Top Left - Stock 4.0L throttle body
Top Right - Stock 2.5L throttle body
Bottom - Rubicon Express 62mm throttle body
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.