When you look at the CJs of yore or even the YJs that preceded them, TJ Wranglers
could possibly represent the best build fodder the off-road enthusiast was ever presented. Unlike the YJs there were no awkward early years with undesirable drivetrains. Right out of the gate in '97 until the model was discontinued in 2006, Jeep's venerable and ubiquitous 4.0L six-cylinder was offered with very little change. Beginning in 2000 Jeep ditched the simple, but crack-prone tubular exhaust manifold for one with integrated, twin catalytic converters, which makes header upgrades somewhat more expensive. Also in 2000, all Jeep 4.0L engines went to a coil-on-plug ignition system, thus doing away with the distributor and plug wires, and then in 2005 the 4.0L got a revised throttle body. Otherwise through the model run, power and torque numbers on the 4.0Ls are similar, with no significant changes. Four-cylinder engines were the sturdy and almost-adequate 2.5L until Jeep's new DOHC 2.4L appeared in 2003, upping the base engine's available power from 120hp/181lb-ft to 147hp/190lb-ft. Regardless, either four-cylinder is disappointing once a lift and tires are added, so it's still best to hold out for a 4.0L model if you plan on bigger tires, adding armor, and doing more than rock crawling with your TJ.
Read more: Jeep Wrangler TJ Buyers Guide - Jp Magazine