Bulletproofing Our IRS
Bulletproof and IRS are two words you don't normally see in the same sentence. The guys making 500-plus hp and more than 500 lb-ft of torque usually run it through a solid-axle rearend. But Corvettes since 1963 have been built with an independent rear suspension (IRS) for good reason. The ride and handling from an IRS are far superior to the antiquated design of any solid- axle rear. Unfortunately, your Vette's IRS was never designed to handle more power than the factory engine developed, so getting it to cope with our big-power big-block started with some very stout race-ready parts. The source—Tom's Differentials.
The parts list from Tom's is extensive, but cut corners here and you'll find yourself stranded on the side of the road. Tom's provided a 3.73 U.S. gearset, an oversized and hardened cross-shaft, a 31-spline spindle kit, a billet 1330 pinion flange, Eaton solid clutches, a hardened spider-gearset, and 30-spline inner stub axles. Throw in the fact that every nut, bolt, and shim are hardened, and you'll see we've basically replaced everything but the case. After all, to do it right, all the "weak links" in the system need to be removed from the equation.
While we were at it, we also tossed the original trailing arms and ordered new arms, bearing supports, and parking-brake components from Lonestar Caliper.
With parts in hand, it was time to move on to the machining, setup, and assembly of our rear diff. The experts at Tom's warned us that there are only a handful of machinists left who have the experience to build an IRS capable of handling this type of power. The reason? It's an incredibly time- and labor-intensive process, and everything inside the diff needs to be set to within thousandths of an inch. At the top of the "recommended" list was Mike Dyer, a 35-year veteran and the owner of Precision Corvette Differential.
Dyer starts by cleaning and media blasting everything, and then boring the new carrier to fit the oversized axles and cross-shaft. He machines a new flat for the cross-shaft lock bolt that is 0.060 deeper, then radiuses the edges and polishes the carrier to remove any casting imperfections. The clutches are pre-scuffed so they're already seated before you turn the first wheel, and the spider gears are shimmed to 0.002 backlash. The ring gear is installed with Loctite, and the axles are machined to 0.005 endplay. Even the tips are polished.
The ears on the case are also machined off for axle clearance, and the bearing-cap holes are drilled and tapped for 1/2-inch bolts. (Dyer machines these HD caps from raw blanks.)
The Timken bearings and races for the carrier and case are installed next, followed by the pinion and carrier themselves. The gears are then set up by pattern—and then the whole thing is disassembled and cleaned to surgical standards.
Finally, Dyer does it all over just to make sure it's perfect. The final assembly includes the installation of the new Torrington stub axle bearings, along with the pinion and crush sleeve; the pre-load is then set to 15 in-lbs. The carrier is then installed and set to 0.007 backlash. As with building a race engine, everything is checked and rechecked.
When we received the complete rear diff and new trailing arms (which Dyer also built), one spin of the axle stubs told us everything we needed to know. The fresh setup had that smooth, almost silky feel that only comes with a machined part that's perfectly on spec. Our project now truly has a Precision Corvette Differential.
While we were at it, we ordered larger-diameter, "nitrous-ready" halfshafts and a new HD main shaft from Denny's Driveshafts. (The Denny's units come complete with heavy-duty universal joints as well.) By comparison, our old half- and mainshafts looked like they came out of a Corolla. Denny's even used our old parts as a template for size, length, and correct yoke spline, to make sure everything will go back together just the way it came out.
So there you have it: big-block power, a super-duty transmission, a bulletproof rear diff, and a set of nitrous-ready halfshafts and mainshaft. Guess where the weak link is now? The rear tires. We're gonna shred 'em like mozzarella cheese.