You tried to do the right thing when you picked up a '91 to '95 Wrangler with the fuel-injected 4.0L. The 242ci H.O. (high-output) engines debuted in the '91 YJ, and they delivered more power, improved fuel economy, and crisper throttle response, not to mention the way-better AX-15 gearbox.
Maybe it's the square headlights or "Yuppie Jeep" translation of the model acronym, but EFI YJs have gotten the short end of the stick when it comes to engine performance mods. Case in point, look at intake systems. Probably the easiest power mod in the book, there are a dozen or so companies pumping out intakes for '97-and-newer TJ Wranglers - but zip, zero, nada for the YJ. That's about to change, as AEM drops its YJ Brute Force cold-air induction system on a well-deserving group of enthusiasts.
The Brute Force intake features a four-layer cotton gauze AEM filter, a CNC mandrel-bent 6061 aluminum intake tube, and a powdercoated heatshield to keep the charge air cool as it enters the system.
Looking at the stock intake system, its confining box design and straw-like intake tube screamed high restriction, so we were expecting to ring up some big numbers on the dyno. Our '91-vintage tester baselined at 145.1 whp at 5,400 rpm and 172.7 lb-ft of torque at 4,000 rpm. The stock box peeled away from the engine with ease, and the AEM system took a little more time to install due to the heatshielding, but it was a straightforward affair, requiring only handtools. Back on the rollers, the AEM system started delivering the goods the moment we hit the sample button. In-depth dissection of the curve shows a 10hp gain at 4,600 rpm, with improvement continuing to register up to peak, where output jumped an impressive 14.9 ponies to 160.0 whp. On the torque side of the power equation, a peak gain of 10.5 was achieved at 4,000 rpm (183.2), but the biggest gains plotted higher up the powerband. Where the stock system gave up the torque on the top end, the Brute Force allowed the 4.0L to hang on better. At 5,300 rpm, torque went from a baseline of 143.1 to 157.7, a 14.6 lb-ft bump.
On the road, the engine feels even more lively at the top of its rev range, conveying a sense of determination that was not previously evident. The next question is, Will this product be the first domino to fall on the way to a new era of acceptance for the YJ? Note to industry: EFI YJ six-cylinder replacement valve cover needed.