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What is the Opti-Spark Distributor?

When GM introduced the LT1 engine in the 1992 Corvette, it was a major advance in V8 pushrod engine technology. Reverse-flow cooling helped it run higher compression ratios on pump gas than ever before. Multi-port fuel injection with a newly designed intake allowed the engine to get incredible fuel economy while maintaining good top end power. Best of all, when GM started installing these engines in the 1993 Camaro/Firebird platform and the 1994 Caprice/Impala SS platform, true high-performance small block power was made available to the masses. For the most part, the LT1 earned every bit of the praise it received.

After people put some mileage on their LT1-powered cars, Opti-Spark Ignitions began to fail at a fairly high rate. Vehicles operated in wet climates were even more susceptible to problems. What was causing problems with the Opti-Spark?

In order to understand the problems with the Opti-Spark Ignition, a brief description of the system is necessary. Quite simply, the Opti-Spark was a new distributor design that debuted on the 1992 Chevrolet Corvette LT1 V8. All LT1 (5.7L) and L99 (4.3L) V8 engines made between 1992 and 1997 were equipped with the Opti-Spark distributor. This would include all 1992-1996 Corvette, 1993-1997 Z28 and Trans Am/Formula Firebird, and 1994-1996 Caprice, Impala SS, Buick Roadmaster, and Cadillac Fleetwood applications. The Opti-Spark distributor was driven directly off the front of the camshaft. Consequently, the LT1 and L99 engines featured many design changes from the "old-style" small block to accommodate the new "front-mount" distributor location.

The term "Opti-Spark" describes the two functions of this distributor: optical ("Opti") engine speed and position sensing, and high voltage ignition distribution to the eight spark plugs ("Spark"). To perform these tasks, the Opti-Spark distributor contains a high-resolution engine speed encoder, a low-resolution engine position encoder, and a standard distributor cap and rotor assembly.

The high-resolution encoding is done with a 360-tooth trigger wheel spinning at camshaft speed. An optical sensor reads these spinning teeth, and creates a simple 0 or 5 volt signal, depending on whether an opening in the teeth is or is not present. As the trigger wheel spins with the engine, this 0 or 5 volt signal becomes a square-shaped voltage signal, or "square wave" in technical terms. This signal is then sent to the engine management computer to determine engine speed.

Once the computer knows the speed of the engine, it needs to calculate the engine position in order to establish spark timing. To accomplish this task, the low-resolution engine position encoder disk is utilized. The low-resolution sensor itself is essentially identical to the high-resolution sensor. However, the low-resolution encoder disk only contains 8 teeth. Four of the teeth are of the same size, and occur at 90-degree reference intervals (these four teeth help to give quick synchronization during start-up cranking). The other four teeth have varying tooth widths. From these variable-sized teeth, the computer uses a fairly simple algorithm to determine engine position.

The data created by the high- and low-resolution sensing system is ultimately used by the engine computer to generate one thing: ignition timing. The timing signal is sent from the computer to the ignition module, which is located next to the coil on LT1 and L99 engines. The ignition module then sends a high current “charge” signal directly to the ignition coil. Once the ignition coil is charged, it fires a high voltage electrical charge to the Opti-Spark distributor cap. The rotor inside the Opti-Spark distributor then distributes the spark to the appropriate cylinder via the distributor cap.

There are two slightly different versions of the Opti-Spark distributor manufactured by GM. The first design version was made between 1992 and 1994. Other than some small vent holes, the first design Opti-Spark had no provision for evacuating built-up moisture. Also, the caustic ozone gases created by the ignition process were also left inside the distributor with no vacuum removal. This design was replaced in 1995 with a vacuum-vented Opti-Spark (1994 B-Body cars received the updated Opti-Spark before the F-and Y-Body cars). Along with the new venting provision, the second design featured a redesigned cam drive and a different electrical connector.