Closing the gap: Corvette ZR-1 vs Porsche 928GT (CAR achive, 1989)

Published: 18 July 2016

► America's finest takes on Europe's standard-setter
► Can the Corvette ZR-1 outdo Porsche's 928GT?
► An archive classic CAR magazine twin-test from 1989

The Corvette ZR-1 plot is simple enough: to lay waste every other performance machine ready money can buy. Here America’s finest takes on Europe’s standard-setter, the new 928GT, the most powerful front-engined Porsche yet. 

The Corvette ZR-1 is the only car in the world that has two ignition keys. The first starts the engine, which then has only about 80 percent of maximum power at its disposal. To free the remaining 20 percent, you turn the second so-called power or valet key which lives in a slot in front of the gearlever. Without it the ZR-1 is barely as quick as an off-the-shelf 240bhp L89 Corvette normale. 

In this better-safe-than-sorry mode, the engine runs only three valves and one injector per cylinder. But as soon as you flick the stubby power key to the left, the unit runs on all 16 injectors and 32 valves. Gimmick or necessity? 'Call it a precautionary measure,' says chief engineer Dave McLellan. 'There are owners who would prefer their spouses or sons not to be in command of the ZR-1's full performance potential.' 

The Porsche 928GT has only one ignition key; nor does it have most of the US car's eccentricities, such as rainbow-hued instruments, selective ride control or six-speed transmission using computer-aided gear selection. But it has a — however tacky —GT badge on its foam-filled, impact-absorbing rump, which indicates that this brand-new 928 derivative is something special. Porsche's senior marketing manager Hans Halbach explains: 'The 928GT is a blend of 928S4 and 928 Clubsport. In other words, it combines the luxury of the S4 with the higher performance of the Clubsport.' 

While the 928GT, unlike the ZR-1, is not a quantum leap above the base model, it is quantifiably quicker, and has an uprated no-nonsense sports suspension. Within the £30,000 to £60,000 price bracket that accommodates most of the practical and sensible 365-days-a-year super coupes, this top-of-the-line 928 is Europe's closest rival to Detroit's hottest iron. The Testarossa, the F40, 959 and Countach QV are all too pricey, and mechanically too far removed. 

The world introduction of the Chevrolet Corvette ZR-1 took place in Carcassonne in the very south of France, a stone's throw from the Spanish border. It's an idyllic part of the world — the sea on one side and the Pyrenees on the other — but it's a bit off the beaten track, which explains why it took us well over 10 hours to get there. The starting point of our four-day journey was Stuttgart, where we picked up an Indian red 928GT on Friday lunchtime. S-JZ 5957 was the first GT to roll off the line. 

Those not familiar with the evolution of the 928 range will find it hard to tell the difference between the GT and the less powerful S4, but a closer look shows that the new car has a twin tailpipe exhaust and slightly wider alloy wheels shod with 225/50ZR16 tyres at the front and 245/45ZR16 rubber in the back. As the wheels are an inch wider than before, and spacers extend the rear track by an extra 0.7in, Porsche had to modify the wheel-arches to make room for the fatter footwear. Other GT-only modifications include a limited-slip differential with a locking ratio of 40 percent, a reinforced final drive, stiffer springs, tauter dampers and a shorter gearlever. 

Unlike the Clubsport— which had no central locking, rear wiper, power seats, cruise control, rear sun visors or remote-control tailgate release—the 928GT is not spartanly equipped. It comes with all the above, and a sound system incorporating no fewer than 10 speakers as well as automatic air-conditioning, tyre pressure sensors, ABS and an alarm system. As a result, the GT weighs 3476lb — that's 2131b more than the Clubsport and only a little less than the 92854 automatic. 

While the power rating of the Clubsport remained unchanged at 320bhp, Porsche says the GT develops 330bhp at 6200rpm, up 200rpm from the standard powerplant. The extra 10 horses are produced by a hotter camshaft, a larger-diameter intake manifold and a free-flow exhaust system. Maximum torque is still the same (311Ib ft), but it now takes 4100 instead of 3000rpm to produce it, As these figures suggest, the difference in performance is marginal. 

The factory claims that the 928GT can accelerate in 5.8 seconds from 0 to 60mph, although, like all official Porsche figures, that is bound to be conservative. Expect 5.5sec for the dash. Although we did not put a fifth wheel on S-JZ 5957, several runs against the stopwatch confirmed that the GT achieved a double-checked 174mph top speed —that's 3mph faster than the official figure.

After the 1000-mile drive, we arrived hungry at our destination, but in good shape. The following morning, Chevrolet rolled out a red ZR-1 which looked truly awe-inspiring in the narrow streets of Carcassonne. The beefed-up version is even prettier than the standard car, one of the best-looking high-performance coupes currently made. The ZR-1 is one inch longer and three inches wider at the rear to accommodate the vast 315/35ZR17 Gatorback tyres. The widening exercise dictated new doors, rear quarters, rocker panels, a revised (now convex instead of concave) rear end and an accordingly modified upper panel.

Also new are the four rectangular tail-lamps, the wider 17-inch rear wheels and the quad-tailpipe exhaust. As a result, the ZR-1 looks particularly aggressive: it sits squat and low, its pointed shark-like nose a distinctive and commendably aerodynamic feature; the car's muscular stance is further enhanced by the extra flesh in its shoulders. 

The engineering changes are plentiful but less obvious. They include hydraulic engine mounts, the Z51 performance handling package that has been around since the current Corvette's debut in 1984, the FX3 electric ride control device co-developed by Bilstein and Delco, large-diameter front disc brakes, a six-speed gearbox supplied by ZF, and the Lotus-developed LT5 engine produced by Mercury Marine in Oklahoma. 

LT5 hides under a clamshell hood which is heavy and probably very expensive to replace. Filling the space between the front wheels almost completely, the V8 is a very pretty motor. Made entirely of matt silver aluminium, the joint effort of Lotus, GM and Mercury is actually 391b heavier than the cast-iron L98. You can tell at a glance that the designers have treated the engine bay with extra care. All auxiliary equipment, all pipes and most reservoirs are hell black, a nice contrast with the silver of the engine itself and the devil red chosen for the lettering and the ignition cables. 

The difference between LT5 and the usual L98 V8 is not restricted to the number of camshafts and valves. The 32-valve unit is really a completely new design which maintains the original 5.7-litre displacement, despite the fact that the bore is reduced while the stroke is increased. Engineering highlights include a sequential instead of simultaneous double-fire fuel injection, two fuel injectors per cylinder, two oxygen sensors instead of one, a narrow valve angle of only 22 degrees, hydraulic valve lifters, thermodynamically highly efficient clover-leaf combustion chambers with incorporated squish lips, a speed density instead of a mass air flow meter, a high-compression ratio of 11.25 to one, a combined three-blade induction and variable valve timing arrangement, and a computerised direct-ignition system. 

Originally, LT5 was intended to produce 400bhp and 400lb ft of torque, but the target has since been eroded to around 360 to 385bhp and to 380lb ft. Because the certification process is not yet complete, Chevrolet refuses to give official horsepower and torque figures. It transpires, however, that the car is likely to be rated at 380bhp at 5800rprn. The maximum torque of 3831b ft is likely to be available at a high 5200rpm. Just as we heard a wide variation of power output data during our stay in Carcassonne, we were given a selection of different acceleration and top speed figures. I have seen enough 0-60mph print-outs of 4.2 seconds to believe that this figure is authentic, but I haven't been able to verify the claimed maximum speed of 190mph. The ZR-1 does not feel that much faster on the motorway than the Porsche 928GT, and its top-end performance is to an extent marred by the extremely tall sixth gear whose ratio is a from-here-to-eternity 0.499. 

Stepping from the 928 into the ZR-1 is initially a bit of a culture shock. The Porsche is a big car that is disappointingly small inside, but there is at least enough space for the driver and his passenger, and there are few ergonomic problems thanks to easily legible instruments and well-placed switches, buttons and levers. I am not too fond of the 928's new in-dash display which is hooked up to the on-board computer, but this fingertip message centre is still much more pleasant than the glitzerama provided by the Corvette. This car is an ergonomic disaster: wiper controls in the door panel, instruments that were obviously designed in the computer department of Toys R Us, the worst switchgear this side of a cross-wired Korean microwave oven, colour schemes that make sunglasses compulsory, and so on.

The only good thing about the ZR-1 interior is that it will be replaced at the end of this year, and I for one sincerely hope that GM will kick out the seats at the same time. Trimmed in the kind of plasticised longlife leather every self-conscious cow would refuse to be associated with, the space-age buckets are adjustable in so many directions that there is not enough room to accommodate all the controls in one place. And that's bad news, because some of the switches and buttons sit exactly where I would like to. 

The Corvette is strictly a two-seater, which is no disadvantage since the cargo area is reasonably big and accessible. Heat intrusion from above and below can be a problem, though, and there are also not enough straps, nets or fasteners to stop the luggage from moving about. But the nastiest aspect of the Vette's boot is the positioning of the 20-gallon fuel tank well behind the rear axle and thus in a potentially dangerous spot. The Porsche can in theory — accommodate up to four adults, and yet those condemned to sit in the back should certainly be dwarfish. The rear seats are OK for kids, though, and they are just perfect for anoraks and soft travel bags. This leaves the actual boot free for the only suitcase it can hold. 

On a sheer horsepower-per-pound sterling scale, the Porsche is poor value for money. Priced at £55,441, the 928GT charges £168 for each of its 330bhp. The 380bhp Corvette, on the other hand, costs in America the equivalent of £29,500, which translates into a very favourable £78 per horsepower. But all is not gold just because it glisters: in those European countries planning to import the Corvette, the price of the ZR-1 is likely to be as high as that of the Porsche. The American Car Centre, in Kingston-upon-Thames, reckons the ZR-1 will cost £55,000 when UK sales begin in October. The cars are likely to be 'personal imports'. The conclusion? Go to America to buy your ZR-1, provided you can find a dealer who does not insist on a backhander. US sales are likely to start this summer with the 1990 model, which has the modified interior and therefore a built-in excuse for a last-minute price hike. 

The price difference between the 928GT and the ZR-1 is largely due to a vast difference in build quality. Both engines are equally well executed, but the fit and finish of the cars they ride in are worlds apart. The Porsche feels as solid as a rock. Its doors close with reassuring finality, the materials have the potential to outlive the owner of the vehicle, and all components—from the steering column shroud to the majestic tailgate are so obviously part of one integrated, solid and meticulously screwed together whole. 

The Corvette, on the other hand, is a laissez-faire sports car whose build quality is best described as nonchalant. Compared with the Porsche, its plastics and carpets look downright cheap, and this impression prevails once you set off and get a chance to listen to the squeaking body panels, the rumbling suspension and the crackling dashboard. These things do matter, but as long as they don't cause bits to fall off or red lights to flash, they are no worse than the tyre noise and the wind roar you'll suffer in the 928GT. For a model that has been in production since 1978, such vices are simply inexcusable. 

Both cars are very fast, but at least up to 100mph the Corvette is quite a bit quicker than the Zuffenhausen GT. From 0 to 60mph, one-and-a-half seconds are a small eternity, and that's what the ZR-1 puts between itself and its challenger. 'Our car is hot on the heels of the limited-edition supercars from Zuffenhausen and Maranello,' says Dave McLellan. 'The only difference is that the ZR-1 is soon going to be readily available at a fraction of the price of any Ferrari and most Porsches.' And 4.2sec from 0 to 60mph —that's indeed only 0.4sec slower than a 959. 

The 3454lb Vette takes off like a rocket. Wheelspin in first and second gear is the easiest trick in the book. Traction is quite superb. Although the rev limiter allows the big V8 to spin to an earth-shattering 7200rpm, you quickly learn to shift just under 6000rpm, which is where the power and torque curves start to flatten.

Despite all the bottom-end thrust, the ZR 1's strength is not the 0-60mph area but the next section, that spans from 60 to 125mph. Third and fourth-gear performance, that's what makes all the difference. Sprinting to 115mph, for instance, takes only 13 seconds flat, and even up in this critical speed range where drag and rolling resistance are taking their toll, the ZR-1 feels nimble and light-footed and always ready to explode. Beyond 130 or 135mph, however, this Great American Sports Car slowly begins to run out of steam. Fifth gear with a ratio of 0.751 is just short enough to retain most of the momentum, but sixth simply cripples the engine. In top, the quad-cam V8 hums along at a mere 1500rpm to maintain a steady 60mph, and even 125mph corresponds only to 3000rpm. 

The main culprits are not the Corvette engineers but the US Environmental Protection Agency. In order to avoid the gas guzzler tax, the Chevrolet brainforce was forced to opt for the longest legs they could find, and they even installed LAGS, which is Vette-speak for computer-aided gear selection. The secret behind GAGS is a massive metal pin that physically guides the gearlever 'during certain light-throttle driving modes' straight from first into fourth. This may yield the required EPA rating, but it's still a nuisance because you find yourself in fourth rather more often than anticipated. There are, however, two ways to overcome the thinking shifter: one is to pause briefly before slipping the rod into second, and the other, definitive cure, is to cut a red wire which your friendly Chevy dealer will be only too pleased to point out. 

After all the nice things we have said about the Corvette engine, it would be easy to lash the 5.0-litre 928 unit for its comparative lack of power and low-end torque. But that would not be fair: even alongside the ZR-1, the 928GT is no sluggard. What helps the Porsche to stay in touch with the ZR-1 on most types of road is the much better spaced five-speed gearbox that provides plenty of useful mid-range thrust. True, the five-speeder from Stuttgart is not as slick and precise as the ZF box, but its ratios are so much more practical. The 928GT also has a more progressive clutch which does, with the exception of first-gear pick-up, relay more feel than the rather dead left pedal of the Corvette. Similar criticism also applies to the brakes of the ZR-1, which are powerful but difficult to modulate because pedal travel is long and not much happens until you have pushed the thing four-fifths of the way into the firewall. Both cars have competent fade-free ABS-equipped disc brakes all round, and both have the handbrake where it should not be next to the door sill. 

The 928GT is a big car and feels it. The massive front end drops quickly out of sight, and the rear visibility is not too good either, because of the various roof pillars and because of the launch pad-like black tail rudder. The very communicative and well-damped power steering of the 928GT is relatively heavy and not all that quick, which means you have to work hard to put this full-size barge through its paces. 

The Corvette is surprisingly easy on the driver. Its six-speed box slots through the gate with much greater precision than the shifter of the 928. Although the steering is amazingly effortless, it does feel underdamped, and it turns in with a kind of see-saw sharpness that needs frequent correction. The all-round visibility is good (the ZR-1 can even be had with a transparent moon-roof), but the instruments reflect badly at night, and the bulbous backlight provides some amazing rear-mirror distortions. 

Like its predecessors, the current Corvette has a welded steel frame clad with glassfibre-reinforced plastic panels. Its suspension is as unconventional as the body-and-chassis configuration. In the front are upper and lower A-arms, coil springs and gas-charged dampers. The rear wheels are located by a multi-link suspension boasting dual upper and lower control arms, a trailing link and gas-pressure shock absorbers. At both ends, the Vette comes with anti-roll bars as well as unique transverse monoleaf springs made of cosmetic materials. The ZR-1 is, in addition, equipped with Selective Ride Control which enables the driver to choose from normal, sport or performance modes. Labelled FX3, this system comprises four electric actuators which sit on top of each shock absorber, where they will change the damping rate when a microprocessor instructs them to do so. Within each mode, there are six different speed-governed damping levels. 

On paper, this suspension is close to perfection. On the road, however, it is not. For a start, FX3 is not really that wonderful, not very much of a surprise since it does not work particularly well in the Porsche 959, either, where it was first introduced. The first disadvantage is that the driver can interfere with the system —suspension tuning is too serious an affair to let amateurs play with, especially while the vehicle is in motion. Secondly, the system is governed merely by speed. Other equally important parameters such as steering angle, turn-in, throttle position and body movement are not taken into consideration. On French country roads, Selective Ride Control is a joke anyway, because the secondary highways and byways are so rough that you get thrown about no matter what the setting. On the track, the difference is obvious, but when the tarmac is flat as a pan, it makes little sense to use anything but the performance mode. 

FX3 is an indication that the ZR-1 was primarily developed for the flat, evenly surfaced roads which are pretty common in America but the exception in Europe. The spring and damper ratings point in this direction, too. It almost does not matter which damper setting you choose, because the springs are so stiff that they virtually override the work of the shock absorbers. The ride quality is almost as bad as it was back in 1984, when the arrival of the Z51 handling package suggested that you counted your bones before fastening the seat belt. Still obvious, too — albeit less pronounced — is the poor communication between body and chassis. The movements of the suspension and the movements of the body don't harmonise particularly well, and this impression varies from bad to worse depending on the road surface. Unfortunately, the ZR-1's handling is also very American. In the dry, the car understeers with the determination of a truffle pig. But on the track —we used Goodyear's impressive facilities at Mireval — it takes only a couple of laps before the ZR-1 will produce the most beautiful four-wheel drifts. 

At the end of the day, it does not really matter that the ZR-1 'outhandies brand X and outcorners brand Y', nor that 'its skid-pad performance is unrivalled'. Even the offer to pay 'one million dollars to anyone who can lap a given race circuit faster than Rick Mears in a Corvette ZR-1' becomes immaterial, because these simulated achievements mean very little on the route nationales or a British A-road. The only truly valid virtue the ZR-1 manages to transfer from the lab to the road is the incredible amount of roadholding. The car sticks as if Goodyear had invented the magnetic tyre, and after a while your neck hurts as much as your spine because of the g-force's constant attempts to treat your head like a punch bag. 

But is that what makes a good car great? We are looking at a different, more driver-oriented set-up for Europe,' admits McLellan. 'But please don't forget that we have not finished the ZR-1's development. First, we have to sort out the US spec car. Then we can look at the European version.' 

Driving the 928GT back to back against the ZR-1 made it obvious that there is still room for improvement. Unlike the Corvette, which has difficulties synchronising the motions of suspension, chassis and body, the Porsche acts like a taut, homogeneous entity. True, it is not as failsafe and forgiving as the ZR-1, but it is much more fun at the limit. After a long neutral phase, the rear wheels will eventually let go, inviting you to an entertaining powerslide. 

The drift angle depends entirely on the road surface and on the courage and ability of the driver. While dancing from bend to bend, the 15mpg 928GT may not be any faster than the 14mpg US coupe, but the fun factor is so much higher. When we changed cars during our charge through the Pyrenees, whoever stepped out of the Porsche always had a smile on his face. The Corvette does not provide the same satisfaction; it is so cold-blooded and effortless. The ZR-1 induces hardly more drama than the BMW Z1 or the Carrera 4, but it does boost your blood pressure where you least need it: on the motorway at 125mph-plus, where its directional stability is not nearly as good as the Porsche's. Although the US car holds the road very well, there is a suspicious high-speed floating motion to its front end. 

On aggregate, the Porsche wins, but it's a close victory. If you are not prepared to work the 928GT hard, take the ZR-1 : it is so much easier to drive fast. At present, however, it is still let down by excessively defensive handling, by the extremely long-legged gearbox and by its teeth-rattling ride. But once these crucial areas have been attended to, and once the new interior has come on stream, the ZR-1 should be fit to reach for the crown. Which is why this comparison may well, in a year's time, yield a new number one. 

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By Georg Kacher

European editor, secrets uncoverer, futurist, first man behind any wheel