One of the downsides of upgrading a car incrementally is the tendency of such an approach to highlight existing weaknesses in the driveline. In the case of performance-modded fourth-generation Corvettes equipped with an automatic transmission, it's the stock rearend that usually fails first. We'll get to the reasons why in a moment, but first, a little history.
In one of those mystifying engineering decisions that could only make sense to a GM accountant, all auto-trans C4s (and all '84 models, regardless of transmission) were factory equipped with the none-too-sturdy Dana 36 differential. Manual cars from '85 through '96, meanwhile, got the significantly brawnier Dana 44.
Place the two units side by side, and the differences are easy to see. In addition to its larger gears and thicker housing, the 44 is equipped with a cover that incorporates a pair of bearing-support blocks. Without descending into engineer-speak, these blocks "preload" the carrier-bearing caps, preventing them from flexing or even cracking under load. Crack the caps, and the unsecured differential will attempt to exit through the back of the housing, often in fiscally ruinous fashion.
Recognizing the limitations of the Dana 36, many a performance-minded automatic-C4 owner has mulled the idea of swapping out the stock differential for its stouter sibling. Unfortunately, the two diffs don't directly interchange, instead requiring a hodgepodge of ancillary parts from both units to create a workable hybrid. And with the demand for Vette-spec 44s trending upward as the cars age, used-parts vendors are asking top dollar for rebuildable cores.
Since our '96 C4 project is all about affordability and ease of installation, we decided to look for another option when upgrading the car's rear. After riffling though our imposing library of Vette-parts catalogs, we found what appeared to be a workable solution in the form of a heavy-duty rebuilt Dana 36 from Zip Products.
Zip's technicians begin each rebuild by degreasing and shot-peening the differential housing to relieve stress in the metal and provide a like-new appearance. A pair of thick aluminum blocks are then welded into the unit's cover to provide support for the carrier-bearing caps, la the Dana 44. Finally, the entire unit is overhauled with quality U.S.-made internals and set up to the same precise tolerances you'd expect from an assembly-line piece.
The result is a better-than-factory-spec Dana 36 with a durability quotient approaching that of a 44. (Author's note: For extreme-horsepower automatic C4s equipped with drag slicks, Zip recommends stepping up to a full-on 44 swap. To ease the job, the company offers an all-encompassing swap kit that comes with the necessary driveshaft, cover, and other supporting bits.) Throw in a 3-year, unlimited-mileage warranty, and the $1,395 price tag (plus a $525 refundable core charge) looks like quite a bargain.
In addition to the aforementioned fortifications, Zip loaded our diff with a set of 3.54 gears in place of the stock 3.07s. (The company uses Richmond or U.S. Gear sets in all of its rebuilt units.) This change alone should noticeably improve the car's acceleration on the street and show up in the form of quicker e.t.'s at the dragstrip. We'll be back to report on that in an upcoming issue, but for now, let's take a closer look at how Zip's rebuilt Dana 36 differential earns its "heavy duty" designation.