Surprising Differences Between the Ultimate C4 Custom Corvettes

Typically the difference between a coupe and convertible C4 Corvette boils down to adding extra chassis bracing to compensate for the loss of rigidity resulting from removing the hardtop. That’s not the case, however, with Dick Guldstrand’s spectacular conversion of the Corvette C4, dubbed the GS-90.

Developed in the mid-1990s, it represented an effort to take the ’90 to ’95 ZR-1 to even greater heights. The GS-90 project proved to be short lived, however, as only nine or so were built, six coupes and at least three convertibles (although some sources suggest that additional convertibles were built). Whatever the exact number, GM eventually decided to cancel the extension of the C4/ZR-1 line, when it was replaced by the new C5 Corvette of 1997.

The tail treatment on the GS-90 Coupe is nearly identical to the Convertible's rear end, and echoes that of the '63 Grand Sport racer.

It’s fascinating to see just how many differences there are in content between the two GS-90 models. Each offers a separate vision, in response to  platform availability and changing tastes in the marketplace. We were able to see them first-hand for a side-by-side comparison, thanks to Charlie Budenz, who is fortunate enough to own one of each. He’s a former docent for the Riverside International Museum (now closed), and an avid Guldstrand fan.

For those not familiar with credentials of Dick Guldstrand, he is affectionately referred to as “Mr. Corvette.” That nickname stems from his successes in road racing, winning three consecutive Pacific Coast Championships (1963-65) in Corvettes. He also won the GT-class at the 1966 24 Hours of Daytona, set a GT-class track record at Le Mans in 1967 and drove Corvette Grand Sports for Roger Penske.

After hanging up his helmet, gloves and driving suit, Guldstrand founded Dick Guldstrand Engineering to build a variety of custom-tuned Corvettes. In the ensuing years, he kept close ties with the Corvette team at GM, often consulting for it in various areas, especially suspension setup. Sadly, he passed away in September of 2015 at the age of 87. So, this comparison of GS-90s serves in part, as a fond look back at the innovations and creativity of Mr. Corvette.

Drop-Top Differences

To understand why the Guldstrand GS-90 coupe and convertible differ so much, keep in mind that during the course of the C4 development cycle, no ZR-1 convertibles were planned, so the base C4 convertible had to be used in building a GS-90 Nassau Roadster, as it was called. The coupe, however, had the advantage of using the ZR-1 platform right from the get-go.

As Budenz points out, when the ZR-1 was first introduced for the 1990 model year, it was already at the head of the pack (which is why it was called, “King of the Hill”), and priced accordingly. Instead of the standard, 250hp L98 V8 that was powering the base C4, it prominently featured an all-aluminum (block, heads and intake) 5.7-liter, 4-cam engine, designed by Lotus (owned by GM). Initially rated at 375 hp, by 1993 it was upgraded to deliver 405 horses. While Guldstrand was impressed with this hotter engine, he felt the ZR-1 needed something more, since the body looked exactly the same as all other Corvettes. He decided on a complete makeover, both inside and out.

Going way more than just one better, Guldstrand hired engine tuner Doug Rippie to raise the Lotus LT5’s factory output from 405 to 475 hp at the rear wheels. In addition, he switched out the transverse left springs for adjustable coilover shocks at all four corners, and also put on fatter rubber, the largest 18-inch Michelin Club Sport tires available at the time, measuring 335/30 and wrapped around wide OZ 3-piece wheels. He also fitted into the chassis a race-style, cross-braced roll bar and fire extinguisher.

As for the exterior, Steve Winter came up with the shape of the carbon-fiber body conversion. Educated in England with degrees in automotive design and engineering, he had previously worked at Porsche, where he helped create the 924, and then later at BMW, Volvo, Mazda and Ford in Australia. After emigrating to the United States, he became head of the automotive division at Design Works USA in California, as well as an instructor at the famed Art Center College of Design.

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In recreating the C4 ZR-1, he drew upon his European experience, smoothing out the hard edges and rounding the flat panels with shapes derived from timeless sports car designs such as Jaguar and Ferrari. He rendered a rounded nose with exposed projector-beam headlights, along with a wraparound rear panel with a dozen taillights. Besides softening the shape, he changed the car’s overall dimensions as well, trimming the length of the car by six inches, while widening the rear four inches.

When Guldstrand saw the design proposal, he embraced the new form as a way to emphasize the dramatic differences in the GS-90’s performance. He then had Detlef Stevenson craft a body out of a combination of fiberglass and carbon fiber. Despite the altered size of the car, the body conversion retained all of the original attachment points, as well as internal structural elements. So new crash testing data wasn’t required since it could be taken from the original ZR-1 EPA/DOT program.

I believe the Nassau is faster out of the hole! – Charlie Budenz

The result was a striking, Euro-styled shape that left on-lookers mystified by what just whipped by. As a historical salute to the ’63 Grand Sports (which Guldstrand drove for Penske, and trounced Shelby’s Cobras in the Bahamas), he painted it Nassau Blue with a white racing stripe. He also incorporated a dozen taillights, the same treatment used on the Grand Sports. Other competition elements included functional scoops and vents, plus a plastic rear window that saved weight, and nicely flowed with the new teardrop shape. All these changes came at a princely sum, however —  $135,000 — about double the going price of a ZR-1 back then. Or a GS-90 buyer could supply his own 1993-95 ZR-1 and have it converted for $75,000.

Having A Hardtop

How did the convertible GS-90 stack up to the coupe? As already noted, the coupe was blessed with a tweaked LT5 mill boasting at least 475 horses, but the convertible started out with only a standard L98 pushrod engine and basic chassis setup. Even so, Guldstrand made sure it would definitely not be a sheep in wolf’s clothing.

To elevate the power, he bolted on a Vortech supercharger, along with inserting a Traco cam and doing some cylinder head treatment, raising the output to 420 hp (439 lb/ft of torque), when running about 10 pounds of boost. A tuned Flowmaster twin-pipe exhaust system venting through two inline resonators facilitated engine breathing as well. While this peak of power was not as lofty as the coupe’s heady level, recall that base Corvettes of this era supplied only 250 to 300hp, so the convertible was certainly no slouch.

Although the GS-90 Convertible doesn't have the 475hp Coupe's modified LT5, a Vortech supercharger ups the output on the L98 to 420 horses.

As for the suspension, it was modified to improve roll centers and add anti-squat to compensate for the increased power. This change was done by replacing the stock bushings on the front fiberglass leaf springs with polyurethane wedges, dropping the front ride height by 0.75 inch. And the rear suspension was lowered an inch by using new bolts and stiffer polyurethane bushings, but without altering the original geometry. Other upgrades included bigger sway bars with Heim-jointed end links, full-race Heim-jointed camber rods, trailing links and a diff ratio of 3.73.

They are intended to be ‘kinetic art’ as I do believe them to be the most stylish Corvettes ever made. – Charlie Budenz

The convertible was also fitted with the same wheel/tire combination used on the GS-90 coupe (wide OZ 3-piece wheels and 335/30ZR18 rear tires). Even though the brakes were factory heavy-duty discs, they were dimpled and channeled to help vent brake pad gases and reduce heat and fade. While all these upgrades were not quite the caliber of a ZR-1, neither was the price. By not including all of the coupe’s components, the Nassau Roadster could be offered at a much lower cost (about $86K, versus a starting price of $135K for the coupe). Yet it still had the right look, and the performance was nonetheless a serious step up. All told, they were still close cousins from the same family tree. So how does Charlie feel about the differences between them?

“Deciding between the coupe and the convertible is a tough one,” he admits. From purely a handling standpoint, he favors the coupe’s chassis enhancements. “The precision steering finesse, combined with the sheer grip of the wheel/tire combo and the more sophisticated suspension links makes for the better handling car. Plus, really being able to steer the nose and simultaneously place the rear end with the throttle is phenomenal. The coupe is my pick!” Even so, he points out that the Nassau Roadster has its own set of engaging characteristics.  “If you really like a wind-in-your-hair, unlimited sky view in the mountains or at the beach, this one is a gem!” he enthuses. “And you get to hear it roar—sounds great.” He notes that when spectators hear the coupe’s high-revving 4-cammer, they come looking to see where the shriek is coming from. But when the basso-profundo pounds out from the convertible’s conventional pushrod V8, they know what it represents, even at low revs. It gives the convertible hot rod street cred.

“In fact, I believe the Nassau is faster out of the hole!,” Charlie adds. “It is fantastic at launch; in the same way as a big-block Corvette.” Yet the GS-90 also manifests traits of a finely tuned small-block Stingray, with a much faster top end. As one might surmise from Charlie’s observations, he has worked hard and spent “mucho denaro” to keep his pair of Guldstrand Corvettes street legal and licensed.

“They are not trailer queens, as I strive to drive them to the events I participate in,” he maintains. “They are intended to be ‘kinetic art’ as I do believe them to be the most stylish Corvettes ever made, and a rolling tribute to the Guldstrand team that brought them into existence.”

Unlike most of the other GS-90s that are locked up in private collections, Charlie’s ultimate goal is to preserve them as operating, original vehicles to be used as intended, where all Corvette fans and auto hobbyists of every kind can see them in motion and hear them in action. They serve as a high-functioning homage to Dick Guldstrand and his talented associates who strove to build super-exotic, coach-built, world-class GT machines.

About the author

Steve Temple

Steve Temple has more than three decades of experience as an automotive photojournalist. He has served as editor of several automotive enthusiast magazines, and also as director of marketing for Shelby American. As such, he is intimately familiar with a wide range of vehicles, ranging from vintage street rods and classic musclecars to modern sports cars. Steve has handled tech and install features on all types of aftermarket upgrades for both cars and trucks.
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