A CLOSER LOOK AT THE ‘HEART OF THE MATTER’
In standard form the 1300 Gordini engine (type 812) was a 1255cc, based on the 1108cc Sierra block and 72mm stroke crankshaft, but with the bore stretched to 74.5mm. Twin 40DCOE Weber side-draught carburettors replaced the Solex items and developed either 110BHP or 103BHP at 6750rpm, depending on which specification was quoted. Maximum torque of 85lbs-ft at 5000rpm was given.
The cylinder head was based on the R1134 1108cc model but with larger 35mm inlet valves. In order to accommodate the relatively large valves Gordini’s approach featured spark plug chambers recessed into the head, and linked to the combustion chamber by two small flame ports. This had the added advantage of spreading the flame front during ignition, resulting in a very effective design. However, this feature also proved to be a bit of a heartache for years to come, in that if the engine overheated the head would more than likely crack in the area between the valves and the flame ports. It is a tribute to South African engineering expertise that pioneering work was done in the early days of MIG aluminium welding by the likes of Frank Shearsby, enabling the repair of otherwise scrap heads.
However, all this wouldn’t have worked if it weren’t for the equally innovative overhead rocker shaft arrangement driven via short-angled pushrods by the existing block-mounted camshaft. This provided many advantages of a double overhead camshaft arrangement, but was considerably cheaper and simpler – and easier to adjust. This head and rocker shaft design served Renault well and the principle was adopted for the engines used in the R16TS, R12 Gordini, R5 Gordini and Alpine, Alpine A110 and Renault-supplied engines for the Lotus Europa and others.
The Gordini engine wasn’t just a Sierra block with a fancy head, though. The block was bored with a 2mm offset to accommodate the larger cylinder sleeves, which together with the higher output necessitated the development of thicker section conrods with a corresponding 2mm offset big end. Pistons were domed, with pockets cut out for the valves, and compression ratio ranged from 10.5:1 (standard) to 11.5:1 and higher for sport and racing.
To accommodate the more stressful demands the camshaft was fitted with phosphor bronze bearing bushes and wider cam lobes as well as wider cam followers, which reduced the initial valve train acceleration shock on the flank of the cam and enabled quite a phenomenal rev range. A larger capacity oil pump and full flow oil cooler were fitted, the lighter flywheel was dowelled to the crankshaft, and a balanced heavy-duty clutch plate was fitted to the flywheel. A 4-into-2-into-1 branch manifold with slightly unequal lengths to give a spread around the max torque range was fitted, and to handle the electrical demands the Gordini was fitted with a Motorola alternator. Larger diameter crank and water pump pulleys found a home, driven by a 12mm (as opposed to 10mm) fan belt. The cooling fan went from four to six blades to help a thicker core radiator stay cool.