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1979 K10 & 1996 Jeep Grand Cherokee Limited - Mail Box

Posted in Features on March 1, 2004
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Editor's Note: If you have any questions, comments, rants, or raves, please feel free to contact us at OFF-ROAD magazine, Mailbox, 774 S. Placentia Ave., Placentia, CA 92870. You can e-mail us at

Broken Differential Gears:Rebuild or Replace?I need some information on the rearend in my '79 K10. On a recent trail ride, two teeth on the rear axle's ring gear were sheared off. I wasn't driving that hard; the engine is a stock 400 small-block and the tires are only 33s. I am debating whether to replace the gears in the stock 12-bolt axle or to replace the entire axle assembly with a stronger one. I've heard that the C-clips in the 12-bolt GM axles are the weak link. Will a 14-bolt fit the rear springs of my truck without a lot of modifications? Or should I look for a different type of rear axle?Sherman Sieversvia e-mail

If your truck's rear gearset snapped two teeth off the ring gear, there were some serious forces at work, but the gear breakage had nothing to do with the C-clips. What likely broke the teeth off the ring gear was either an excessive load transfer of some type or a direct impact to the axlehousing. There's no need to replace the 12-bolt axle since it will easily handle the power of the stock engine and the 33s, as it is already equipped with 30-spline axles. Also, the aftermarket heavily supports the 12-bolt, so there's a host of differentials, brake upgrades, and so on. The function of a C-clip is to hold the axles in position, and, typically, the only time a C-clip will break is through a severe side impact, such as a direct hit to the tire and wheel from a rock or other solid object. You can purchase a C-clip eliminator kit, but it will require the installation of new bearings, and you'll either need new axles or will have to have the old axles machined for the C-clip eliminator hardware. If it were our choice, we'd rebuild the 12-bolt with a good limited-slip differential and new gears, and install the C-clip eliminator kit if we had enough money left after that.

Custom Air Filter Size GuideI'm going to build a custom air filter box and air intake tube for my F-150 because I haven't seen any aftermarket products that I really like. Also, I plan to mount the filter box at the highest possible point within the engine compartment in order to keep water out of the intake. In fact, I may even place the filter box inside the cab, which I've seen done on other trucks and appears to be the most water-resistant placement possible. My question concerns filter element size. How big of a filter (in square inches) do I need for a 351ci V-8 engine? The V-8 is moderately built with a performance cam, a small amount of cylinder head porting and larger valves, roller rocker arms, an Edelbrock intake manifold, an aftermarket fuel injection, an exhaust system with headers, and an ACCEL ignition. Is there some formula I can use based on the engine's approximate power output, or should I use a filter with a size that's intended for a big-block engine?Josh HartmanChino, California

Good question, Josh, and the answers are relevant to any vehicle that's being fit with a custom one-off air intake system. We asked the knowledgeable folks at K&N Engineering for help, and here's the formula you'll need to calculate the proper air filter element size for your custom filter:

The rpm figure in the preceding formula should be the highest rpm the engine will rev to, or the rpm where peak torque occurs. For this formula, we used 351 ci of displacement, 3,800 rpm as the torque peak, and came up with a recommended filter size of 53 square inches.

Bad Cat; Good CatI recently purchased a '96 Jeep Grand Cherokee Limited with a 5.2L V-8. My mechanic tells me that the catalytic converter is plugged and needs to be replaced. When I told him I thought I would buy an aftermarket high-flow converter, he said the only catalytic converter that would work is the OE model, and that the engine's computer wouldn't work with the aftermarket converter. The performance catalytic converter is much cheaper than the OE converter, and I thought the performance converter would work better with the high-flow exhaust system I plan to install later this year. What do you think I should do?Craig KellyIrving, Texas

In our experience, almost any truck runs better with a quality-manufactured performance catalytic converter in place of the OE model. One of the best aftermarket manufacturers of catalytic converters is DynoMax, whose parent company is Walker Exhaust; Walker Exhaust manufacturers nearly 80 percent of OE converters, so you know you're getting an excellent product. Once you've purchased a Walker/DynoMax catalytic converter, you can team it with a DynoMax exhaust system and muffler for the best possible exhaust flow. Your Cherokee's performance will be noticeably improved with a complete high-flow exhaust system and catalytic converter. Give DynoMax a call at (734) 384-7806, or visit the company's Web site at

area in square inches= cid x rpm

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