As the Z/28 was nurtured and developed ostensibly for the rampant factory involvement in Trans Am racing, it was bound by a 305ci engine displacement when the Big Four automakers were beginning a heavy upswing to engines that were internally and physically larger than ever before. The 301's 4-inch bore x 3.00-inch stroke netted an actual 301.59 ci. Chevrolet marketers simply stretched the truth a few millimeters to match the occasion. Thor's motor has a 0.030-inch clean-up bore, expanding its insides to a schoche more than 306 ci. The 302 also had kinship with drag racers of the day. Most of the lower-class gasser and modified production cars stuck with this displacement like gospel and their drivers stabbed the clutch at a heady 9,000 rpm.
Thor has done precious little else to the Z/28 and that is where the crux of this report lays. The engine got most of the attention to make driveable torque as well as high-rpm horsepower. It produces 466 hp at 6,800 and 400 lb-ft at 5,300 rpm at the crank - about the same verve as the race-ready Trans Am 302 back in the day. John Villegas sent the rotating parts and cylinder block to Jim Grubbs Motorsports in Santa Clarita for the machine work while McKenzie's Racing Cylinder Heads CNC-ported the camel-hump castings (2.02/1.60 valves) in nearby Oxnard.
The idea was to make the 306 as stock appearing as possible, but to roil its guts like a Modified motor straight out of the '60s. Villegas laid the groundwork with ARP studs. On the forged steel arm he set reconditioned forged steel connecting rod pinned to 12.5:1 JE pistons. He filled the cam tunnel with a custom-ground Comp solid roller (0.630-inch lift on both valves, 256/258 degrees duration at 0.050, 106-degree LSA) and joined it up with Comp pushrods and valvesprings. The rocker arms are pressed steel, as original. A 6-quart GM oil pan houses a Melling pump and Cloyes timing gear whirs behind the front cover. Every other engine accessory is original or facsimile of the same, even the fuel pump.
Villegas included an MSD 6AL box and ceramic-coated Doug's Headers with 1 7/8-inch primaries that lead to an X-pipe, Flowmaster 40 Series mufflers, and an aluminized 21/2-inch Flowmaster exhaust system. The crown, of course, is that cross-ram intake manifold (PN 3940077) and those staggered 4295 Holley 600-cfm carburetors. Stan Younts at Stan's Restoration in Lexington, North Carolina, supplied the manifold and the cowl cool-air feed assembly, as well as some other bits of Z/28 engine unobtainium. The rest came from Rick's First-Generation.
There were several iterations and revisions of this exotic induction system depending on the year of use and user (Penske, Yunick, TRACO, etc.). It was only offered as a dealer-installed option (first available in 3/68) and never included on any assembly line vehicles. Right off, it was worth 25 hp over the single-carburetor setup. These manifolds came out of Winters Foundry, the outfit that put its trademark "snowflake" on almost all Chevrolet's aluminum castings regardless of application. Cost of a plenum air intake and duct system (furnished loose, as in the trunk inside a burlap bag) was $479.25. Tubular exhaust headers, also furnished loose, were $779.40.
Thor would have no less than an M22 'speed beneath the boards. It's attached to a GM 403 bellhousing over a Centerforce 10.5-inch-diameter clutch assembly and steel flywheel. Ron Gates Transmission upstate in Sunnyvale, California, rehabbed the four-speed, making sure all the gear whine was just right. Coast Driveline in Ventura refurbished the original prop shaft with new Spicer U-joints, then went through the 12-bolt, fitting an Eaton positive traction differential with 4.56:1 gears! Yeah four-and-halfs with a 1:1 final drive, just like we did it in the bad old days.