V8

mercedes-560sec_engine_bay

V8 history

If you own, or are considering purchasing a classic W126 Mercedes, you may be interested in the events that led to the first Mercedes V8 engine.

Mercedes first developed a V8 engine before the war as a reaction to rules implemented by the Italians for the 1939 Tripoli Grand Prix. At the time regulations had allowed a cylinder capacity of up to 3,000cc. The new rules allowed only 1,500cc single seat cars to take part.

This change was a problem for Mercedes for a racing sector that they had dominated. Undaunted the manufacturer set to work on the W165 built especially for the new rules. These successful cars came both first and second in the race driven by V8 engines.

The V8 was most likely result of the lack of time available to the engineers. A smaller engine was needed than the V12 used in the W154 and the V8 was a stripped down version. The bore was reduced from 67mm to 64mm and stroke set at just 58mm producing an engine with an output of 256HP using a single stage compressor. The V8 was planned in September 1938 and under construction by November 1938.

The smaller V8 car was extremely agile and received the ‘thumbs up’ from the two Mercedes drivers Caracciola and Lang when they tested them at Hockenheim.

Unfortunately, the W165’s were lost with the advent of the war. There had been a plan to revive them in 1951 but the temporary closure of the F1 series ended that initiative.

The V8 racing engine returned in the W196 which won the new F1 World Championship in 1954 and 1955 driven by Juan Miguel Fangio.

V8 in a touring car

By the sixties Mercedes were building a V8 6,332cc V8 engine with Bosch fuel injection and a chain driven camshaft. Each line of cylinders had its own camshaft and the engine produced 250HP at 4,000rpm.
This engine was used in the Grosser Mercedes a car weighing between 2,470 and 2,640kg and measuring up to 6.25m in length. Even with this immense bulk to pull, the engine still managed to deliver a top speed of 205kph.
2,677 600 models were produced up until 1981.
By 1968 the same engine was being used for the 300SEL, a smaller, but still luxurious touring car. Designated the ‘300 SL 6.3’ the car boasted a top speed of 200kph achieved with an automatic transmission and weight of 1,740kg.

Smaller V8

A new smaller capacity V8 was then designed and built by Mercedes with 3,500cc with 92mm bore and 65.8mm stroke. Again with twin camshafts the engine produced 200HP on a compression ratio of 9.5:1.
Interesting, the weight of this engine was 229kg, just 18kg more than the 2,800cc 6 cylinder engine used by Mercedes at the same time. This was the smallest capacity V8 build by Mercedes up until that time still strong enough to drive the SL coupe and cabriolet to 210kph (205kph with automatic transmission).

W126 S Class

The S Class V8 range was launched in 1982 with 3.8 litre and 5 litre versions. Although powerful engines they were efficient for their time thanks to new electronics controlling starting and fuel injection.
When the 5.6 litre engine arrived with the 4.2 litre in the 1985 facelift it produced an output of 300HP.

5.6 litre S Class and rivals

At the time there were some more powerful production engines available such at the 340HP unit in the Ferrari 412, the 390HP in the Tessarossa, and the Aston Martin at 438Hp.
All these vehicles were effectively racing cars for the road not touring saloons and coupes. Of similarly large engine saloon cars the Rolls Royce V8 was 6,750cc but output was thought to be no higher than 200HP.

Why a 5.6 litre engine?

Why did Mercedes build another 5 litre plus engine when the one they had worked perfectly. Pollution regulations definitely played a part.
When the S class range was being produced the West German Government at Bonn were looking to fight pollution with a number of measures with the focus mainly on protecting German Forests.
Both the US and Japanese markets had strengthened anti-pollution legislation and the EU was looking to take similar steps.

The use of the catalytic converter reduced pollution but cut output by some 10%. This still left the 560 version with an additional 68HP and gave Mercedes a top of the range model to compete with the highest end of the luxury car market. (The lowest priced Rolls Royce of the time was nearly twice the price of the 560 SEL long wheel base saloon).

Increasing cylinder capacity

Increasing the cylinder stroke to 94.8mm from 85mm increased the cylinder capacity of the new W126 560 SEC and SEL models.

The new engine had 8 rather than 6 counter weights, a different position height, exhaust manifold, and air filter. The cylinder head was light alloy reducing the engine weight by 20%.
The improved design of the new engine helped to reduce both emissions and fuel consumption. The EZL electronic system used a micro-processor to find the most favourable ignition point depending on load. The KEIII fuel injection system adjusted fuel injection with load and engine speed and temperature to further assist efficiency.
All the V8 engines in the S class without the catalytic converter can use both leaded and unleaded petrol and those with the converter unleaded petrol only.

Other changes in the 560

The new 560 models had fixed caliper unitary discs as opposed to the floating calipers in the previous model. The brakes are 49% lighter and the wheels, fitted with 15 inch tyres, allow for larger discs.