Nuts & Bolts - September 2014
Your 4x4 Tech Questions Answered
9-Inch Front Axle
Q I am building a Jeep Wrangler YJ with an eye towards survivability. So far I have swapped in a healthy 351, NP435 transmission, an NP205 transfer case, and a Currie fabricated Ford 9-inch rearend. I still have the Dana 30 front. I have considered a Ford 9” front so that I can keep a set of spare dropouts geared low with spools for fun, and a set of modest gears (3.73s) for general use. My conundrum is what to do for the outers. For fleet interchange reasons I would like to stick with the 5-on-41⁄2 wheel bolt pattern, as they are also one of the fusible links on the build. What I mean is that U-joints and studs are cheap and easy fixes, while gears and axleshafts are not. How much tire will the Dana 30 outers handle compared to a Dana 44? I never plan on going any bigger than 35-inch tires, and then only because they are “required” for the Ultimate Adventure. Unless 34-inch Super Swamper LTBs would be acceptable? Maybe?
A You are correct in thinking that your Dana 30 front axle is living on borrowed time. With V-8 power on tap, low gearing, and 35-inch-tall tires, one stab of the throttle at the wrong time will result in bad noises and broken parts. A fabricated Ford 9-inch front axle like one from Currie Enterprises (currieenterprises.com) is a good option for the reasons you state: Third members are easy to change if you want to swap between trail gears and street gears, not to mention a ring-and-pinion that is much stronger than what is found in a Dana 30. You also get rid of the weak center axle disconnect mechanism that is present on your Jeep’s front axle. However, if you insist on keeping your Jeep’s 5-on-41⁄2 bolt pattern, you won’t have many options for strength upgrades from the knuckles out over what you can upgrade with your current Dana 30. As for your fusible link theory, keep in mind that broken axle joints often take out axleshafts and break ball joints at the same time, while sheared lug studs are not an easy trail fix.
Unless your YJ happens to be a 1995 with ABS, the stock axleshafts use puny 5-260X U-joints, which are weaker than the 5-760X joints found in ABS-equipped YJs, TJs, and later XJs, not to mention most Dana 44s. These same joints are also what Currie uses with its direct-replacement 9-inch front applications. It’s easy enough to swap in axle stub shafts that will accept the larger 760X joints and work with your stock unit bearing wheel hubs, and these are even available in chromoly. This is a significant strength improvement, but the stub shafts are still going to be the weak link. Another option is to upgrade to an aftermarket hub conversion from Yukon Axle & Gear, which uses a stronger stub axle and has the added bonus of serviceable wheel bearings. However, the 5-on-41⁄2 hub kit uses relatively weak locking hubs like those used on Rangers and Bronco IIs for several years.
At the end of the day, a fabricated 9-inch front axle with a 5-on-41⁄2 bolt pattern wouldn’t be much of a strength improvement because some of the weakest links from your Dana 30 would remain. Sure, there’s a much larger ring-and-pinion and larger inner axleshafts with more splines, but from the knuckles out it won’t be much different from even an upgraded Dana 30. By stepping up to a 5-on-51⁄2 (or larger) bolt pattern, your strength options improve significantly. Internal spline locking hubs like those from a Dana 44 become an option, and Currie can even build a hybrid 9-inch that uses Dana 60 axleshafts and U-joints with a 5-on-51⁄2 pattern. Since you want to build your Jeep with an eye toward durability, this is really a better option. As for Ultimate Adventure requirements, you could ask Fred Williams or Rick Péwé for an exemption, but 35-inch tires have always been the minimum and I doubt that will change anytime soon.
Gears vs. Turbo
Q I own a ’99 Ford F-150 4x4 extended cab with a 4.6L V-8 and an automatic transmission. This is my daily driver and has 245,000 miles on it. I run 305/70R16 tires. I pull a trailer from time to time but enjoy mild off-road adventures as well. I have a 19,000-mile engine waiting to drop in, and I want to rebuild the transmission when I install the engine. I am debating on installing 4.10 gears or a turbo system. I want to try and get as many miles per gallon as possible but gain performance as well. I haven’t found many reasonably priced turbo systems for my truck, but Mustang systems are cheap and easy to find. Is it possible to make a Mustang turbo system work without major changes? Another option is a straight axle swap. I have a Dana 70 rearend but no front axle. Is there a certain year that works best and is easy to find, or can I use any Ford front axle? Any advice would be helpful.
A If mileage and power are the primary goals, I’d lean toward a gear swap. Your truck likely has somewhere around 3.50 gears, and with your truck’s current tires being about 3 inches taller than factory, the engine is being forced to operate below its optimum powerband, which hurts performance and fuel economy. Swapping in lower gears such as 4.10s will help restore most of that lost power. And even though regearing axles is expensive, it’s still going to be cheaper than a turbo kit. There are a couple of turbo kits on the market for your truck along with a few supercharger kits, but these can run upwards of $4,000 not including installation. A gear swap will likely run half that amount. It’s probable that you could make a Mustang turbo kit work, but there will be some significant differences in underhood packaging, meaning that you are looking at a lot of plumbing modifications at minimum. Also keep in mind that turbos and superchargers aren’t going to do a thing for mileage, and most of them require running premium fuel.
A solid axle swap is not going to be easy, but it is possible and several people have done it with an F-150 like yours. There are two basic schools of thought. One is to find a suitable donor front axle with a driver-side output to match your truck’s transfer case and close to the same width as your truck’s rear axle. If you can find one, a Dana 44 from a pre-’80 SuperCab F-150 with leaf springs would be just about the perfect candidate, as it would be very close to the right width and would be mostly free of brackets and castings that you would otherwise have to cut off the axle when fabricating new suspension. However, these axles can be very hard to find and the wheel bolt pattern will not match your truck’s rear axle. The other school of thought is to swap in a set of axles from a similar year Super Duty. The Super Duty axles are a little wider than your F-150 rear axle, hence the need to swap both of them, but the Super Duty rear axle should have the tone ring and sensor that is necessary to keep your speedometer functioning correctly. A third option would be to simply have an axle built to your specifications, which really won’t be that much more expensive when you consider the expense of modifying and re-gearing a donor axle. For suspension, you can duplicate a pre-’80 F-150 coil-spring suspension, fabricate your own four-link suspension, or run leaf springs. Each of these options has its own compromises in terms of ride quality and ease of fabrication. Regardless, a solid axle swap is a huge undertaking with a heavy amount of fabrication and custom parts involved, so you want to be sure you have the necessary tools and know-how before diving in.
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