Getting to Know the Opti-Spark
Understanding and Modifying This Much-Maligned Ignition System
By Ray Bohacz
Editor's Note: Way back in March of 1998, GMHTP
ran "Opti-Spark" by Ray Bohacz, an engineering-level dissection of GM's
Opti-Spark distributor. This evolutionary ignition component was a
bridge between traditional distributors and the DIS-type, coil-on-plug
systems that we know and love today. Unfortunately, LT1 owners have had
long-term reliability problems with the Opti-Spark, and even though it
has been around for 11 years now, many enthusiasts are still in the
dark when it comes to identifying, troubleshooting, or upgrading them.
The first half of this story is a similar version of the one Ray wrote
in 1998; the last half focuses on model identification, trouble signs,
and new technology that will allow Optis to last longer and perform
To be worthy of the famous LT1 designation the
latest rendition of this engine needed to exceed the pre-emissions
version's power generation while meeting today's demanding standards to
be considered world-class. Specific mass, size, fuel consumption,
emissions, start-ability and torque-band-width all needed to be
improved over the L98 engine that it replaced--all while surpassing
previous reliability levels.
Early on in the LT1's development the design team
recognized the inherent deficiencies of the then-current L98 and the
pitfalls it would cause in arriving at their goals. To this cause the
LT1 employed advanced theories such as reverse-flow engine cooling and
a gear-driven water pump. The Opti-Spark first appeared on the 1992
Corvette and then progressed to the fourth-generation F-body in 1993.
The same ignition is also found in the full-size B-body cars when they
switched to LT1 power for the 1994 model year. During the Opti-Spark's
reign there were three distinct versions of the distributor, but they
all functioned in the same manner. The differences were found in the
drive mechanism along with the design of the housing and vent system.
Time Versus Angle-Based Ignition
to the Opti-Spark most conventional ignition systems referenced the
delivery of the spark through a time-based method. These systems
functioned on a prescribed time-delay period after passing a reference
point that was usually an interrupt signal from a magnetic reluctance
sensor, more commonly known as a pick-up coil. In contrast, the
Opti-Spark uses multiple optical sensors working in conjunction with a
two-track slotted disk to have the ability to identify in one-degree
increments the position of the crankshaft. This is done by
incorporating both a high-resolution outer track of 360 slots and a
low-resolution inner track with eight varying-width slots that identify
By recognizing both the leading and falling edges
of the high-resolution signal, the ECM knows the position of the
crankshaft within one degree. A light beam is created by dual LEDs and
photo transistors and reads the low-resolution slot signals at TDC for
each cylinder. The number of high-resolution pulses observed prior to
reaching the falling edge of a low-resolution slot is the method used
for identification. The light passing through each of the slots
generates a 5-volt transistor-to-transistor logic signal that is
recognized by the ECM.
Using both the leading and falling, commonly
referred to as the trailing edge of the square wave output generated by
the high-resolution sensor, effectively encodes 720 high-resolution
edges to keep track of the crankshaft's position in lieu of the four
acknowledged positions that a conventional ignition is referenced from.
By comparison, a magnetic reluctance sensor (unless equipped with an
ancillary signature pulse) has no knowledge of which cylinder is
firing. But more importantly, it has 45 degrees of distributor rotation
that is not recognized and is unaccountable.
The Opti-Spark ignition system breaks tradition in
a number of unique ways beyond its optical triggers. The following
technology is represented in this design:
The combination of two systems in one. A high-resolution engine timing system and a low-energy secondary distribution network.
The ability for the ECM to have total control over
the ignition timing through an angle-based spark advance function along
with the ancillary control of altering the cranking start-up ignition
timing based on the ambient air temperature.
Robust reductions in spark scatter during
instantaneous transient acceleration of 10-rpm-per-millisecond or
greater intervals in comparison with time-based methodology.
The ability to benefit from a more aggressive timing curve without entering the spark knock zone.
A large cap design with a widened rotor tip,
allowing for all of the secondary voltage to be delivered radially
without employing a large rotor tip clearance. This reduction in rotor
tip clearance also pays dividends in decreased radio frequency
Individual cylinder timing and knock retard capabilities.
Accurate drive positioning directly from the front of the camshaft.
No need for mechanical timing adjustment.
The LT1 ignition
system could actually be referred to as a hybrid that is a cross
between a time-based conventional ignition and a corresponding
duration-configured distributorless design. Prior to the development of
the Opti-Spark the exact position of the crankshaft was never
recognized by the ECM, that data was not available. The input of data
in one-degree increments of crankshaft rotation into the ECM's
programming now allows for more accurate control of engine functions.
In a standard L98 ignition, which is an electronic-spark-timing
high-energy system, control of the spark advance curve is handled by
the tables in the PROM once an engine speed of 400 rpm was surpassed.
Below that rpm the control of the ignition timing was independent of
the ECM and was a function of the module and the installed position of
the distributor. Testing by GM proved that at an ambient temperature of
-35 degrees C, the time-based ignition would have a start time of
approximately four seconds while the angle-based LT1 would fire in 1.5
seconds. This is the result of knowing the exact crankshaft position
along with the ability to vary the ignition timing on crank as a
function of coolant temperature.
To a calibration engineer concerned with meeting
EPA cold-start emissions standards this is invaluable. When the engine
is cranking, fuel is being administered at a rate of inject pulses
twice as often as it is during engine run along with substantially
longer pulse widths. The more fuel injected during crank, the higher
the hydrocarbon emissions and the longer it will take the catalytic
converter to light off once combustion starts. By decreasing the cold
start crank time, the extremely critical first two minutes of emissions
output is greatly reduced. This is a major concern during the EPA cold
start test cycle.
A combination of a decreased rotor gap, along with
the use of a conductive ink to route the spark path in the plastic
encapsulated distributor cap, allows for minimal voltage losses and
longer burn times at the plug from the increased available energy. Burn
times are referenced in degrees of crankshaft rotation that the
ignition system has the ability to keep the spark plug ignited. Any
gains are extremely desirable and increase the conversion of chemical
to mechanical energy. Energy losses that are absorbed by the larger
necessary rotor gap in a conventional distributor consumes a portion of
the coil's output, leaving less voltage to bridge the gap of the spark
With the ECM capable of using input data sensors
to calculate the required spark advance curve, the start of the coil
saturation period is converted from a time-based to angular function.
The timing signal that begins with coil charging and ends with the
secondary discharge of the ignition coil can now be interfaced with
crankshaft position to offer individual cylinder timing control and
knock recognition. Reading the leading (rising) edge of the
low-resolution optical signal along with dual knock sensors (some
models), the ECM is able to identify detonation on individual cylinders
and offer timing correction to those in need.
Spark scatter is defined as a variation between
the actual timing signal and the value commanded by the ECM. It is also
known as cycle-to-cycle and cylinder-to-cylinder timing variation. This
can occur in a number of distinct scenarios, but scatter is the most
common during transient instantaneous acceleration of the engine. It is
a result of a stack-up of tolerance in the distributor drive gear and
changes in timing chain tension that are caused by localized
acceleration between cylinder events.
Spark scatter detracts from the aggressiveness of
the programmed advance curve. This is because an engine with increased
scatter values tends to drift toward the spark knock zone during rapid
acceleration. Due to the variation in timing, an additional buffer
needs to be built into the spark calculations on conventional
distributors. By reducing timing error not only can additional advance
be used, but the effect that scatter has on power, fuel economy, and
emissions is minimized. Ignition timing error is most apparent during
crank when the slow speed of the engine accentuates the non-circular
motion of any internal combustion engine. Ignition variation during
this time can be skewed up to 30 degrees with a conventional
distributor. With the Opti-Spark the timing variation is a maximum of
one degree of error.
Due to its unique installation location and other
design features, the LT1 ignition offers no mechanical timing
adjustment. The Opti-Spark not only paid dividends in decreased
assembly time during engine manufacturing, but also removes the
inherent tolerance that a mechanically adjustable distributor creates
during engine build procedures and infield service.
Same System, Different Look
many aspects of the automobile industry, the Opti-Spark endured a few
evolutionary changes during its six-model-year run. The changes were
not in how it operated, but to the drive mechanism, housing, and vent
All Opti-Spark systems have a removable
distributor cap that allows access to the rotor. According to our
sources at Jay Fisher Pontiac-GMC who specialize in modifying GM EFI
cars, there are no service parts offered for the early design. The
later-style allows replacement of the cap and rotor as a service item,
but keep in mind that to access these components the water pump needs
to be removed. As good as the Opti-Spark is, routine maintenance is not
one of its strong points due to its location.
Early Opti-Spark systems had a tendency to collect
moisture that would interfere with the function of the optical sensor
and cause the engine to run very poorly. This condition would often
induce cross- and misfire similar to a cracked distributor cap on a
conventional ignition. Two distinct faults were at work when this
occurred. The first was inaccurate primary interrupts from the moisture
affecting the optical sensor's ability to clearly define the leading
and falling edges of the 360-slot wheel. The second was more
rudimentary. Electricity taking the path of least resistance, the
high-energy secondary voltage would sooner follow the moisture down
into the cap than bridge the spark plug under cylinder pressure. The
high voltage would burn through the rotor or the slotted wheel,
stopping the engine in its tracks.
To eliminate this
problem, later designs incorporated a more efficient venting system
that pulled air through the distributor housing from the throttle body.
This was supposed to eliminate once and for all the moisture issue.
changes were made over the years to the engine-side of the housing and
the size and attachment of the drive mechanism. Due to this there are
three different timing case covers used on LT1 engines. Abe Bergian,
motorsports service manager at Jay Fisher Pontiac-GMC, explained that
the 1992 to 1994 versions of the Opti-Spark on both F- and Y-cars had
no vent and used the timing case cover with a small distributor drive
hole. 1994 B-cars used a new timing case and companion Opti-Spark that
had increased venting, a serviceable cap and rotor, and a new-style
drive attachment to the camshaft. This style was then switched to the
F- and Y-cars for the 1995 model year. With the arrival of OBD-II for
1996, the timing case was again modified to accept a crankshaft sensor
for misfire diagnostics but the Opti-Spark was unchanged. The
older-style unvented design can be updated to the final version but
would require a new-style timing case, Opti-Spark distributor, and
changes to the cam drive mechanism. In addition, a longer dowel pin
needs to be installed in the camshaft to drive the distributor. The
first iteration used a traditional short Chevrolet cam dowel pin.
the ranks of the LT1/LT4 faithful a love-hate relationship has been
established with the Opti-Spark. Some owners have experienced a rash of
continuous failures while others have seen the system function
perfectly for 200,000 miles in taxicabs and police cars. Both the
author and GMHTP could not find any reason for the repeated failures
some experience. The question is often posed to GMHTP for a way to
remove the Opti-Spark and convert to another-style ignition system.
This of course can be accomplished with the use of an aftermarket ECU
such as ACCEL's excellent Generation 7.0 DFI that is configured to use
a crankshaft trigger. But if the retention of the GM engine management
system is desired, the use of the Opti-Spark is mandatory. Fortunately
for LT1 aficionados two new products, the LTCC from Ramchargers and the
DynaSpark from Dynotech Engineering Inc, provide a welcome alternative
for those torn between continuously buying OEM GM Optis or converting
to a stand-alone engine management system (See sidebars)
conclusion, it is the author's opinion that the Opti-Spark is an
excellent design that has been a victim of misunderstanding and poor
mechanical procedures. There have also been reports of oil leaks into
the distributor and this is usually the result of neglecting to change
the seals in the timing cover when a cam swap is done. Today's
tight-tolerance lip seals should be changed whenever the timing case
cover is removed, but they often are put back into service and start to
leak shortly after re-installation.
GMHTP we hope that this has given you a newfound respect for this
special ignition system. It offers features and benefits that allow the
stunning performance of the LT1 engine that up until now have not been
DynaSpark: Opti-Spark Made Reliable
The DynaSpark was designed to increase distributor reliability over an
OEM Opti-Spark--which meant redesigning many of the external and
internal components. The case is manufactured entirely of billet T-6061
aluminum plate stock for increased structural rigidity under extreme
heat and cold. It is anodized red for two reasons--corrosion resistance
from the elements and aesthetics. A deeper optical sensor mounting
pocket was used to facilitate air circulation around the sensor for
more effective sensor cooling. This greatly adds sensor life and
accuracy. It is engineered to eliminate the need for the OE, white
rinite insulator, which also eliminates one entire leak-prone perimeter
seal entirely. The bearing bore provides a press fit for the bearing,
which gives improved bearing support and provides a better foundation
for higher rotor rpm capability. Internal component mounting boss holes
are now blind as opposed to the leak-prone, OE pass-through mounting
holes. The case perimeter now has a provision for a captured rubber,
high temp O-ring, instead of the flouro-silicone, leaky OE piece.
bearing is now held securely in place by a hardened steel snap ring
that provides a full 360 degrees of bearing retention surface area, as
opposed to the OE twin-screw, stamped bearing retainer that rusts
heavily, breaks frequently and provides only 40 percent of bearing
retention surface area. The DynaSpark has its own semi-permanently
sealed electrical connector harness tower that eliminates the leaky OE
distributor connector entirely. The distributor drive cavity is now
sealed to the timing chain cover with a rubber, high-temp O-ring to
eliminate water/moisture intrusion into the backside of the case.
case of distributor drive seal failure in the timing chain cover, oil
cannot get forced internally into the case--thanks to a weep hole in
the bottom of the distributor drive cavity that allows the oil to
escape without contaminating the distributor housing.
The entire case is now cross-vented, instead of the plug-prone localized venting of the OE unit.
modified distributor cap and voltage insulator enhances and improves
cross-flow cap venting (which eliminates crossfire/misfire). Other
features include a riveted rotor contact, a lightened rotor, heavier
duty, stainless steel rotor mounting screws, and a semi-permanently
sealed cap that must hold a 15-inch vacuum for 10 minutes before being
allowed to "pass" for shipment. The entire rotating assembly is spin
balanced and the rotor drive is correctly and accurately indexed to the
#1 cylinder. Also of note, the electrical harness is now integral with
this distributor and it comes complete with the new revised vacuum
Dynotech Engineering is working on two other projects: an upcoming Gen.
I distributor for the '92-94 LT1s will share the same upgraded internal
components and refinements of the Gen. II model. And the "Gen. III"
distributor will be rotor-less and capable of 8500 engine rpm. The Gen.
III is designed to work with the LTCC and individual LS1/LS6 ignition
coils and will have all of the same improvements as the Gen. I and II,
but with a billet aluminum cap in place of the OE plug wire cap and a
lightweight reluctor wheel support. Owners of the Gen. I and II
DynaSpark Distributors can always upgrade their current unit at a later
time to the Gen. III unit by purchasing the "high rpm kit upgrade",
which will allow them to utilize individual ignition coils and the
LTCC. You can view the DynaSpark at www.dynotech-eng.com.
LTCC: Opti-Spark Alternative
The LTCC (LT1 Coil Conversion) grew from the need for a high-energy
ignition system for the LT1 that did not use the distributor section of
the Opti-Spark. The optical portion is reliable and is needed to keep
the stock PCM happy. By using the Opti-Spark Hi Resolution and Low
Resolution signals to feed the LTCC, the interface can decode which
cylinder is being fired by the PCM and direct the timing signal (EST)
to the appropriate coil. The LTCC also calculates its own dwell (coil
charging time) and can begin charging the next coil in the firing order
before firing the current coil. This allows full spark power at very
high rpm. The LTCC can be run to 8000 rpm, much higher than most anyone
runs an LT1. For high rpm operation (>6000 rpm), it is a good idea
to remove the rotor from the Opti-Spark as it tends to shatter.
LTCC system consists of an interface unit and plug-in wiring harness.
Other than connecting to 12 volts to feed the coils and the EST wire,
all connections are direct plug-in using OEM weatherproof connectors.
The LTCC features a spark-based rev limiter (2 stage) and a timing
retard that has built in curves for turbo, N2O, and supercharger
applications. Both the retard and rev limiter can be enabled full-time
or triggered. The LTCC has a trigger wire that can be configured to
activate the second rev limiter stage or the timing retard.
In addition to the LTCC kit, the user needs to obtain 8 LS1 coils, plug wires and a way to mount the coils.
wide array of header styles, valve cover dress-ups, and turbo systems
has made the design of universal coil brackets a challenge. As of this
date no truly universal bracket is available. Installations have been
done on top of the intake plenum, and even on the frame rail (race
application). The interface is a gasket-sealed aluminum box and can be
mounted underhood. See the LTCC at www.bailey-eng.com
Is Your Opti-Spark Failing?
LT1 owners have dealt with the Opti-Spark distributor for 11 years now,
but there is still much confusion regarding the causes and symptoms of
a failing unit. Tapping into the PCM with a scan tool is a good way to
start, but sometimes no codes will be set. Before diving into the
Opti-Spark, be sure to verify that your grounds are good and the coil
and wires are not the source of the problem, as they are much easier to
* Car suddenly dies and won't restart
* Starts but immediately dies
* Extended cranking to start
* Rough idle
* Trouble reaching higher rpm
* Black smoke from exhaust
* Poor performance with car warmed up
* Weak plug wire spark
* Codes 16, 36, and 42 may be set
Jay Fisher Pontiac-GMC
PO Box 595