Corvette LT1 MSD Distributor - The OptiSpark Option

MSD comes to the rescue with its Pro-Billet LT1 distributor

Chris Endres Oct 6, 2006 0 Comment(s)
Vemp_0611_01_z Corvette_lt1_msd_distributor Distributor_cap 2/5

MSD's new distributor cap is part of the Pro-Billet LT1 Distributor package. Molded from DuPont Rynite, a material that exhibits superb dielectric properties, the cap also has redesigned traces to minimize the chance of cross-firing.

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.

Vemp_0611_02_z Corvette_lt1_msd_distributor Robust_sealing_system 3/5

MSD incorporated a more robust sealing system and a large ball-bearing assembly to ensure a long service life unaffected by moisture and debris. The distributor is available for both early- and late-style LT1s.

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.




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