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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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.




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