Precise ignition timing is essential for high-performance engine tuning and ensuring your small-block or big-block is delivering every horsepower within its capability. Since the era of poodle skirts, flattops, and Ed Sullivan, that’s been handled by a camshaft-driven distributor.
A gear at the rear of the camshaft meshes with another on the distributor shaft, spinning it and a rotor beneath the cap (at half the speed of the crankshaft’s rotation) triggering—or distributing—electrical energy from the ignition coil to each spark plug per the engine’s firing order. As mechanical systems go, it’s a feather in the cap of man’s engineering prowess, particularly when you consider the speed engines are capable of achieving.
There are cons to this type of ignition system, too. Regardless of whether you’ve ditched original-style points for an electronic conversion or changed to an HEI-type distributor, timing accuracy typically erodes over time due to wear in the associated components. The cam and distributor shaft gears wear after constant contact, for example, while a stretched timing chain at the front of the engine can affect the camshaft’s rotational relationship to the crankshaft, throwing off the timing. And at high rpm in a performance engine, the camshaft can deflect inside the cylinder block and “spark scatter” inside the distributor cap can affect the timing and spark distribution, too, affecting timing in both cases.
Of course, you can always adjust your engine’s timing periodically, but that doesn’t always address high-rpm timing issues and doesn’t help at all with spark scatter. A more contemporary method of ensuring spark-timing accuracy across the rpm band is basing it on the crankshaft’s rotation rather than the camshaft. Such “crank-trigger” systems have been used by car manufacturers for nearly 30 years—and while converting a classic small- or big-block to such a system isn’t a new idea, it’s one that is growing in popularity because of the timing precision it offers for high-performance engines.
And let’s be clear here: A crank trigger system offers the greatest benefit for higher-horsepower engines designed for high-rpm performance. We’re talking mostly about engines used primarily on the strip or circle track, but you’ll still see the benefits of more precise timing and less need for periodic adjustments when used on a street engine.
Another benefit of a crank-triggered ignition is the opportunity to move to a distributorless system such as the OE-style coil-per-cylinder design used for years now on production vehicles. In fact, if you’re running EFI, the controller will handle the job of the distributor, sending the spark energy to the appropriate plug, thereby eliminating the need for the distributor altogether and allowing for a more contemporary appearance of your classic small- or big-block.
1. How does a crank trigger work?
The systems are relatively simple in design and similar to how OEM ignition systems are triggered. Basically, a trigger wheel with spokes, studs, magnets, or holes is mounted to the harmonic balancer and spins with the rotation of the crankshaft. As it does, the spokes, magnets, or holes pass a pickup sensor that sends a signal to the ignition box or EFI control unit indicating the rotational angle of the crankshaft. That crank-angle signal triggers the precisely timed spark for the appropriate cylinder.
Typical crank trigger conversion kit shown—this one is from FAST. They generally include the trigger wheel that mounts to the outside of the balancer, a signal pickup sensor, harness connectors that run to an ignition box and/or an ECU, and the mounting hardware. Pretty simple stuff. Unlike other systems that use magnets imbedded in the wheel, FAST’s system features tabs on the outside of the wheel that are read by the inductive sensor. It’s a design similar to OEM systems.
MSD’s “flying magnet” crank trigger design is the opposite of FAST’s, with magnets in the wheel rather than the pickup. They pass a non-magnetic pickup to trigger the ignition signal, which the company claims offers greater accuracy because there is less chance for false triggers.
The pickup sensor is shown here in line with one of the embedded magnets in MSD’s system. The sensor “reads” the magnet as it passes by, triggering the ignition signal. Each system has an air gap specification between the sensor and wheel that must be set during installation to ensure the trigger signal is picked up accurately.
2. What about an LS-style 58x ignition system?
Chevrolet Performance, Holley, and other aftermarket manufacturers offer crank-trigger ignition systems based on the 58x, or “60-2,” trigger systems used on later LS engines. In contrast to the previously described trigger wheels, the wheel in this system uses essentially one tooth for each six degrees of rotation. The sensors read the space where the two “missing” teeth would be to determine the crankshaft angle. This system works pretty much the same and is just as effective or more so than the other systems, but requires an EFI controller to direct spark in a fuel-injected application or an aftermarket ignition box such as the MSD 6LS-2 for carbureted engines. They’re designed for use with LS-style individual ignition coils—aka coil-on-plug.
This is one of Holley’s LS-style crank trigger systems, which uses a 58x, or 60-2, trigger wheel. It also uses a Hall effect pickup sensor rather than a magnetic pickup. As with most electronic distributors, the sensor acts much like a switch, because it produces a constant voltage signal that is interrupted by the presence of a magnet. In this case, it’s the ferrous metal wheel’s teeth. The two missing teeth serve as a reference for determining the No. 1 cylinder.
Here’s a 58x trigger wheel—also known as a reluctor wheel—installed on a big-block Chevy engine. It’s from Chevrolet Performance’s kit and is similar in design to the Holley system; and like it, it mounts on the crankshaft hub behind the balancer. That’s a significant difference from other bolt-on systems that mount on the balancer. Regardless, the crankshaft pulley is pushed out marginally, which affects the accessory drive system.
When the engine turns over at start-up, the trigger sensor “reads” the missing teeth on the 58x reluctor wheel to determine the position of the No. 1 cylinder in order to initiate ignition. It’s similar in design to earlier 24x systems, but provides more references to the ECM for more precise data on the crank angle position.
Chevrolet Performance’s big-block kit (PN 19260247) is designed for sequential-type EFI systems, including a specific timing gearset with 4x cam gear and complementing sensor. The kit can be used with non-sequential and carbureted engines, too. It also includes a specific timing cover with sensor mounts.
3. What happens to the distributor?
The spark-timing function is eliminated, requiring the removal of the points or Hall effect sensors. Converting to a crank trigger also means locking out the centrifugal advance of the distributor, which means if you want to run a timing curve in the engine, you’ll need to do it through a programmable ignition box or, with EFI, via the calibration software. The distributor can then be retained to serve as the spark energy conduit between the ignition coil and the spark plugs. When doing so, it also retains its all-important oil pump-driving functionality, as it continues to be driven by the camshaft.
With the crank trigger system installed, the distributor is retained to drive the oil pump and transfer spark energy from the coil to the spark plugs. Its ignition-triggering “guts” must be removed. It can also be removed entirely for a distributorless system.
4. How do I make timing adjustments with a crank trigger?
Without the distributor directing the timing, the adjustments come by altering the position of the pickup arm to advance or retard the spark. That is typically done with the engine off rather than the conventional method of moving the distributor while the engine is running. The standard timing pointer is still retained and a timing light is used to check the settings with the engine running after the adjustment has been made.
The pickup sensor shown here on MSD’s flying magnet system is similar to others in its adjustability. By sliding the sensor fore or aft on the mounting bracket, the timing of the trigger signal is altered. Simple.
Installation of a crank trigger doesn’t change the need for a timing light and pointer on the balancer for making adjustments, but rather than adjusting the distributor while the engine is running, the adjustment is made with the pickup sensor when the engine is off. Then, you start the engine, take the reading and make adjustments accordingly. And yes, it’s a little more time-consuming this way.
5. So, can I remove the distributor entirely?
Yes, you can convert the engine to a distributorless—coil-on-plug—ignition system with a crank trigger, but you’re going to be doing it with an EFI setup because you’ll need the ECU to distribute the spark energy. An ignition box such as an MSD 6AL, for example, works only with a distributor. The other consideration is how to drive the oil pump, which is an issue when you yank out the distributor—but more on that below.
Although it’s possible to convert a small- or big-block to a distributorless system, spark direction can’t be handled with a crank trigger by a conventional aftermarket ignition box. MSD’s 6LS series boxes, however, can handle LS-style 24x or 58x systems that also incorporate a camshaft sensor. If you adapt that type of trigger system, the box can be used with a carbureted engine. Otherwise, you’ll need EFI and a compatible ECU to convey the spark signal.
6. Is it difficult to convert to coil-on-plug?
Not really. The coils and mounting brackets are readily available and getting less expensive all the time. You’ll need the appropriate ignition box, as mentioned above. With that, it’s merely a question of wiring; linking the crankshaft and cam sensors from the 58x-style trigger system to the ignition box, which handles the signal distribution to the individual coils. After that, you’ve gone distributorless!
The connectors from the crank trigger kits make conversion to a fully distributorless ignition a simple plug-and-play proposition. An EFI system may require calibration refinement when used with a camshaft sensor, but today’s self-learning systems will typically set the timing perfectly, based on the initial start-up—assuming the sensor is positioned correctly.
Removing the distributor means replacing it with individual coils, like those used on LS engines. In fact, the LS coils are perfect for small- and big-block engines; and they’re good for about 1,000 horsepower. All that’s required is an appropriate mount on the valve covers or somewhere else near the spark plugs. Try eficonnection.com for mounting brackets. The system shown here has custom mounts.
7. How do I drive the oil pump without a distributor?
The simple solution is installing MSD’s oil plug (PN 8513). It was originally designed for a front distributor conversion on big-block engines but it serves the purpose for simple crank trigger conversions on small- and big-blocks. It simply drops in place of the distributor to seal the hole in the intake manifold and features a shaft gear that meshes with the cam gear to drive the oil pump like the distributor. Easy stuff.
MSD’s oil plug (PN 8513) simply drops into the distributor hole in the intake manifold on small- and big-block engines. An adjustable collar enables it to fit engines with non-standard deck heights. It also has a bronze drive gear so it’s good to go with roller-type camshafts.
8. What is a “cam sync”—and do I need one?
A camshaft synchronization signal—cam sync—tells the controller which cylinder is No. 1 to ensure the proper ignition sequence and is only required on EFI-equipped engines running production-style sequential injection. That means you don’t need to worry about it for a carbureted engine or one running a throttle body-type EFI system.
Got one of these TBI systems or a carburetor? You don’t need a cam sync sensor in addition to the crank trigger system. You will if you’re running sequential or port-type injection.
With most EFI systems, the controller will take the crankshaft signal and send it to the spark plugs via the individual coils in a distributorless setup. In that regard, no additional ignition box is required, but not all controllers have an ignition driver, so it’s important to double-check before swiping your credit card for one.
9. Are there any special installation requirements for a crank trigger system?
Yes. The crank trigger wheel may affect the drive accessory pulley spacing, which is something you’ll want to check before bolting it on your engine. And on Chevrolet Performance’s big-block system, the instructions call for slight machining of the block and balancer. Also, if you’re changing to coil-on-plug, you’ll need to mount the coils on or near the engine, but with the aftermarket brackets available, that’s a bolt-on affair.
Regardless of the system type, adding a crank trigger to a small- or big-block will push out the position of the crankshaft pulley when it’s reinstalled. That means spacers or shims will be required to properly align the accessory drive system. It’s unavoidable.
Chevrolet Performance’s big-block system uses a keyway in the reluctor wheel for effortless alignment with Top Dead Center, but it also requires a keyway-equipped crankshaft and damper—something to consider if your engine isn’t already so equipped.
Apart from the somewhat hidden sensor locations, there’s no external clue on this big-block that it’s running a contemporary 58x-style crank-triggered ignition system. It’s mounted behind the damper and under the unique timing cover.
10. In the end, will a crank trigger ignition deliver more power?
In a word, no. Its advantage lies in offering greater consistency with your tune and less need for timing adjustments, especially for high-revving engine combinations. For many—like us—that’s all the reason we need.
This project engine still wears the distributor, but will be fully converted to a coil-on-plug system to make the most of the crank-trigger ignition’s capability. In this case, rather than more power, the goal was more precise timing across the rpm range, with less need for periodic adjustments.
Opti-Spark—What the Hell was That?
The introduction of the second-generation LT1 engine used in late C4 Corvettes, 1993-’97 Camaro Z28s, and the Caprice/Impala SS brought a unique and often troublesome ignition system dubbed Opti-Spark—although its official name was Angle Based Ignition Timing System (ABITS).
Anyone familiar with those vehicles recognizes the Opti-Spark module, which looks like a distributor cap mounted on the lower front of the engine. It was sort of a cross between a conventional distributor and crank trigger technology, using a “signal disc” to report the camshaft angle to the engine controller.
The signal disc was driven by the camshaft. As the disc rotated, an optical sensor read two rows of slots on the disc. The first, or outer, row featured 360 same-size slots—one for each degree of rotation—while an inner row featured eight different-sized slots the engine controller identified as the different cylinders. Like a conventional distributor system, Opti-Spark also used a rotor to distribute spark energy from the ignition coil.
GM initially touted the system as a “100,000-mile” ignition system, but real-world driving typically proved otherwise. Its placement low and at the front of the engine made it especially susceptible to heat and moisture—and early models weren’t even sealed and vented. As such, they were notoriously unreliable, and when the distributor required servicing, the crankshaft pulley and water pump had to be removed. Ugh.
A smart idea poorly executed, the Opti-Spark will forever rank as one of the General’s darker moments in ignition engineering.
Early fourth-generation F-bodies, along with the LT1-powered 1994-’96 Impala SS and the 1992-’96 Corvette used the unique front-of-camshaft-driven ignition timing system known as Opti-Spark, which suffered poor reliability and complicated maintenance requirements.
Replacement Opti-Spark distributors such as this one from ACCEL (Holley) are worlds better than the original with higher-quality bushings and seals, along with proper venting. It’s rare that a car is still running its original, but if so, preempting the inevitable with a replacement is a wise idea. MSD also offers a billet unit with adjustable timing.