As General Motors' flagship automobile, Corvette has always been the recipient of GM's latest and greatest innovations. Usually, this is a good thing. Occasionally, however, the latest isn't all that great. Such engineering pratfalls as the "Cease Fire" Injection debacle of the early '80s spring to mind, as do the half-baked FX3 suspension and the oddball 4+3 Nash crash box. The LT1 C4 aficionados among you will quickly add OptiSpark to this list, and with good reason.
Falling back on the early '70s LT-1 name (minus the hyphen), the '92 LT1 was a technological stutter-step for Chevrolet. (Although the reuse of the LT-1 name leaves us wondering if GM had finally run out of those catchy three-character alphanumeric RPO codes it holds in such high esteem.) Chevy's small-block had gone fundamentally unchanged since its introduction in 1955 and was certainly getting long in the tooth. While the LT1 retained the same basic internal geometry as previous small-blocks, it offered 50 hp more than its predecessor, the Tuned Port-injected L98. At the time, the 300hp (net) LT1 was the most potent small-block Chevrolet had ever produced, besting even its legendary namesake. (Although rated at 370 gross horsepower, the original LT's true output is believed to have been in the 270-290hp range.)
With the all-new aluminum LS1 still some five years from production, GM saw the need to take immediate steps to keep from falling behind in the horsepower wars. To this end, the LT1 had a few noteworthy innovations such as reverse-flow cooling, which routed coolant first to the cylinder heads. This allowed higher bore temperatures and reduced ring friction because it improved cooling around the valve seats and spark-plug bosses. It also allowed a bump in compression courtesy of freer-flowing, high-compression cylinder heads and a complementing camshaft. And, yes, the infamous OptiSpark ignition system was also introduced here.
GM's OptiSpark is an optically triggered ignition distributor used on the LT1, LT4, and L99 (4.3L V-6) engines. It is mounted on the front of the engine, above the crank snout and behind the water pump. At the time of its introduction, the OptiSpark system was met with much enthusiasm, as it seemed an extremely accurate timing mechanism. These days, its mere mention is met with the sort of disdain normally reserved for an envelope from the IRS. In principle, the OptiSpark design is actually rather clever. But, as is so often the case with the General, the bean counters got involved and stripped away the reliability in the name of saving some minuscule fraction of a cent per unit. More on this later, but suffice it to say that the compromises proved ruinous.
Two main sections make up the OptiSpark. The optical section of the distributor contains a disk with 360 slots in it, each representing one degree of crankshaft rotation. An optical sensor creates and relays a signal as the slots pass through the sensor beam, providing the PCM with exact crankshaft location. The PCM uses this information to time plug and injector firing. Optical sensors are inherently robust and seldom cause problems in the OptiSpark.
Stacked on top of the optical section is the cap-and-rotor assembly, which is fairly conventional in design. The OptiSpark distributor uses a "correct-a-cap" scheme that places the plug-wire terminals on the proper side of the engine in order to ease wire routing. This was done in response to space limitations on the front of the LT1. Achieving this required the cap's terminal traces to be molded extremely close to one another. This can lead to arcing at high rpm, particularly when used with high-voltage aftermarket ignitions.
The environment in which the OptiSpark is asked to perform sets it up for failure. Mounting it low on the front of the engine practically guarantees that it will be constantly assaulted by heat, water, and debris. While heat and debris should not be dismissed completely, it is moisture that has proved to be the number-one killer of OptiSpark distributors. Further exacerbating matters is the fact that the '92-'94 model year OptiSpark distributors are not even sealed from outside elements. Later versions were given a seal and a vacuum-drawn venting arrangement, but this was a stopgap measure that proved only marginally effective.
Moisture from an outside source or even condensation inside the OptiSpark corrodes the optical components or causes shorts within the cap-and-rotor assembly. Moisture inside the OptiSpark can originate from several sources. One common cause of OptiSpark irrigation is water-pump failure. Even if the pump does not fail completely, higher-mileage units often exhibit leaky seals, which allow coolant to escape from the pump-body weep hole and directly onto (and into) the distributor underneath. Care must be taken when driving through high water, as the unit can easily be submerged. Automatic car washes with undercarriage wash can also soak the unit. The simple act of rinsing the engine with a garden hose has killed many OptiSpark distributors as well. Never, ever spray your EFI engine with water.
Other weaknesses exist in the OptiSpark as well. The bearings are suspect and prone to premature dereliction of duty. Usually, bearing failure can be traced to a misaligned or distorted drive mechanism caused by an incorrect camshaft dowel pin. The cap and rotor should be considered maintenance items just like on a regular distributor. Using an aftermarket ignition amplifier or coil can accelerate wear to these items. Higher voltage here often improves performance but causes ozone buildup inside the assembly, shortening component life.
Long considered the crme de la crme of aftermarket ignition components, MSD Ignition's products are highly sought after by everyone from street-car enthusiasts to the highest levels of professional racing. With a reputation for top-notch parts and first-class customer service, it's no surprise that MSD took a fastidious approach when designing its new Pro-Billet LT1 distributor.
As the name implies, the Pro-Billet housing is machined from a solid billet of aluminum, unlike the OEM housing, which is cast. This makes a more precise component that is stronger and fits perfectly in place of the factory piece. The drive assembly uses a robust 1.580-inch ball bearing to mitigate any premature wear caused by dowel-pin misalignment. The housing has an extra-thick mounting flange, a replaceable O-ring seal, and an additional hold-down bolt for the cap to eliminate leaks. It also features an OEM-style fresh-air vent.
Perhaps the most important feature of the Pro-Billet distributor is that it gives you the ability to adjust the ignition timing. An easily accessible adjustment screw near the top of the housing allows the timing to be advanced or retarded up to seven degrees. This will particularly benefit owners of modified engines that could profit from a slight timing adjustment but otherwise do not need a major PCM code re-work.
Rather than rely on an OEM-style optical pickup, MSD created its own high-resolution unit with superior accuracy at high rpm. The pickup incorporates shaft-encoder technology similar to that used in CNC machinery. Where stock OptiSpark performance typically erodes significantly above 6,500 rpm, the MSD is good to go all the way to 10,000 rpm. Preliminary reports from the field bear this out, with more than one user reporting noticeably more stable performance as the revs climb.
As with all of MSD's distributors, the Pro-Billet LT1 sports the company's trademark red cap. While a splash of color always makes you feel good about a new performance part, the important stuff is what's hidden under that cover. The Pro-Billet's cap housing is injection molded from DuPont Rynite, a material that provides extreme strength and excellent dielectric properties. This reduces the chance of breakdown, which can cause a misfire. The terminal paths have been improved and are encased in a durable epoxy compound for additional protection from cross-fire. The rotor is also a new molded piece with a brass/stainless rotor-tip design.
The MSD Pro-Billet LT1 distributor sells for $499 and is available in both early ('92-early '94) and late (late '94-'97) versions. The distributor is supplied complete with all the components required for installation and has a one-year warranty. Replacement OptiSpark distributors from GM commonly retail for over $400, and dealer service departments have been heard to quote over $1,000 to perform the switch. Sure, the MSD piece is a little more money than an OEM replacement from GM, but as we've shown, you get a premium product that should far outstrip the original in both performance and longevity. Watch for an installation feature on Editor Heath's '96 street sweeper in the near future.
Think your Opti is on its way out? Some common traits exhibited by a faulty OptiSpark include:
*Excessive cranking required to start
*Starts but dies immediately
*Rough idle with backfiring or sputtering
*Poor performance with car warmed up
*Poor high-rpm performance
*Poor gas mileage
*Weak plug-wire spark
*Car suddenly dies and won't restart
The best way to determine the root of your LT1 Vette's ignition-related problem is through the use of a scan tool such as Autotap. Often, the PCM will detect the fault and flag it with an appropriate error code. This is not always the case, however, particularly if the problem is intermittent.
Failing that, verify that all grounds are good. Also check the condition of the spark plugs, wires, and coil. Spending a few extra minutes to properly diagnose the problem could save you a lot of time, frustration, and more than a few Benjamins.