The goal was to produce the lightest possible 911s for rallying and racing, and the first step was trimming as much weight as possible from a stock 911S coupe to establish a starting point for FIA Homologation. Then, the car could be further lightened. To achieve the target curb weight of 840 kg (1,848 lbs) that would be legal for a “base” 911S coupe, the factory created “Option M470 (Delete).”
All 911Ss were fully equipped with top-of-the-line trim and comfort items; in other words, they all had Option M470; deleting that “Option” simply meant that the car was built without any of those items, which brought the dry weight down to just over 2,040 pounds. That was accepted by the FIA as the legal weight of a base 911S. From that new, lower starting point, Porsche’s Competition Department could then remove even more avoirdupois for a class-legal weigh-in.
Lighter materials were used wherever practical. Rivard says “about 40 special sets of thinner-gauge (0.70 mm) sheet metal were produced for the factory race and rally cars.” In his book “Excellence Was Expected,” Karl Ludvigsen places that number at about 30. The special thin sheet metal, exclusive to the works-built cars, was employed for the seat pan rear and side panels and rear inside panels, the roof and quarters of the works-built cars. The factory, says Ludvigsen, also produced about 100 packages of additional lightweight parts and made them available to privateers.
Standard steel bumpers were replaced with aluminum alloy, there were no bumper guards, and the steel engine lid was replaced with aluminum. Along with that thinner-gauge steel for the roof and inner structure, the factory racers were fitted with doors made of an aluminum alloy made rigid by light-gauge (0.75 mm) steel framing. The polished aluminum horizontal grille bars on the engine cover were tossed in favor of an even lighter mesh screen, and the engine lid name and model badges, by themselves just a few grams each, gave way to a paper-thin decal.
The stock steel front and rear fenders were widened with softly-rounded flares to accommodate wider wheels and larger tires. Because Fuchs did not offer an 8.0-inch-wide wheel with the desired offset at that time, many STs were fitted with Minilite alloy wheels in the rear and are often seen with them today. Fiberglass was used for the front and rear fascias, and a plastic front lid was offered as an option. Rubber hold-downs saved a bit of weight over the stock 911 latch assemblies, and thus the factory omitted both the wire release cables and T-handles and the thin steel tubes that housed them.
All window openings except for the windshield were fitted with Plexiglas and the interior was stripped. Thinner Glaverbel laminated glass was available as a windshield replacement. There was no carpeting, glovebox door, passenger-side sun visor, joint sealing or undercoat. The ashtray assembly was removed, as were those welded-in tubes for the hood release cables, seat-slide supports on the central backbone, the standard seat belt anchor points, heat duct tubes, front and rear cover locks, fog-lamp recess covers, front torsion covers and the caps over the rear torsion bar ends, even the dust-caps on the Fuchs wheels.
A few important items were added, in particular, a transverse brace between the front strut towers, and in some cases, a carefully-fitted roll-over bar. For rallying and racing, either an 80 liter (21.1 gallon) or 110-liter (29.0 gallon) plastic fuel tank with through-the-lid filling capability, along with a space-saver spare tire were fitted. The result was a car that was nearly as light as the 911R. Lightest of all the STs, and indeed the lightest 911 ever constructed, was the one special car prepped for the 1970 Tour de France Automobile sports car race. That car was shaved to just 1,720 lbs, or 789 kg, by employing a number of titanium and magnesium parts.