seemed less understeer, as well. The ride was outstanding. Credit that to a package of suspension modifications that bring the best out of base springs and the F45 shocks.

Our surprise grew to amazement when we looked at more test numbers Chevrolet supplied. Taken at Moroso Motorsport Park in Florida with Andy Pilgrim driving, maximum lateral (max. lat.) acceleration went from 0.91g to 1.00g. That’s nothing compared to the road race results.

The Supercar averaged a blistering, 6.12 seconds a lap quicker around Moroso’s 2.25 miles than a stock, F45 car and a little under six-seconds quicker than a stock Z51. That quantum leap in lap time was not all horsepower, either. The car was also tested with just low-restriction air filters and exhaust and it still beat a stock Z51 around Moroso by three seconds.

Want more? In the same test session, Chevy had Andy run against a Porsche 911 Turbo-S and a Dodge Viper GTS. He smoked the Porsche and virtually equaled the Viper, in spite of the Chrysler’s racecar-stiff tuning and mega-brakes. The averages were, Corvette Supercar: 90.40 sec. Porsche: 91.60 and Viper: 90.36. Not bad for a hot rod C5 with a little ole, normally-aspirated 350.

So, what makes this car’s admittedly soft chassis work so damn well on a race track? Three simple changes. First, a different tire/wheel package, Goodyear Eagle F1 GS-Fiorano (say "Fee-o-rahn-o") tires on four Fikse (say "Fick-see"), forged aluminum wheels.

The front tires are 265/40ZR18s and rears are 295/35ZR18s. Goodyear Fioranos are O.E. on Ferraris and, from a performance standpoint, a noticeable upgrade over the stock, Eagle F1 GS EMTs. We interviewed Warren Croyle, Goodyear’s Product Manager for high-performance tires. He told us the big differences are: 1) size, of course–265/40s and 295/35s put more rubber on the road 2) Fiorano’s have less tread depth and a more "blocky" tread pattern–makes for less tread squirm during aggressive maneuvering 3) they have a stickier tread compound–somewhere between the F1-GSes and a showroom stock race radial and 4) Fioranos are not EMTs. Their sidewall design is more appropriate for a performance application not requiring run-flat capability.

The wheels are four, Fikse FM5s. In recent years, Fikse has become a top choice of Corvetters for high-end wheels. The fronts are 9.5x18s with 7.5 inch backspacing. The rears are 10.5x18s with 8.5 inch backspacing. They weigh 18 lbs. (front) and 18.5 lbs. (rear). Fikses use a three-piece design having forged, CNC-machined, 6061-T6 aluminum centers and spun aluminum rims. The standard finish, such as on the Supercar’s wheels, is polishing and clear-coat, but other finishes are available. An important feature of Fikse C5 wheels is they accept O.E. tire pressure monitoring system sensors. Some other wheels on the market do not.

The Supercar was lowered 3/4-in. all the way around. Lowering reduces the center-of-gravity and improves appearance.

Finally, the front stabilizer bar was upgraded from a 23mm, base/F45 unit to a special, 25.4mm bar built by Hotchkiss Performance, an aftermarket suspension specialist in California. C5 sharpies know O.E. Z51s bar are also 25.4mm and will ask, "Why not use that?"

W-W’s Jeff Beitzel, who did a lot of the chassis tuning himself, told us that, with Goodyear Fioranos, traction–especially the front with not only a sticker tire, but a substantially wider tread, too–increased such that the front end rolls a lot more rather than slipping as it would with the stock tires.

A significant increase in front roll stiffness was required to reduce body roll. It was more than the Z51 bar offered but W-W did not want a larger bar than 25.4 because it would require aftermarket, polyurethane mounts. For improved noise, harshness and vibration characteristics, on all C5s GM uses stab. bar mounts having a low-friction lining. Urethane mounts increase friction. Wheel-to-Wheel wanted to retain the production parts’ NVH advantage so a larger bar was not acceptable.

Hotchkiss prototyped a hollow stab. bar out of tubing having 6.35mm wall thickness versus the stocker’s 4.0mm so bar rate rose significantly without a diameter increase. The special bar in stock mounts represents some sharp thinking by Wheel-to-Wheel and Hotchkiss.

The car’s alignment is set to the performance end of the factory tolerance range. Considering input we’ve received from GM’ engineers regarding C5 alignment for autocrossing, those specs are really not all that aggressive.

The only quality we didn’t like in Supercar’s ride and handling was occasional bottoming in the rear when driven hard on our mountain test route. That kind of road surface can impart more drastic, low-frequency ride movements than would be expected on race tracks which tend to be smoother. If it’s been lowered more than 1/2-in., during such ride movements the base/F45 rear suspension can be prone to bottoming. On the upside, C5 bump stops are designed with a spring rate of their own, best described as giving a "soft landing" to the suspension when it bottoms. Yes, the spring rate goes up upon bottoming, but apparently not to the point you are thrown way off line when it happens. Perhaps a recalibration of the shock absorber valving strategy might help.

Jeff Beitzel wanted a car that could perform very well on a racetrack, but not the stiffer ride of Z51’s higher rate springs and more aggressive damping. Perhaps these wants were shaped by Bietzel’s doing a lot of ride work locally. Wheel-to-Wheel is located in Michigan, a state with some of the worst roads in the country. This tends to skew chassis tuning more towards ride.

Generally, we’re skeptical of a suspension tuning strategy like this because, in the past, it’s not proved realistic for our tastes, however, with the C5’s ride decoupled from handling by design and innovations like the Hotchkiss stab. bar in stock mounts and the Fiorano/Fikse package, it seems Beitzel’s strategy works and works well.

The rest of the Corvette tuner community should take notice: with C5, great handling does not always require stiff springs. In fact, it seems not to require even stock Z51 springs. Put your money in Fioranos, Fikses and a Hotchkiss front bar, people.

So, Where Do We Go from Here?
First, it ought to be clear the potential for the production C5 to unseat the Dodge Viper as the American Supercar is there.

Secondly, also clear should be that current C5 owners can have Viper-squashing performance and handling right now with only moderate modifications. Better yet, as Chevy’s hot rod C5 proves you can go snake squashing with an engine of stock displacement; the cost seems reasonable.

Third, the Corvette Supercar project’s stated goal: "…to improve a current Corvette using aftermarket parts and established procedures." was met with ease. Obviously, the LS1 can responds well to those "established procedures." The result was a startlingly quick hot rod.

As we said before, the Supercar is a case were some of the things clouding our view of future briefly clear away. Back to Corvette Brand Manager, Dick Almond words when the Supercar was first shown to media. "Our experiments not only provide enterprising owners with a menu of surefire modifications, they also offer a realistic look at the types of improvements we’re investigating." Almond was referring to the production Corvette.

Ask yourself why can the LS1’s MAF sensor, intake manifold, fuel system and valve sizes be capable of supplying fuel and air to a 450hp engine as easy as they do to a 345hp engine? Was GM Powertrain planning ahead?

Does this point to a future, production engine in the 425hp class? It just might, though we suspect such an engine will not be normally aspirated but supercharged. Why?

New exhaust emissions and fuel economy restrictions loom in the short term. Corvette will see small changes to address those issues in model year 2000 when it meets low emissions vehicle requirements. In the mid-term, say MY01-03, things could get even more restrictive if Vice President Al Gore is successful in his bid for the Presidency. Gore is no friend of the automobile and if elected, we can expect a veritable orgy of liberal-sponsored, motor vehicle/environmental legislation. In spite of those uncertain clouds on the horizon, we still think there will be a Corvette, although the government and the insurance industry will do their best to make the privilege of owning America’s Sports Car more expensive.

Our educated guess is emissions and fuel economy concerns will have GM will going the blower route to additional performance. As good as the LS1-Plus is, we don’t think its camshaft would make it past Federal certification.

When would we see this C5 blower motor? Our guess is, in the fall of ’02, when Chevrolet announces the Fiftieth Anniversary Corvette, you’ll find it under the hood.


Box 48080
922 Southwest 151st St.
Seattle WA 98166

Goodyear Tire and Rubber Co.
See your local Goodyear dealer


Hotchkiss Performance
Suite 13
12035 Burke St.
Santa Fe Springs CA 90670

K&N Engineering
PO Box 1329
Riverside, CA 92502


Wheel-to-Wheel, Inc.
570 Exec Dr
Troy MI 48083

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