Along with the body mods and extreme weight loss, these special cars enjoyed numerous mechanical changes from stock. Since the 2.2-liter cars would be at some disadvantage in the up-to-2.5-liter class, the STs needed larger engines. The FIA allowed oversized bores, but the 2.2’s stock stroke of 66 mm had to be maintained. New Biral cylinders (cast-iron with alloy cooling fins) one millimeter larger in bore diameter (87.5), or the same size as those used on the 908 race car’s engine, brought displacement up to 2,247cc. This would become known as the “2.3.”
Close attention was paid to the engine’s internals, with polished connecting rods, 906-spec camshafts, and careful balancing. With 10.3:1 compression, twin-plug ignition, and Bosch high-butterfly mechanical fuel injection with a twin-row, six-plunger pump, the 2.3 Type 911/20 engine produced a hefty 250 hp at 7,800 rpm, along with 188 lb-ft of torque at 6,200 rpm. In 1971, the engine was enlarged to 2,381cc, producing 260 hp.
A 2.5-liter racing engine—based on the 2.4 from 1972—appeared next, taking further advantage of the class displacement limit. This Type 911/70 measured 86.7 × 70.4 mm for a displacement of 2,492cc, and was quite muscular, producing 270 hp at 8,000 rpm and 191.6 lb-ft of torque at 5,300 rpm. Flywheel issues reported with that engine prompted a redesign, which reverted to the earlier and more reliable 66 mm stroke but an enlarged 89 mm bore.
Accurate production data for these special lightweight cars is difficult to pin down; various sources offer a range of numbers of between 35 and 44, not including the 1970 Tour de France car. Landenberger says the factory’s production records show just 41 911STs built to this specification between September 1969 and July 1970.
The late Patrick Paternie’s “911 Red Book” states that 35 STs were built between 1969 and 1971, plus that one super-light example for the 1970 Tour de France. Paternie lists seven 911STs specially numbered for rally purposes in 1970: 9110300001, 002, and 003, plus 300102, 949, 950, and 1127. Writer John Starkey says there was a total of 36, while another source says there were just 18. Paternie says the first three cars in this group of STs, numbers 300001, 002, and 003, were fitted with Type 911/22 2.2-liter engines. 2.3-liter racing engines (Type 911/20) were fitted to numbers 102, 949, and 950.
There may also have been some others, says Swiss researcher and ST owner Marco Marinello. He has compiled a list of 44 chassis numbers from 1970 for what are called the 930 kg (Rally weight) cars. “When I look at the 44 1970 cars and look at the known VIN numbers of 2.3 race cars, I think about 15 to 20 were 2.3 race cars and about 20 to 25 were lightweights,” says Marinello. “In 1971, I count 20 lightweights in total and eight had the 2.3 engine, Option M491. The 2.3 was much more a race car then the 1972-only 2.5, which was mostly a 2.4 steel shell.”
Of those 44 examples from 1970, he says, perhaps a third would be prepared for rally duties, another batch was built for circuit-racing. Those had widened front fenders and quarter panels, while the remainder, perhaps no more than eight to 10 units, retained their original “narrow” bodies and stock fuel tanks, and were intended for factory executives or special customers.