Mercedes-Benz M130 engine
The Mercedes Benz M130 Engine was the last and largest of the ‘mid-sized’ Single Over Head Camshaft (SOHC) straight-6 cylinder (inline) engines produced by Mercedes Benz. The ‘mid-sized six’ started life as the 2.2 litre M180 (80mm bore × 72.8mm stroke = 2,196cc/134ci). It was the first "over-square" engine - with a cylinder bore greater than its stroke - that the manufacturer installed in a production car.
The unit was unveiled at the Frankfurt Motor show in April 1951 alongside the new 3.0 litre M186 ‘big’ straight-6. These engines were used to power the newly introduced 220 and 300 models of the Mercedes range. While sharing many design features such as staggered valve arrangement and rockers running off a single overhead camshaft driven by a duplex cam-chain, the engines were of completely different design with little or no inter-changeability of parts.1
The 'Big Six'
The 3.0 litre ‘big six’ was produced from 1951 until 1967 with no change in its 2,996 cc displacement derived from a slightly under-square 85 mm x 88mm bore and stroke. The various versions of the engine (M186 – M198) produced from 113 – 212 bhp (158 kW) as compression ratios rose and the number of carburettors multiplied or were replaced with fuel-injection. These engines powered the 300 Adenauer, 300S/Sc, 300SL, 300SE Fintail/Coupe/Cabrio (W111/112), 300SE/SEL (W108/109).2
M180 to M130
The first M180 engine was rated at 80 hp (60 kW). It powered the W187 220 Sedan, Coupe and Cabriolet (Type A and B) from 1951 to 1955, the W180 220a from 1954 to 1956 and the W105 219 from 1956 to 1959.
The W180 Ponton 220S Sedan, Coupe and Cabriolet of 1956-1959 benefitted from the up-rated M180.924 which delivered 124 hp (92 kW). In 1958 Bosch mechanical fuel injection was added to the 2.2 litre six and the engine, now giving 113-134 hp, was redesignated M127. The M127 was fitted to the last of the Pontons: the rare 1958 W128 220SE, of which fewer than 4000 were produced.
In 1959 the first series of W111 ‘Fintail’ 220SE and 220SEb models was introduced, with the carburetted M180 in the 220b and 220Sb and the M127 used in the 220SEb. The addition of the lower case ‘b’ was added to differentiate the Fintails from the earlier Pontons.
From 1965 the W111/W112 sedans superseded by the new W108/109 chassis (apart from the W111 230/230S, produced from 1965 to 1968 to use up the old fintail bodyshells). All models in 1965 were given up-rated engines, the M180's bore being increased by 2 mm (82mm bore × 72.8mm stroke = 2,307 cc/141ci) for the 230 and 230S. The new Mercedes-Benz W108 250S and 250SE gained new 2.5 litre (2,496 cc) variants of the mid-size six - the carburetted M108 and fuel-injected M129 - as did the W111 250SE two-door models. The extra capacity was obtained by lengthening the stroke by 6 mm (82mm bore × 78.8mm stroke = 2,496 cc/152ci). The M129 was fitted with a mechanically controlled six-piston fuel injection pump. A change from four to seven main bearings was necessary in order smoothly to handle the resulting increase in power. A larger capacity oil pump was also specified. Connecting rods were slightly shortened, reflecting the 6mm longer stroke in a block of unchanged overall height. The redesigned cylinder heads incorporated larger ports, and the valve diameters were increased by 2mm. On the fuel injected cars a six-plunger pump replaced the previous car's two plunger pump and the injectors were repositioned to give a more direct angle towards the inlet valve heads. The engine cooling fan now had six blades instead of four and incorporated a viscous coupling which activated the fan only when engine speed exceeded 3,000 rpm or the radiator water temperature reached a preset limit.
In 1968 the ‘New Generation’ or /8 models were introduced, along with a change to the venerable mid-size six. The M180 engine of the 230/230S was continued in the W114 230 and 230.6, while the M108 of the 250S underwent a few changes and was renamed the M114. This was also available with Bosch D-Jet fuel injection, as fitted to the 250CE. The M129 of the 250SE were superseded by the M130 engine of the 280S and 280SE. At this time the 'Big' six was discontinued and the 1968-1971 300SE/300SEL used the identical M130 engine used in the 280SE and 280 SL.
The final incarnation of the M180 line of engines was the 2.5L M123 which was seen in the W123 chassis 250, fitted with the notorious Solex four barrel.
The M130 was the ultimate variant of the mid-sized six, obtained by increasing the 2.5-litre versions' bore by 4.5 mm using the same stroke (86.5mm bore × 78.8mm stroke = 2,778 cc/170ci). This was the maximum practical enlargement of the engine given the limitations of the block as evidenced by the deletion of water passages between the cylinders.
Mid-sized six-cylinder head design
The mid-sized six carries over the 1951 190SL's four-cylinder SOHC general design, with non-cross-flow (reverse-flow) ports all located on one side of the head casting.
The combustion chamber design is very similar to the pre-WW2 Chevrolet "Blue Flame" OHV six-cylinder, with a "dog leg" L-shaped chamber. The exhaust valve is deeply recessed in a pocket containing most of the chamber volume, and the much longer intake valve is nearly flush with the deck surface. This design of inlet tract, with the large valve opening directly into the cylinder from an individual, semi-downdraught port, provides excellent gas flow resulting in the high volumetric efficiencies of these engines (157 and 158 BMEP from the 2.2L and 2.8L fuel injected versions)
1 Six Appeal, Mercedes Enthusiast, May 2007, pp 52-58