Ignition
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Ignition Upgrades

The stock 4.0 ignition system is already very good right out of the box but there are a few upgrades that can be carried out to improve its reliability and longevity. A poorly-functioning ignition system will not completely burn the air/fuel mixture in each cylinder resulting in reduced performance, gas mileage, and poor emissions. Ignition system upgrades will not, by themselves, produce a horsepower/torque gain but they will regain HP/TQ that was lost due to poorly-functioning components.
The '99+ Jeep Grand Cherokee WJ and the '00+ Jeep Wrangler TJ/Jeep Cherokee XJ 4.0 engines were upgraded to a distributorless ignition system with individual coils per cylinder, so the ignition upgrades (except for the spark plugs) outlined below do not apply to these models.

Autolite Copper Core Spark Plugs

Part no. FRM-985
These Autolite copper core spark plugs feature a cold-formed steel shell with precision-rolled threads, a full copper core electrode to provide better performance, and a one-piece terminal post to add strength. Autolite copper core spark plugs use a copper-glass seal that bonds the insulator and terminal post center electrode together. This ensures full voltage at the spark gap and a 100 percent leakproof, gas-tight seal.
Autolite Professional Series Spark Plug Wires

Part no. FRM-96624
These Autolite spark plug wires are designed to meet or exceed OES design requirements by providing the necessary silicone content, boot design, and accessories (i.e. retainer clips, heat shields, and extension bases) where required. The high-tech nylon reinforcement in the wires provides increased terminal retention and improved jacket adhesion. The tougher silicone jacket is less likely to tear away from the insulation.
Accel "HD Black" Distributor Cap

Part no. ACC-120330
Accel distributor caps are the best way to help out your ignition. The compression molded caps with brass terminals are stress relieved immediately after molding in special fixtures to help ensure dimensional stability. The OEM distributor cap is good for 60,000 miles but has aluminium terminals that have a lower electrical conductivity than brass, resulting in a lower energy spark.
Accel Performance Distributor Rotor

Part no. ACC-130229.
The high conductivity corrosion resistant brass inserts and stainless steel rotor springs provide unrestricted spark energy flow.
MSD Blaster Coil

Part no. MSD-8228.
MSD Blaster 35,000V coils are designed with improved materials and windings to produce a stronger spark. This improved spark helps burn the fuel mixture more efficiently, resulting in a smooth idle and improved mid-range rpm power.
The OEM coil is a reliable and durable unit, therefore consider the MSD Blaster coil as a good stock replacement.