Back in 1991 when Chrysler introduced the H.O. 4.0L in Cherokees and Wranglers, it was a marked improvement over the offerings of the previous year. The Wrangler had suffered with the electronically controlled, carbureted 4.2L, while the Cherokee had it a little better with the injected 4.0L that used the French Renix computer and fuel-injection system.
As good as the later Chrysler-based 4.0Ls are, they can be made better with a few simple bolt-ons. If you read the four-cylinder power story in this issue, "Four-Banger Power," then you'll realize why we elected not to dyno our vehicle after each part was installed. Instead, we trundled (does anyone really trundle? Actually, we weaved through traffic like a lunatic) down to The Dyno Shop in Santee, California, to generate our before-and-after power numbers.
For the most part, save an intake change here or a cylinder head revision there, H.O. 4.0Ls are fairly similar and will respond well to the same modifications. Even if you're a pre-'91 4.0L Cherokee owner, some of these modifications will apply to you and will help you make some decent power. Here's Jp's formula for an extra 20 hp and 18 lb-ft on a 130,000-mile 4.0L.
Step 1: Spark and Arc
There's no point in making mods that will cram more air and fuel into the engine if you don't have the spark to touch it off. We called Performance Distributors for one of its 4.0L Firepower Ignition Kits for '93-'99 4.0Ls (PN 4.0KIT). The Firepower kit includes a set of the company's excellent low-resistance LiveWires plug wires, a Screamin' Demon ignition coil and a new high-performance cap and rotor. We also ordered a wiring harness adapter for use with '98-'99 4.0Ls (PN 31728-AH) that hooks the Screamin' Demon right to the factory coil connector with no crimping or splicing. The Firepower ignition will allow you to open your plug gap up to 0.065 inch. We opened ours to 0.055 inch.
Seat of Pants: Cranking time during startup was reduced to less than a full engine revolution and idle quality seemed to smooth out. No real power increase, but engine felt smoother throughout power curve.
Step 2: Suck Big Time
Next, we removed the factory airbox that we had installed for our baseline dyno numbers and reinstalled the K&N FIPK (Fuel Injection Performance Kit), PN 57-1518, that we've been running for about 18 months. The K&N FIPK replaces the entire stock airbox and ducting. The kit comes with everything you need, including the heat shield, and is 50-state emissions legal. It installs easily, and the high-flow filter and duct work increase and straighten the air flow to the engine. We cleaned the filter about 10,000 miles before our dyno testing, but K&N claims they actually flow slightly better with a little gunk on 'em, as long as they're not totally obstructed.
Seat of Pants: Way noticeable power improvement from off-idle to redline. Engine spools up much faster and readily climbs above 5,000 rpm. Power you can feel.