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Distributor-less Ignition Systems
The distributor has been a key piece of an engine’s ignition system since the dawn of the internal-combustion era. We now have the option of eliminating the tried-and-true distributor as an engine component. Since four, six and eight cylinder all used distributors, it is important to understand what the distributor does. In its early days (the 1980s and earlier), the distributor was a large part of the overall ignition system. In later years (the 1990s through 2000s) the distributor allocated spark to the various cylinders, based on the engine’s firing order. It took about 30 years for this evolution to take place. There is much more to a distributor-less ignition system than simply discarding the distributor.
MPI requires many sensors, about six or seven, typically including an O2 sensor, a MAP sensor, a throttle-position sensor and a crank sensor. The distributor-less ignition system uses some of these same sensors for calculating various advance curves.
The original distributor was considered a points-type unit and was the universal design for engines in the 1950s and ‘60s. There were single-point and dual-point designs, with dual-point units popular for performance and racing applications. The timing mechanism was facilitated by the arm of the points rubbing on the multi-sided cam, mounted on the center shaft of the distributor. The number of sides of the cam is based on the number of cylinders in the engine – eight sides for eight cylinders, and so on. Since the points rubbed on the cam, point gap was very important to performance and wear – requiring constant adjustment. Inside the distributor, the points and condenser were adjusted and replaced regularly. Centrifugal advance was facilitated inside the distributor by a spring and weight assembly. The vacuum advance was actuated by a vacuum pot attached to the side of the distributor housing, with an arm extending inside the unit and attaching to the advance mechanism. The vacuum canister or pot was connected to the carburetor (vacuum source) by a rubber hose. The engine’s initial advance and the engine’s maximum advance were defined by the location of the distributor in the block. Changes were accomplished by rotating the distributor in the block, then clamping tightly in the new position. The specific location was found by using a timing light and aligning marks on the vibration damper.
Chrysler introduced the electronic ignition in 1971. Thekey part of this new ignition was the electronic distributor which replaced the points and condenser of earlier units. The timing mechanism was now done by a reluctor, with points like a star wheel – one for each cylinder. The matching piece to the reluctor was the pickup. The gap between the pickup and the reluctor was important, but no wear occurred as no parts actually touched. These basic electronic distributors (A-engine) became industry standards and were used in all production engines and in performance and racing applications. While the timing function was still done by the distributor, the rest of the ignition functions were done by an engine control unit (ECU). Externally and fully assembled, these electronic distributors looked very similar to the older point-type units. The service cap and rotor packages like P5007856 fit both the electronic units like *P3690430-1-2 and the OEM versions. Caution: there are slightly different caps and rotors designed for race distributors (no vacuum-advance pot on the side, housing not aluminum). While the coils were similar, the electronic system’s ballast resistor could be a dual-ballast which was used in most production ignitions, or a single-ballast (02495531 restoration), for most common performance applications. Mopar® will have three electronic ignition kits coming soon, to convert old point-type ignitions to electronic ignitions: *P3690426 – 383/400 big block kit; *P3690430 (distributor), *P3690427 – 426 HEMI®, 426 wedge, 413/440 big block kit; *P3690431 (distributor) and *P3690428 – small block kit; *P3690432 (distributor). Packages include vacuum advance electronic distributor with high-performance advance curve, wiring harness, electronic control unit, ballast resistor and installation instructions.
Multi-point fuel injection (MPI) was introduced in production applications with the 1984 Turbo 2.2 engine. The fuel system side of the computer requires many sensors including a TDC (top-dead-center) sensor (or crank sensor) which is typically located on the flywheel or ring gear. Both the centrifugal advance and the vacuum advance curves are directed by the computer, along with maximum advance. There is still a distributor, but it is a solid-shaft unit which only distributes spark to cylinders. This design picks up some of the features of the earlier crank-trigger race ignition, with the trigger wheel now around 15 inches in diameter.
The first of the true computer ignitions used in production was the lean-burn system that was introduced in about 1972, developed to comply with stiffer emissions standards. These ignitions used a solid-shaft distributor. In these systems, the advance – both centrifugal and vacuum – was directed by the computer. Also in the early 1970s, on NHRA Pro Stock race cars, Chrysler introduced the crank-trigger ignition which took the timing (triggering mechanism) function out of the distributor and placed it on the nose of the crankshaft, which allowed the trigger wheel to be 7 inches in diameter rather than 1.5 inches. Also in this time frame, Chrysler introduced the dual-plug HEMI® engine (426 Gen II) which used a special dual-distributor drive and two distributors – one for each set of plugs.
The original distributor had many functions including initial spark advance, centrifugal advance, or vacuum advance – as well as distributing the spark to various cylinders through the proper firing order. In many early distributor developments, what changed was the timing mechanism for the spark. Key parts of the distributor are the cap and rotor which need to be changed occasionally. The coil and ballast resistor are part of the ignition system, but not part of the distributor.
The distributor-less ignition was actually introduced on four- and six-cylinder engines, but it really got attention with the introduction of the 5.7L HEMI® Gen III engine in 2003. All 5.7L HEMI engines are distributor-less along with all of the 5.7’s siblings – the 6.1, the 6.4, the 392 and the 426 aluminum. This means that there is no distributor and no distributor hole in the block. All of the advance curves and the initial and total-advance functions are done in the computer, for the 2006 5.7L Charger/300 with the 5.7L HP cam. Another feature of distributor-less ignitions is the use of multiple coils – typically one coil per spark plug or one coil per two plugs. Normally, the coils are mounted very close to the plugs themselves.
The performance parts offerings for complex electronic systems are constantly changing based on customer requests/requirements and racing applications. For ignitions and fuel control electronics there are many, many options and I have only discussed some of the more popular ones. For more information and added details, please contact the Mopar Performance Tech Line at: 1-888-528-HEMI (4364) or go to the website www.mopar.com.
*Not legal on pollution-controlled vehicles or vehicles registered for highway use.