Porsche already had the advantage of a 97mm bore in house, which was used for the 3.3 Turbo, so coupling these cylinders with the stock 3-litre’s stroke of 70.4mm would result in a capacity of 3,122cc. Nonstandard pistons would be required from Mahle, but otherwise the kit would be a classic parts bin eff ort. However, because this scheme was in clear contravention of Fuhrmann’s policy of winding down the 911, it is believed approval had to be sought from Ferry Porsche himself before manufacture could start, and the green light was obtained on the express condition it remained an extremely discreet modifi cation, invisible to the uninitiated.
The project came under the auspices of Rolf Sprenger, Porsche’s Bosch injection specialist who was then in charge of the repair workshop at the Kundenzentrum in Zuffenhausen. The object was to match the output of the earlier 1973-74 2.7 at 210bhp. Supervised by master mechanic Helmut Pietsch, powerkit production started on customer cars: besides the larger bore, work on the cylinder head allowed a significant increase in compression ratio from 8.6:1 to 9.5:1, which demanded use of 98 RON fuel.
Under the engine’s external ‘tinware’ were the cylinder cooling fins of the Turbo 3.3 and 12 Dilavar studs – then state-of-the-art material that was less prone to thermal expansion than the standard steel items – bolted the head to the block on the exhaust side. The 1975-76 2.7 had been subject to a rash of breaking head studs and the use of Dilavar studs, on the exhaust side at least, would become Porsche practice on the later SC. For the 3.1-litre, the K-Jetronic fuel injection was as fitted to the SC, but a higher pressure Bosch fuel pump was specified.
A larger oil radiator went into the front wing and in the interests of an increased top speed, the 915 gearbox had a higher fifth gear, another modification that Porsche would make standard on the 188bhp 1980 model year SC. The upgrade was not marketed as such: it was never a dealer fit item and, as far as we know, no sales literature was issued.
Porsche relied on word of mouth to communicate to dealers that if customers asked, the factory would supply and fit a performance kit. Those 911s delivered were referred to in their documents as an SC-L model and an accompanying letter confirmed that this was a 3.1-litre factory conversion. The letter went on to indicate that while the modification did not affect the standard warranty, if buyers experienced an engine problem relating to the upgrade, their recourse was to the Reparaturwerkstatt at Porsche not their dealer, as the latter ‘would not have the parts’ to effect a repair.