First Look: Corvette ZR1 — Something old, something new
by Greg Raven
“It’s been a long time coming. It’s going to be a long time gone.” Crosby Stills and Nash
Ever since Chevrolet reskinned the Corvette in 1984, an informal debate has raged about its true position in the world: Is it finally world class, or is it still merely a pig in plastic? Is the Corvette a settle-for car that will find a home only with those without the price of admission for a “real” exotic car from Europe, or is it the ultimate achievement of American technology — offering a taste of the best of everything at a down-to-earth price?
The soon-to-be-released ZR1 Corvette is sure to fuel this debate. It appears on some levels to be nothing more than an engineer’s fantasies run amok. Company hotshots have been getting their giggles for years by putting big motors in production cars. This time they just got carried away, and the big motor in question comes from the drawing boards of the wizards across the sea at Lotus Engineering.
To say the ZR1 has been eagerly awaited is to explore the limits of understatement. Until recently, Chevrolet was so conscious of press attention that they officially denied the car existed and would answer no questions whatsoever. The ZR1 is a significant car, and Chevrolet was finding it difficult to perform the testing and retesting required on a project of this magnitude in the fishbowl of public scrutiny, so they declared the whole topic off-limits.
Now it appears that the ZR1 project is a bit more complicated than first imagined. Even though Chevrolet has raised the curtain on the first prototypes of the car, there are still enough bugs in the system that none of the press has been allowed a ride in any of the 55 prototypes built to date, let alone a drive.
Knowing that pretty pictures would not do much towards satisfying curiousity about the ZR1, Chevrolet has released gobs of technical details. Here, we find that the ZR1’s LT5 powerplant is an odd admixture of good old American brute force (cubic inches galore) and several de rigueur Euro-style high-tech pieces, all seemingly brought to life by the near magic of the Lotus-designed combustion chamber.
Because the ZR1 Corvette and the regular production Corvette share virtually everything except for the engine, the rear tires and wheels (wider in the ZR1), and the body (distintively rounded off on the ZR1), the engine — the only domestically produced dual overhead cam, 4-valve, all-aluminum V8 — gets most of our attention.
To start with, it is stunningly attractive, and even the most casual glance will find that the Lotus-inspired packaging gives the impression that there may well be ten pounds in a five pound sack. The distance from the outside of one cam cover to the outside of the other was fixed by the dimensions of the existing engine bay, but somehow Lotus managed to work within these restraints while at the same time stashing the alternator and air conditioning compressor in the front half of the engine valley. Not only are these units tucked away, they are also rigidly mounted, as is the power steering pump, which cuts down on vibration and increases reliability. The gear reduction starter motor, ignition, and all four ignition coils reside in the valley at the rear of the motor. The catalytic convertors are tucked up close to the heads in order to preserve as much of the exhaust heat as possible for better convertor efficiency.
Sitting atop the motor like a 16-legged arachnid is the induction system. The head of the spider houses the main throttle body, consisting of a single 22 mm primary butterfly and dual 59 mm secondary butterflies. Running on the primary alone, Chevrolet claims the engine makes 30 horsepower and will push the Corvette along at 70 mph. With the secondaries open, which happens after 80° of primary throttle opening, 200 hp is available.
The intake runner length and diameter are tuned to produce peak power at 6,000 rpm. Each cylinder has a primary and secondary intake runner. The primary runners are always open. The secondaries, however, are controlled by the engine electronic control module (ECM), which also governs the 16 fuel injectors.
This arrangement of intake manifold runners allowed Lotus to design one fairly tame profile to operate the valve fed by the primary runner and a slightly wilder profile for the valve fed by the secondary runner. Camshaft advance and retard is accomplished through a vernier adjustment at the cam sprocket similar to that found in the Porsche 911. The cams have an oil passage through their centers to carry oil to the bearings and lobes.
Getting back to the intake runner butterflies, upon receiving a signal from the throttle position sensor that the driver wants more power, the ECM examines signals from the engine rpm sensor, coolant temperature sensor, and MAP sensor.
If everything specs out, the ECM will open the eight secondary intake runner butterflies and energize the eight secondary fuel injectors, putting the engine in its full power mode. Chevrolet has not said exactly what “full power” means, but the best guess is that the LT5 will produce 300 lbs/ft of torque at 1000 rpm, 390 lbs/ft of torque at 4100 rpm, and 380 hp somewhere near 4100 rpm; 50% more hp and 30% more torque than the current L98 motor. Even with all its new hardware, the LT5 is only 6% (39 pounds) heavier than its predecessor.
The performance of the LT5 is said to be sufficient to propel the ZR1 through the quarter-mile in the high 12s, from zero to 60 in less than 5.0 seconds, and to a top speed of roughly 180 mph. Although nobody is talking much about it, Chevrolet has also set a goal of a zero to 100 to zero time of under 13.0 seconds, which would make the ZR1 better in this respect than the 427 Cobra. Sources indicate that the ZR1 will already break 15 seconds.
Sensitive to the fact that cars in this price range are sometimes driven by valets and others, Chevrolet has fitted the ZR1 with a separate key-operated switch that prevents the secondary intake port butterflies from coming into play, limiting the output to the previously mentioned 200 hp. Don’t look now, but all over America there are marriages headed for divorce over this issue, guaranteed.
The space restrictions of the Corvette engine bay helped to dictate the design of the head. The twin 39 mm intake valves are canted 10° from the centerline of the cylinder and the 35.2 mm exhaust valves are set the other way by 12°, for a total included angle of 22°. Having the camshafts set closely together meant relatively small cam sprockets, which normally would have made it very difficult to achieve the 2:1 reduction necessary between the crankshaft and the cams. Lotus solved this problem by driving the cams via duplex roller chains that are in turn driven via an intermediate shaft that is driven by a silent-type chain off the crankshaft. This allowed them to set the camshaft sprockets equal in size to the driving gear on the intermediate shaft, as the driven gear on the intermediate shaft is twice the size of the drive gear on the crankshaft.
Because of the demands placed on the crankshaft in any motor that puts out this kind of torque, the 93 mm stroke LT5 crankshaft is forged, nitrided, and internally balanced. The main and rod bearings get their lubrication through the crankshaft, which is cross-drilled to deliver oil from the gerotor oil pump at the front of the engine to all the critical parts. The crankshaft also incorporates an ignition wheel, a steel plate into which are machined nine notches for the ECM sensor (one for each cylinder and one to denote TDC).
The intriguing aspect of the bottom end of the LT5 engine is the block construction. Here, Lotus has chosen construction similar to that of some of the Ferraris and the Porsche 928 (among others), in that the lower half of the main bearings are integral with an aluminum saddle section. The caps themselves are nodular iron, which makes one realize just how far metalurgists have come. The aluminum saddle is poured around the pre-made cast iron bearing caps to make one solid unit.
This construction lends a lot of strength to the cylinder block, but it can also pose some interesting machining problems. Enter Mercury Marine, the people who bring you all those high performance boating engines. Mercury Marine has what is reputed to be one of the highest tech manufacturing plants around when it comes to machining aluminum. As it turns out, they have used similar construction (aluminum top bearing cap and cast iron bottom bearing cap) in some of their own motors, so they knew exactly what was needed to properly align bore the crankshaft bearing caps.
Having Mercury Marine to manufacture and assemble the LT5 engine generated other benefits as well. Not only is Mercury Marine at the top of the field in working with aluminum castings, but their problem solving abilities paid big dividends in bringing the LT5 to production. Consider, for example, the camshaft, which runs directly in the the aluminum head without the benefit of bearing shells. Align boring the bearing caps requires spanning a 22-inch distance with a small-diameter boring bar. Partially because of the complexity of the machining and partially to keep track of common mode failures, Chevrolet initially will be exchanging any motors that fail mechanically.
In choosing an alloy for the block, Lotus considered the high silicon content aluminum used by Porsche in the 944/928, but discarded it as being too brittle. Instead, they opted for an aluminum block with free-standing slide-in 3.9 inch Nikasil (nickel-silicon) treated liners. If you are wondering why Chevrolet chose to go to a smaller bore and longer stroke than the L98 motor of the same displacement, it was to preserve the 4.4 inch bore center found in millions of existing small block Chevy V8s while making room for the different construction. This different construction, by the way, makes it virtually impossible to increase the displacement of the LT5, although Lotus has strongly hinted that we haven’t seen the end of the horsepower possible from this package.
As it is, the output of the LT5 is inspiring. Running at 11.25:1 compression, the combustion chamber is where the action is. But then, coming from the company that brought us the Lotus 16 in 1971 that was running 11.0:1, you almost expect the unexpected. The combustion chamber is shaped somewhat like a cloverleaf. The cast aluminum shallow-dish pistons give the combustion chamber a semi-Heron configuration with “lots of squish,” according to Lotus.
This leads to a fast, cool, fuel burn that requires no more than 20 to 22° of spark advance for proper ignition. As you can imagine, this goes a long way towards eliminating pre-ignition. The engine is outfitted with a knock sensor, but the LT5 is not really knock sensitive, again according to Lotus. When it does detect knock, the ECM retards the entire bank of cylinders on which the knock occurs. Lotus is satisfied with this arrangement, however, because they don’t foresee much of a problem with pre-ignition. You get the impression that Lotus would have rather done away with the bother of a knock sensor completely.
Another amazing aspect of the combustion chamber design is that early versions of the head were running too cold, forcing Lotus to raise the temperature of the exhaust ports. Clearly, Lotus has done something special to get this compression ratio, more than enough cooling, and the ability to run on 91 octane fuel. We can only hope that they publish an SAE paper on how they did it.
The speed/density fuel injection is also new for the Corvette. With the rumors of internal dissatisfaction with the mass/air-flow system on the L98 motor, this is hardly a surprise. The speed/density system looks at intake air charge temperature, a barometric sensor, the throttle position, and the difference in air pressure across the throttle butterfly to meter fuel to the motor.
The lack of obstructions (compared to the mass/air-flow set-up) is said to lead to better volumetric efficiency. I don’t expect you to believe this, but Lotus claims volumetric efficiency “in the high 90s,” which is a pretty big number. Whatever the actual figure turns out to be, the look-up table for the fuel injection is adaptive, meaning that it observes the engine’s performance and adjusts itself to compensate for the real-world running efficiency.
The ignition system is different for the LT5, as well. Each of the four coils feeds two cylinders, one of which is on the compression stroke while the other is not. Coil firing is set by the timing wheel on the crankshaft, but an additional timing wheel and sensor arrangement on the camshafts tell the ECM which of the two plugs to fire. This is an interesting bit of complexity that Chevrolet claims will extend spark plug life, even though other manufacturers with similar two-plug-per-coil set-ups neither bother with this issue nor suffer from electrode erosion.
The ZR1 will be available only with Chevrolet’s new six-speed ZF gearbox, which will replace the awful “4+3” transmission found in the current Corvette. The new gearbox was needed to handle the additional power of the LT5 gearbox, but because they were designing a new tranny anyway Chevrolet decided to address the problem of fuel economy as it relates to manual transmissions.
What they came up with is a solenoid that knows when the car is being driven gently, and changes the shift gate so that the upshift from first gear goes not to second but to fourth. After intervening, the circuit goes to sleep until the next time the vehicle is brought to a full stop, allowing the driver to shift into any gear he wants any time he wants it.
We had a chance to drive a ZF-equipped Corvette (all manual transmission 1989 ’Vettes will have the ZF transmission) briefly, and the intrusion of the solenoid is nowhere near as bad as it sounds, and the improvement over the “4+3” is heavenly. However, there will no doubt still be those who will chafe under this system, leading Chevrolet to caution prospective owners not to disconnect the red solenoid wire that would disable the solenoid.
With the differing torque curves of the L98 and LT5 engines, one would expect different gear ratios for each application. Not so. Both cars share identical transmissions. Gear ratios were arrived at by looking at launch rpm at the low end, fuel economy at the top end, and evenly spacing the remaining four gears in between. Neither car will suffer much from this arrangement, but it is boggling to realize that the ZR1 could be quicker still, given a properly matched set of gears.
In the suspension department, the ZR1 gets bigger rubber in the rear and a three-way adjustable Delco suspension that in the soft setting is slightly firmer than the current base suspension, in the middle position approximates the Z52 mid-range suspension, and in the top position gives the type of control you are likely to need in a car with this much horsepower. This suspension is not unique to the ZR1; purchasers of L98-equipped Corvettes can request it as an option as well.
So you can see that the ZR1 is not the definitive answer to the ongoing debate. With a first-year production run of 1500 cars and a price in the neighborhood of $50,000, ZR1s are not the American sports car for everyman. And, with final testing still dragging on, rumored reliability and durability problems that continue to plague the project, and a painful shortage of the ZF transmissions, Chevrolet may not meet it’s target of a Spring 1989 roll-out for the ZR1, so it will be a while before we can tell you if their rather startling decision to turn the Corvette into a two-tiered car line is the right one.
As eager as we all are to see and drive the ZR1, however, we are willing to give Chevrolet the time to do the job right the first time. The worst thing they could do would be to release the car before it is really ready.
I’d be willing to bet, though, that their competition does not share my enthusiasm.
Published in Performance Aftermarket (book? magazine? special?)