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How To Build It - The 2007-Present Jeep Wrangler JK

Posted in Features on February 26, 2014
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It’s hard to imagine that the Jeep Wrangler JK has been around for about seven years now. As a reader of Jp, you may think that we hate JKs, but that is far from the truth. We like JKs and recognize their position in the latest evolution of our beloved open-top Jeep. No one here denies the attraction and functionality of the JK in either two- or four-door configuration. Why don’t we do more tech with them? Well, in all honesty, all of us are Jeep-poor. That is, we own so many Jeeps between the three of us that we could set up a dealer lot. Add to this the still fairly high price of JKs, and you’ll notice the problem—we’d pretty much all rather own a mix of 14 different Flatfenders, XJs, TJs, CJs, early CJ-5s, FSJs, MJs, ZJs, WJs, and YJs before ponying up for one JK to use for tech articles. We do have our much-abused ’07 Rubicon, but unfortunately that Jeep needs a new clutch, tranny, and possibly a new rear axlehousing.

As soon as the JK fad calms down a bit with all the moms and mall crawlers and prices become more realistic, know that we are and have been thinking about building JKs. We are just not following through with the builds just yet. For now, let’s get started with a few hypothetical builds so we can tell you what to look for and what to avoid when it comes to the new Wrangler. Here are a few tips and tricks for building either a two-door JK or a JK Unlimited. Oh, and maybe we do have some more JK tech headed to the pages of Jp soon. Keep your eyes peeled.

The 3.6L Pentastar engine—is this the answer to the anemic and pathetic minivan 3.8L stop gap engine? So far, it seems so. The 3.6L Pentastar is the most powerful factory engine available so far in an open top Jeep with 285hp and 260 lb-ft of torque.

Spinning tires in the muck is gonna take some horsepower. For that we’d look to the ’12-and-up JKs with their more potent 3.6L Pentastar. Sure this little engine is powered by alien technology, but it works and is way more drivable than the 3.8L minivan engine. If you are stuck with an older JK, try to find one with lower odometer readings or be ready to put a new engine in your JK. Another option is to dig into engine swaps for the JK. Many JKs now carry a Hemi between their front tires, and some swaps are so clean that they look almost like the factory intended it that way. Still, that is a costly option that can sometimes double the cost of an already-expensive Jeep. We usually push people towards the Gen III GM V-8 engines as a more affordable option and enough of these things have been done that the kinks are worked out of the swap. Either way, we’d look into the cost of trading to a ’12-and-up before dropping huge chunks of change on an engine swap. Why? Well, we’ve seen a 3.6L JK beat an LS-powered JK in an unofficial drag race. Sure, both Jeeps were modified, and gearing may have not been the same, but still the 3.6L is a hoss.

We know the 3.6L is stronger than the minivan engine, but only time will tell if the Pentastar has more longevity in a JK than the 3.8L. Also, a few ’12 JKs had issues with one of the heads malfunctioning and developing a tick, but this repair should be covered by the warranty and was reportedly corrected for later ’12 year model JKs and newer ’13-and-up JKs.

As for the rest of the drivetrain, we generally prefer the auto transmissions in a JK. Why? Well, the NSG370 six-speed is proving to be a lighter duty transmission than the five-speeds it replaced. Having said that, the four-speed auto in the first few years of the JK behind the 3.8L is also is not that robust, but may be better and stronger than the manual.

Axles found in a four-wheel drive JK (yep, there are a few ’07-’10 two-wheel-drive JKs floating around) will be a high-pinion Dana 30 front axle for the non-Rubicon JKs, a Dana 44 front for the JK Rubicons, and either a Dana 35 rear (’07 JK Xs and Saharas only) or a Dana 44 rear axle. Front and rear Dana 44s should be good for tires up to about 37 inches as long as you don’t throw the heavy steel armor catalog at your Unlimited. The Dana 30 should be good up to a 35-inch tire and maybe 37s with chromoly axles. Both front and rear axles are known to bend when abused under a heavy JK, but there are several options to beef up the housings or move to aftermarket axlehousings. Don’t forget the rather thin inner knuckle. The top of the knuckle will bend if you beat on a heavy JK off-road or seek temporary low altitude orbit. Snorkels are also available and will help keep your engine together despite deep water or mud otherwise be wary of the JK’s air box and intake location or you will hydrolock the engine. Aftermarket suspension parts range from budget boosts to full long-arm kits with coilovers. We recommend keeping your JK nice and low, just tall enough to clear the size tire you want to run, or keep the vitals out of the mud and water.

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The JK Rubicon is probably one of the quickest ways to get into rockcrawling. These things with lockers and the 4:1 NP241 Rock-Trac is basically a turnkey crawler available at your Jeep dealership. A cheaper alternative would be to grab a low mileage used two-door 4WD 3.8L-powered Sport or X with the 42RLE four-speed auto, add lunchbox lockers, a Rubi-Crawler from Advance Adapters, some aftermarket axlehousing parts to prevent bending, some thicker than stock skid plates, rocker guards, a little lift, and some 35s and go hit the rocks. As long as you keep the JK lightweight, it will work well. The Rubi-Crawler is a pretty cool little auxiliary gear box that fits in the tailhousing of the otherwise dubious 42RLE transmission found behind the 3.8L in JKs. This part adds a second selectable 2.72:1 low range. Add that to the factory NP241’s low range (of 2.72:1), and you can crawl with ease with a super low, low range of about 7.4:1.

Also the draw of a family four-door JK in the rocks can’t be denied. The longer wheel base is great for those steep climbs. We usually suggest keeping a Jeep as low as possible while still clearing as large a tire as possible, but with a four-door we usually recommend a little more lift to help with that long belly keep off the rocks on breakover angles. Also, we have seen a few steering box sector shafts break on heavy JKs with large (37s and larger) tires. If you run big tires on a heavy JK, be ready to face this issue if your steering box is factory.

Getting back to axles, the JK’s factory axles, when properly built up to resist bending, can handle up to about a 35-inch tire in the rocks. Anything larger than that and you might be able to squeak by with chromoly axleshafts and RCV front shafts if you have the Dana 44s, but even 37s will be pushing it. If you have a big heavily armored four-door JK be ready to drop some serious coinage on big axles. These big heavy four-door JKs are the reason Dynatrac builds and sells quite a few ProRock 80 custom axles.

The added wheelbase of the four-door JK seems to work pretty well out west, but like anything, that length can hurt on breakover angles or when sharp turning is necessary. Still, the attraction of having extra seats and space is undeniable, and the four-door JK may have helped keep Jeep alive during the most recent economic recession.

A light two-door might be alright with the 3.8L, but with a four-door it’s time to step into a 3.6L or look into engine swaps. The other bonus of the 3.6L is the five-speed auto that lives behind it. Unlike the 42RLE used behind 3.8Ls, this transmission should prove to be pretty bulletproof. This tranny, known as the W5A580, is from the same family of Mercedes transmissions that Mercedes uses behind some AMG V-12 engines. The W5A580 is also found in other Chrysler vehicles that are powered by the Hemi. It also has a pretty low First gear for an auto of 3.59:1. Need we say more?

As far as the suspension of the JK, it is without doubt the latest and greatest evolution of the multi-link solid-axle suspension Jeep has been playing with for years in other models. The JK’s suspension is pretty darn nice in factory trim, and the large wheel wells mean not much lift is needed to clear pretty big tires. This and the fad of adding heavy armor to big ol’ JKs definitely has something to do with bending axles. Add high-end progressive rate coils from the aftermarket, fancy reservoir or bypass shocks, air bumps, and even full coilover conversions, and the JK’s suspension will work just about as well as any solid front axle truck can in the whoops. If that’s not enough, we’ve even seen an independent front suspension conversion for the JK. The jury is out on whether that conversion is a good idea or not.

JKs are über popular with mall crawlers and parents who think they make them seem outdoorsy or edgy. That’s fine with us. In a few years the fads should settle down and there will be tons of used JKs with easy road miles and good factory or aftermarket parts. Until then, rely on us to relentlessly make fun of the blingy, big wheel, overly tall, or stupid JK builds.

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