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Ignition systems have moved from breaker points and distributors to coil-on-plug (COP) designs that eliminate spark plug wires entirely. By 2026, COP continues to dominate new gasoline vehicles because it reduces energy loss, improves combustion timing accuracy, and lowers maintenance. Distributor-based systems and conventional spark plug wire sets remain common in older vehicles and some enthusiast applications, especially inline six-cylinder engines like the Jeep 4.0L found in ZJ/WJ Grand Cherokees and MJ Comanches. Replacing spark plug wires on those 4.0L engines is a straightforward task: use quality suppression wires, route them away from hot exhaust components, and follow the factory firing order. Modern high-voltage silicone boots resist heat and ozone, but routine inspection is still recommended. Whether upgrading to a high-energy coil or simply restoring stock performance, choosing the correct ignition components and maintaining clean connections are key to reliable starts, smooth idle, and maximum fuel efficiency.