1994 Chevy Corvette Convertible - Rocket Science

Want To Build An 11-Second C4 The Right Way? One Reader Shows You How

William Wolfe Aug 2, 2010 0 Comment(s)
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I graduated from high school in 1964, at the very beginning of the muscle-car era. After being honorably discharged from the United States Navy in 1969, I purchased a '69 Pontiac GTO automatic rated at 350 hp. My best time at the dragstrip was 14.5 seconds at 101 mph, and I actually won many drag races. Today, there are V-6 family cars that would give that GTO a run for its money and still get 28 mpg on the highway. There was a time when everyone thought the '60s were the Golden Age of performance cars. I now realize that today is the pinnacle, and I am a willing participant.

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I've always admired the Corvette for its timeless style and performance. I remember being 13, looking over a '60 Corvette roadster, and thinking it was the hottest car I'd ever seen. I knew then I wanted to own a Vette; I just didn't realize how long it would take. Finally, on June 15, 2004, I purchased a 1994 Chevy Corvette convertible with 58,513 miles. The car met all my criteria: it was very clean, it had a six-speed, and it was Torch Red. My first trip to the dragstrip that summer netted a 13.801 at 101.12 mph-respectable, but not particularly fast.

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In September of 2006, I decided to build a C4 that could match the quarter-mile performance of a new Z06 without using a power adder like a turbo or a nitrous system. It also needed to be reliable and fun to drive. Selecting and ordering parts was nearly as much fun as actually working on the car. Each component had to be carefully selected and matched in order to achieve 11-second e.t.'s. My project was completed April 2007, after I spent the entire winter in the garage.

I started with the drivetrain. A 6,500-rpm redline with 6,300 to 6,400-rpm shift points seemed reasonable and should have enabled an 11-second car to achieve about 120 mph in the quarter. The rolling circumference of my tires was measured to be 79.8 inches, so to go through the lights at 6,300 rpm in Fourth gear would require a 3.91 differential and result in a trap speed of 121 mph. I had the differential rebuilt with all new bearings, seals, clutches, and 3.91 gears. I also replaced the U-joints in the halfshafts with heavy-duty units. The driveshaft had 0.013-inch run-out on the differential side and 0.006-inch on the transmission end. Using 3.91 gears, the driveshaft would spin at 3,880 rpm at 75 mph; this was way too much run-out for that speed. As a solution I purchased a lightweight, custom-built carbon-fiber driveshaft that measured less than 0.001 run-out. Gear changing is done via a B&M Ripper shifter that reduced the 3-to-4 shift distance from 6.25 to just 2 inches.

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My next step was to select a suitable rotating assembly. I chose an Eagle stroker kit that included a forged 3.750-inch 4340 steel crankshaft, forged 4340 H-beam rods, and 0.030-inch oversized SRP 4032 forged pistons with full floating pins. This increased the displacement from 350 to 383 ci. These parts were bolted into the line-bored block using billet-steel four-bolt bearing caps and ARP Pro Series studs and nuts. On the transmission side, I used a 12-pound, SFI-certified Ram aluminum flywheel and hydraulic release-bearing conversion kit to replace the heavy dual-mass flywheel. The other end of the crank uses an SFI-certified ATI aluminum Super Damper. Oil pressure is provided by a Melling high-performance pump. The oil pan is an oversized Milodon unit equipped with a Diamond Stripper windage tray. The rotating assembly of this motor could safely reach 9,000 rpm and still hold together.

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The valvetrain was selected to be as aggressive as possible while still providing acceptable driveability. I used Comp Cams roller lifters with 1.6-ratio Pro Magnum roller rocker arms, providing an intake-valve lift of 0.576 inches. The camshaft is a Comp 280 XFI stick that has 230-degree intake duration and 236-degree exhaust duration (at 0.050), along with a 113-degree lobe-separation angle. This cam moves the power band of the motor into the 4,700- to 6,400-rpm range. This is where the engine stays during a quarter-mile run, thus allowing longer, harder acceleration in each lower gear.

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The next decisions involved the top side of the motor. The intake tract received a K&N air filter, a ported BBK 58mm throttle body, and a Granatelli 1,050-cfm MAF sensor. Next, I chose the LT4 heads and intake manifold. TPIS CNC-ported the heads, which were also treated to 7/16 ARP rocker studs, Comp Cams guide plates, and Comp beehive 26918 valvesprings with titanium retainers. With this setup the valves have a seat pressure of 137 pounds closed and 320 pounds open. (Standard LT4 springs have a redline of 6,300 rpm and a seat pressure of 100 pounds.)

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After being CNC'd and port-matched, the heads flowed 290 cfm at 0.600-inch with a port volume of 210 cc. A combustion-chamber volume of 61.2 cc results in a compression ratio of 11.0:1. The hollow LT4 intake and sodium-filled exhaust valves weigh 82 and 72 grams, respectively, compared with the smaller LT1 valves, at 113 and 96. The heavy seat pressure, combined with the lightweight valves and titanium retainers, results in a valvetrain that can safely rev to 7,000 rpm, even with the aggressive fast-ramp, high-lift Comp cam. (The rev limiter is currently set to 6,500 rpm, to ensure engine durability.)

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The exhaust system consists of ceramic-coated Hooker Super Competition 1.75 inch long-tube headers and a stainless-steel Corsa exhaust system. The sound is impressive-reasonable at cruise but great at wide-open throttle.

The ignition is handled completely by MSD. There were two versions of OptiSpark distributors, so I upgraded to the later, '95-up version. The Vette now has an MSD Pro-Billet distributor, 6A control box, Blaster coil, and custom-length 8.5mm Super Conductor plug wires. The plug-wire layout was hand fabricated to work around the headers.

I programmed the Vette's stock computer myself, a job that required some thoughtful preparation. A wideband O2 sensor is mandatory for tuning, and a G-Tech/Pro RR accelerometer is very useful for comparative testing. I welded extra O2 sensor bungs on both collectors for use with the wideband O2 meter. The program I used-LT1 Edit-allows control of every conceivable part of engine management, from speedometer calibration and fan-engagement temperatures to spark-advance mapping and air/fuel-mixture control at any rpm. The program also provides real-time feedback and data logging for engine parameters. Of particular interest were parameters such as coolant and oil temperatures, knock-sensor count, rpm, throttle position, and injector pulse width. I programmed the open-loop WOT mixture to run at slightly less than 13:1 AFR. Even with a compression ratio of 11.0:1, the knock sensors indicate no sign of detonation.

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It's interesting to note that the absolute redline of this engine is determined not by the valve-train or lower end, but rather by the size of the fuel injectors. An engine operating at 6,000 rpm would have the injectors open 100 percent of the time at a pulse width of 20 milliseconds. It's usually not desirable to have a 100 percent injector duty cycle, though something close to that is fine. Using the smallest usable injector with high fuel pressure provides the best spray pattern.

I replaced the 24-pound LT1 injectors with 32-pound units and upped the fuel pressure from 47 to 55 psi. Pulse width is now 18 milliseconds at WOT at a 13:1 AFR. (Interestingly, I flow-tested all eight injectors and discovered a 36-pound unit in the box, proving you can't leave any details to chance.) The 18-millisecond pulse works perfectly for a 6,500 rpm redline, but to rev any higher would require larger injectors.

The Vette also lost more than 100 pounds of front-end weight during this project, thanks to the removal of the stock catalytic converters, the air pump, and the EGR. Replacing the dual-mass flywheel, the cast-iron headers, and the front pulley with lightweight components also helped. I even removed the spare tire, lightening the rear by 32 pounds. Carrying 10 gallons of fuel, the car weighs 1,560 pounds at the rear and 1,660 pounds at the front, for a total of 3,220 pounds. Remember: removing weight from a car is as good as adding horsepower.

With the exception of the machine shop, I performed all the work on this project myself. My goal of breaking into the 11-second range was exceeded, with a best time of 11.721 seconds at 119.21 mph. The sound at idle is impressive, and with the aluminum flywheel the motor revs up almost instantly. On a cool day the car will engage the traction control in Third gear at 75 mph any time the gas is floored. The only way to get any decent traction in First or Second is to first heat up the Goodyear tires. There are no rough spots in the power band, and the engine will pull smoothly from 1,600 rpm. That slam-you-into-the-seat, raise-the-front-end feeling is backed up by the readings on the G Meter: the Vette has hit 0.74g in First, 0.59g in Second, and 0.45g in Third.

I take my car out almost every weekend when the weather is nice and enjoy showing it off at car shows. My dream of owning a Vette roadster has been fulfilled, and all my expectations exceeded. This car was not meant to stay in the garage-it was built to be driven.

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