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1991 Jeep Wrangler YJ - 4x Forum

Posted in Features on May 1, 2002
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Restricted Sixes: The Final Chapter
Q: This is in regard to the Nov. '01 and Feb. '02 "4xForum" topic Restricted Sixes. The reader asked about a restrictor cone in the air intake tube of 4.0 inline-sixes found in many Jeeps. I was surprised to see that a company such as Turbo City, which sells a custom air intake didn't know about this commonly removed piece in the stock equipment. It is visible if you take off the airbox lid as though you were going to change the air filter and look into the air tube. It is easily removed by disconnecting the air tube from the airbox lid, giving access to the cone. You then use a couple flat-head screwdrivers and an extra hand doesn't hurt to remove the cone. If you haven't yet, you should drop in a K&N filter element and enjoy the extra power. I have done this several times but was only able to personally test before and after results on one of the vehicles. The vehicle was a '91 YJ, I believe, with a K&N filter element and an MSD ignition, there was definitely a seat-of-the-pants improvement in the upper rpm power, and you could hear a difference in the engine. I hope this gets to your readers to help out some more Jeepers, even though I'm a Toyota man myself. Keep on 'wheeling.Jeff Nevesvia e-mail

In the Nov. '01 and Feb. '02 "4xForum," there is a discussion about removing the restrictor in the air inlet tube on various Jeeps to increase airflow and horsepower. The restrictor is part of the emission system's evaporative control system, and it should not be removed, with good reason. There is a charcoal canister (it looks like a painted tin can) that traps gasoline vapors from the fuel tank when pressure builds up in the tank. When the engine is turned off, the charcoal in the canister stores these vapors. When the engine is started, the restrictor creates a venturi effect, which creates a low-pressure area in the intake system that allows the trapped gasoline vapors in the charcoal canister to be fed into the engine and properly combusted. You'll notice that there's a vacuum hose just behind the restrictor that goes to the canister. Without the restrictor, there won't be sufficient pressure differential created to properly evacuate the canister of stored vapors, and the charcoal will eventually become saturated, allowing the vapors to ultimately escape into the air. At the risk of sounding like an ecological extremist on a soap box, it is both illegal and irresponsible to remove the restrictor, regardless of how much (perceived) horsepower is gained. We should tread lightly on the air that we breathe as we do the trails that we explore.Greg FriedmanSanta Clarita, California

A: Jeff and Greg, we thought we'd heard the end of the Restricted Sixes saga, but judging by the amount of e-mail and letters we've received regarding the matter, it became obvious that some degree of closure would be necessary. First off, yes, there is a restrictor cone in the air filter box of some Jeep YJs ('91-'95, from what we're told). It's located in the air tube that's molded into the box where the air inlet tube attaches. The only reason Turbo City didn't acknowledge its existence is because most of its performance kits don't incorporate the stock air filter box, not to mention the fact that its series of kits offer more horsepower gain than achieved through the removal of the restrictor. As far as the danger and ecological impact of removing the cone, we haven't heard of any Jeeps blowing up or read any studies about the ramifications of its removal upon the environment. But, however you slice it, the cone is there, and there for a reason. A reason, or maybe more appropriately, reasons that we've heard multiple versions of and still can't give much validity to. Why the cone is there may never be truthfully known, but since it's there, and since you now know of it, use your best judgement weighing all potential dangers and environmental impacts and either remove it or don't. Personally, however, we'd let it be and call Turbo City. Turns out the company has quite a bit of experience concerning these matters.

Questions Or Comments?
Write to us at 4 Wheel Drive & Sport Utility, 774 S. Placentia Ave., Placentia, CA 92870, christianl@mcmullenargus.com.

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