Ignition Design
Words: Larry Shepard
The premise that all engine components must work together as a team has been discussed in many previous Tech Insider columns. By the time you get to the ignition, you have all the parts for almost a complete engine so you may be in a hurry to finish the project, but the ignition components should be considered just as closely as all of the other engine parts—even though they may not have the high-ranking images of camshafts or cylinder heads. In a greatly over-simplified statement, the ignition’s job is to light the fuel-air mixture in the combustion chamber by jumping a spark across a gap in a plug within the chamber. The chamber itself and its related shape and compression ratio is defined by the cylinder head, block, piston (top), crank (stroke and displacement) along with related tuning issues like air flow (intake and exhaust manifolding) and camshaft/valvetrain (lift, duration, overlap). There are issues relating the ignition to the engine’s output, rpm, but I’ll try to focus on general application to help limit the size of the article and make it easier to understand. A general rule to follow: as you increase your engine’s output and peaking rpm, you will need to continually upgrade the ignition system.
First, let’s look at the newer engines like the 5.2L/5.9L Magnum® (1992–2003) and the current 5.7L/6.1L HEMI® (Gen III). These newer V-8 engines are MPI engines which means that they use multi-point fuel injection. In these MPI engines the fuel injection computer also controls the ignition. So a high-performance (HP) ignition requires a HP controller. These engine controllers tend to be very specific as far as model year, engine size and general hardware like P5153734AC, for the 360 MPI kit. This part number is mainly designed for crate engines rather than production vehicles. Controllers are also called “EMS” which stands for Engine Management System. For the 5.7L and 6.1L HEMI engines (crate engine versions or a similar approach) there are programmable EMS units like P5153608 (carb induction) and P5153528 (electronic fuel injection) which allow more flexibility.
From here on, I will discuss the older engines that use carburetors for induction and a separate ignition system. The key component in the ignition system is the basic distributor. The distributor determines whether you use a point-style ignition or an electronic-style ignition. I would strongly recommend the electronic. Points were used in the distributor in both production and racing applications up through the early 1970s when the electronic ignition was introduced in small blocks and then big blocks. Point-systems have problems with high-energy, high-rpm and hi-output conditions and require constant maintenance. The electronic system opens the door to solutions to these issues. The small block electronic distributor is P3690430 (273/318/340/360) while the big block electronic distributor is P3690432 (413/426W/426H/440). The distributor’s location is dictated by the cylinder block and each is driven off the camshaft. The electronic distributors listed above are made of die-cast aluminum and feature a vacuum-advance unit on the side. There are also one-piece billet aluminum electronic distributors designed mainly for racing, like P5249775 (big-blocks), and these do not have any vacuum-advance. Note: an interesting side-light is that all of the HEMI Gen III engines (5.7L, 6.1L etc.) use a distributorless ignition. This is not a concern with the production-based computer/controller (MPI – multi-point-injection) but if you want to use a carburetor and a standard ignition, a special front cover which has a boss for a distributor is needed.
The real heart of the electronic ignition system is the control box. The control unit defines the rest of the parts and how they are wired but an electronic control unit requires an electronic distributor. There are two popular control units – the orange box P4120505 (street applications) and the chrome box P4120534 (racing applications). In making an ignition selection, I would strongly suggest starting with a kit – like P3690426 (small blocks) or P3690428 (all big blocks including 426 HEMI). These kits include the electronic distributor, the control unit and wiring harness and will save you money. There are many special upgrades/racing related control boxes like the MSD 6, P5155516, that are available but a detailed description is beyond the scope of this basic story.
Even though the distributor rotor does not touch anything inside the distributor cap as it rotates, it is a good idea to replace the cap and rotor together on a regular basis – like once per year. The current that passes from the rotor to the individual plug wire terminal causes deposits and arcing. Higher energy ignitions tend to aggravate the problem. One new high-performance distributor cap and rotor for 426 HEMIs, big blocks and small blocks is P5007856 (light brown in color). Another cap and rotor for the 1992–97 Magnum V-8 is P4876255AB (black). While most V-8 caps look similar, they are not interchangeable. Inspect them closely when replacing.
Inside each distributor is a set of small springs that are part of the distributor’s advance mechanism. Recurving the distributor, defined as changing these springs, was quite popular in the 1960s and ‘70s but doesn’t seem to be on many performance-tip lists today. In the muscle car era, installing the fast-advance springs P2932675 into various distributors was quite common. A typical production distributor might not become fully advanced until 3,500/4,000 rpm. The high-performance distributors similar to those mentioned above featured a faster advance curve and may have been fully advanced by 2,000 rpm. The fast-advance springs were designed for racing and got to full advance around 1,500 rpm. The racing aspect of the advance hasn’t changed but the street application has. The fuel/gas that is available for street use is now oxygenated and premium is only 93 octane – at best. This means that you need a curve that is somewhat in between the ones listed. You want the advance at 3,500 rpm but you have too much at 2,000 rpm. To fine-tune the curve, you need the timing curve adjustment kit P5153446, which has six primary springs and three secondary springs plus more hardware.
The early production electronic ignitions used a dual ballast resistor P4529795. Most of the HP electronic ignitions used in the last 30 years use a single ballast resistor (1 ohm) P5206436. The exception is when the race coil P3690560 is used when you need a special ¼ ohm resister P2444641. For the coils themselves, there is a resto coil 02495531 which looks just like the original production coils, and the race coil P3690560 plus the blaster coil P5155514. There are many aftermarket coils available. Check with the manufacturer for the proper ballast resistor with any specific coil. Some special race control boxes require special coils plus unique wiring or ballast resistor selection. Follow the manufacturer’s recommendations. To get the current (amps) from the coil and distributor to the spark plug, you need spark plug wires. There are many styles for many different applications. The typical production wire is about 7 mm. Some high-performance wires are 7.5 mm and some race wires are 8 mm or larger. The resto wires are fully assembled and orange in color, like the originals on HP engines. For example the 440 big block set is P4529792 and the 340 ones are P4529797. There are 7.5 mm HP wires for each engine but my favorite is the 426 HEMI set P4876775 because of the very unique spark plug ends used on the 426 HEMI. The 8 mm pre-cut set like the big blocks (383-440 wedges) features radio suppression. Other versions are available. For custom length applications use P4529032 wire set and cut to length. Tip: replace the plug wires on a regular basis – perhaps ever 1-2 years, depending on the number of plug changes. If you use the wire removal tool P5249718 perhaps you can double this cycle.
The spark plugs fit into the cylinder head and the thread length is defined by the head. There are two basic styles: the 3/4" thread as used in the small blocks (318/340/360) and the 426 HEMI, and the short 3/8" thread as used in the big block wedge engines (383-440). For general street use, use a 12 heat-range plug like P4876926 (the 3/4" thread) or P4876927 (the 3/8" thread). Both are sold in sets of four.
One of the little tricks to the ignition system which is often overlooked is the voltage regulator. There are two basic styles of regulators: the electronic, introduced in 1970, and a mechanical one used in 1969 and earlier vehicles. The basic electronic voltage regulator for the 1970 and newer cars is P4529794. The constant output version is P3690731 (electronic). The trick comes in on the earlier 1969 and older models that use a mechanical voltage regulator. For most dual-purpose applications and general bracket racing, I would recommend using the production mechanical regulator, available at your auto parts store. For serious racing you can upgrade to the constant output model P3690732.
Mopar offers several more coils, controller units, computers, plug wires, wiring harnesses and starters. For additional information, refer to the latest Mopar Performance catalog or visit www.mopar.com Space limits my discussion; if you have specific questions, please call the Mopar Direct Connection Tech Line at 1-888-528-HEMI (4364).
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Mopar Tech Insider Ignition Design - PDF
Larry “Shep” Shepard is a retired Mopar Performance engineer, author of numerous Mopar Performance books and a Michigan Motor Sports Hall of Fame inductee.
