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.
Step 3: Smoking Pipes
We did a lot of research when choosing a header and after-cat exhaust system for our Cherokee. We needed 50-state legality, we wanted complete stainless steel construction and we insisted that it not be loud. In our opinion, there's nothing worse than a loud exhaust on anything except a V-8. We found exactly what we were looking for in Gibson's stainless 4.0L header (PN GP400S) and after-cat exhaust (PN 617200). Since we're lazy, we took our Cherokee to Gibson in Corona, California, where technicians Guy Parsons and Shawn Seidelman installed the header and exhaust in about three hours. Not surprising, since Parsons is the company's header designer and Seidelman designs the after-cat systems.
The header hooks to the downpipe with no problems, but on later H.O. engines with the rounded intake manifold, you'll need to make sure the intake and header aren't contacting each other as the bolts are torqued down. Gibson's install instructions should cover everything the do-it-yourselfer will need to know.
The stainless steel after-cat exhaust features a mandrel-bent tailpipe, a polished stainless exhaust tip and a stainless steel chambered muffler. Allow us to gush here for a moment. This exhaust system just floors us with its power and sound levels. From inside the cabin it's every bit as quiet as the stock exhaust, with only a slight burble noticeable from outside the vehicle when idling. The fitment is great, with no rattling or contact anywhere.
Seat of Pants: Dramatic difference in power. Off-idle response is very snappy and almost violent, with much improved acceleration across the board. Feels like there's much more low-end torque as well as high-rpm horsepower.
Step 4: Big Mouth
Most people associate Rubicon Express with its line of suspension systems, but the company also carries one bitchin' 4.0L throttle body. The Rubicon Express 62mm throttle body (PN RE1062) is milled from a solid chunk of billet aluminum to exacting tolerances, then it's anodized bright red. It comes almost ready to bolt on. You just need to transfer your TPS (Throttle Position Sensor), IAC (Idle Air Control) valve and MAP sensor. A block-off kit is included in case your stock throttle body doesn't have a MAP sensor. The company even includes the proper Torx wrench for the silly factory anti-tamper screws.
The throttle arm has provisions for the automatic tranny kick-down linkage, cruise-control cable attachment and two different throttle cable attachment points: one for stock-like throttle engagement, and another for increased off-idle throttle resolution. This is where we attached ours, and it basically just makes the butterfly open a little faster off idle for better acceleration.
Seat of Pants: Increased throttle opening really felt when punching throttle from a dead stop. Very little power difference felt in midrange, but more noticeable in upper rpm ranges.
Step 5: Computer on the Brain
The all-time easiest install we've ever tackled is the Hypertech Power Programmer III for '96-'04 Wrangler, Cherokee and Grand Cherokee 4.0Ls. You simply plug into the diagnostic port under the driver-side dashboard, turn on the Power Programmer and follow the instructions on the screen. The Power Programmer allows you to do lots of things, so here's the list: You can choose between two different engine calibration programs for 87 or 92 octane, adjust your top speed limiter to 116 mph or 128 mph, increase the rev limiter in 100 rpm increments, adjust the speedometer for different tire sizes and even read and clear factory DTCs (Diagnostic Trouble Codes). When you need to take your vehicle in for servicing, you simply plug the Programmer back in and return the vehicle to stock settings in a matter of minutes.
The Power Programmer is really calibrated to work on stock vehicles, so the gains on our modified vehicle wouldn't be as noticeable as on a stock vehicle, but we installed it anyway. We entered the 92-octane tuning, increased our top speed limiter to 128 mph and increased our rev limiter to 5,900.
Seat of Pants: No seat-of-pants difference noticed, but power runs up steep hillclimb noted consistent 5-mph improvement at top of hillclimb over modified vehicle with stock computer calibration.
Step 6: Back to the Dyno
Once again, Dyno Shop owner Mark MacNeil strapped our rig to the rollers to see what our efforts had accomplished. Unlike our Wrangler, which went severely rich in the mid-rpm levels, our XJ went over on the lean side right in the sweet spot of the power curve. Leanness builds horsepower to a point, but it can also kill engines. Our air/fuel ratio was dancing close to dangerous levels, with a high of 14.3 recorded from about 4,000 to 4,600 rpm. Ideally those numbers would be closer to 12:1 at wide-open throttle. We could have tried an adjustable fuel-pressure regulator to get a little more fuel in the engine, but our later '99 engine features a returnless fuel injection system that would require dropping the fuel tank. Too much hassle.
Again, MacNeil suggested the Unichip his company installs as a way to optimize the air/fuel ratio and timing events for an increased 10-percent more power and torque over what we generated. In the long run, that's probably what we'll do, but for now we'll just be careful if we hear detonation and make sure to always run premium fuel.