Rearend

The GTO uses a modified (control arms instead of leaf springs) Hotchkiss rearend with a hypoid ring gear and pinion. Component parts have been beefed-up since ’63, and should give no trouble even under severe competition at the dragstrip, assuming normal maintenance, of course. The differential case is a one-piece unit with a cover plate at the rear. Steel axle housing tubes are pressed and welded into the cast case. The semi-floating rear axles are big enough for very rough usage and are supported at the outer ends by pressed-on heavy duty ball bearings. The pinion shaft is also big and is located by two tapered roller bearings.

There are seven different axle ratios available for various model cars. Identification of a ratio is from a code stamped on the rear of the righthand axle tube adjacent to the carrier or on the left rear brake drum surface. There will be two letters in this code identification, the first identifying the type of rearend (W-Standard, Y-Safe-T-Track, X-Standard with metallic brakes and Z-Safe-T-Track with metallic brakes) and the second denoting ratio. There are nine axle ratios listed, but only eight of them generally appear in print. They are B-2.56, D-2.93, E-3.08, F -3.23, G-3.36, H-3.55, K-3.90, and L-4.33. The ninth is a 4.11 to 1 ratio, normally listed as an optional option, along with the 4.33.

Pontiac calls its limited-slip differential the Safe-T-Track, and lists it as optional on all ’65 Tempests, regardless of axle ratio. A tag attached to one of the third member cover plate bolts identifies such an option. Very similar to the standard differential, the Safe-T-Track has an additional set of cone clutches and pre-load springs. In use, this assembly transmits power to the wheel having most traction, unlike a standard differential, which does just the reverse.

The Safe-T-Track is an extremely free-running unit, with no apparent binding on tight corners or deceleration. When the 335-horsepower engine is ordered, the standard axle ratio will be 3.23. With the 360hp engine, the ratio will be 3.55. This holds true for both the three- and four-speed manual and automatic transmissions.

Transmission

A three-speed synchromesh transmission is listed as standard equipment in all Tempests. A cast iron case is used for the three-speed, enclosing helical gears made from drop-forged steel blanks, heat-treated and shotpeened.

The GTO offers the Hurst floor shifter as standard appointment, and two ratios can be purchased in the three-speed configuration. The first is 2.58:1, 1.48:1, and direct. The second is 2.42:1, 1.61:1 and direct.

Probably of most interest to any automotive buff is the four-speed alloy-cased gearbox. This is available as an option only and consists of two cases, the front holding the four forward gears and the rear case housing the Reverse gear. A fully synchronized design, the four-speed comes with a Hurst floor shifter and two different ratios. The wide-ratio box has 2.56:1, 1.91:1, 1.48:1 and direct, while the close-ratio transmission (available only with the 3.90 rear axle ratio) has 2.20:1, 1.64:1, 1.28:1 and direct.

For general driving in city traffic with only an occasional fling at the drags, the wide-ratio gearbox would probably be most satisfactory. This would allow more flexibility in any particular gear. The close-ratio box is for any condition requiring constant engine rpm, such as drag racing, hill climbs, slaloms, etc.

The floor-mounted Hurst shifter is located right at the edge of the bucket seat, and features an extremely short throw from First to Second, a slight spring-loaded jump to the right, and a slightly longer throw between Third and Fourth. Reverse is spring-loaded all the way to the left and forward. This short-throw shifter can be a bit tricky at first, but quite comfortable with experience. For a sports car type of bash, it’s ideal. A two-speed torque converter automatic is available, with a max torque multiplication off the line of 3.87 to 1. The governor is set for automatic upshift at 5,200 rpm.

Engine

The GTO is equipped with two basic ohv’s, both displacing a cool 389 cubic inches. The only difference between the two is a little matter of 25 advertised horsepower, derived at through camshaft timing and carburetion.

The 389 engine is a short stroke, 90-degree V design, with a reasonably thin-walled alloy cast-iron block. The bore is 4-1/16 inches, while the crank strokes a neat 3-3/4 inches. The flat-top aluminum pistons have a compression ratio of 10.75:1 and are fly-cut in the crowns for valve clearance.

The block has five main bearing supports, providing a very strong bottom end for the cast pearlitic malleable-iron crankshaft. The crank is fitted with heavy-duty Moraine-400 aluminum-on-steel main and rod bearings. Journal diameters are 3-inch, which means that a good amount of modifications can be made to the engine without undue worry about the bottom end falling out.

The heads are identical side for side, and are designed to flow water completely around the valve seats for better cooling. The valveguides are integrally cast in the head and consume the major length of the valve stem, cutting down on excessive valve sideplay. All ports are large in stock form, indicating a minor bit of shaping and clean-up only.

The valves are big, with intakes measuring 1.92-inch and exhausts of 1.66-inch. The intakes have a 30-degree seat angle; the exhausts use a 45-degree seat for better heat dissipation. Lift is set at 0.395 to 0.417-inch, which is quite reasonable for a hot street machine. The cast-iron camshaft has lobes ground at an angle to cause the lifters to rotate and is supported by five bearings. Special high-performance hydraulic valve lifters are included. The 335 engine has a valve timing of 273 intake, 289 exhaust (duration in degrees), 54 degrees overlap. The 360 engine features an intake of 288 and exhaust of 302, with an overlap of 63 degrees. The dual exhaust with low restriction mufflers and resonators is stock.

The 335hp engine is fitted with a single four-barrel carburetor. The bigger 360 engine has three two-throats with a mechanical progressive linkage. The throttle bore area of the single quad is 7.62 square inches compared to 12.19 for the triples. Under the conditions we’ve just listed, the 335 produces 431 lb/ft of torque at 3,200 while the 360 register 424 at 3,600. Under almost any set of rules, the 389 GTO engine must be rated as very good in showroom trim, and superb with a little “hot rod english.”

A transistor ignition is available, as is a seven-blade declutching fan. Chrome valve covers and carb cleaner sheetmetal adds to appearance, if not performance. To take care of any unusual heating problems, heavy-duty radiators are listed as options.

Body

The bodies are of standard sheet steel, with no optional aluminum or fiberglass parts available for racing purposes. As it is, the car can be ordered without underseal or other sound-deadening material, which cuts about 130 pounds in total weight; further refining in the diet department should trim another 80 pounds easily. According to the printed material, the hardtop with small engine weighs 3,462 pounds and the coupe weighs 3,443. The big engine adds 16 pounds to this total, due to the heavier manifold and two extra carbs. Another 10 pounds is added by the automatic trans. As tested, our car weighed 3,660 on the strip scales, 1,995 pounds of this on the front end.

The GTO is distinguished by a pinstripe along the side, following the general upper outline of the fender line. The wheelbase of all models is exactly 115 inches, and overall length is 206.1 inches. Tread is 58 inches in keeping with the wide-track theme, both front and rear. Overall width is 73.4 inches; height for the closed body is 53.5 inches. Height at the cowl is 37 inches and ground clearance is 6-1/4 inches at the lowest point with the Red Line tires installed.