350SL, 380SL and 420SL: The history of the M116 engine and should you buy a Mercedes-Benz 420SL?

In our showroom we currently have a selection of 4.2 litre V8 Mercedes 420 SLs, in a variety of different colours. The 4.2 litre V8 is a refined and capable power plant, distinctly different to the larger 500SL and smaller 300SL engines, with a character of it’s own and a specific personality… but where did this engine come from?

The Mercedes-Benz 420SL was introduced in 1986, available in the R107 SL range until the end of production in 1989.

This new 4.2 litre V8 engine was the final incarnation of the M116 engine, first seen in 1969 with the 300SEL 3.5.

What is the Mercedes-Benz 420SL…
And where did it come from?

In the late 1960s Mercedes-Benz needed a V8 engine to offer more power, something the US market was accustomed to. The M116 engine was a 3.5 litre V8 producing 197hp – it would be used in the early R107 350 SL and 350 SLC, as well as the W108, W109, W111 and W116 coupes, saloons and convertibles. Utilising a cast iron cylinder block with aluminium heads, it had one chain driven overhead camshaft for each cylinder bank and rocker arm operated valves.

Where it began… The 3.5 litre V8 engine as fitted to the 350SL and 280SE 3.5. This engine is full of character, charming and raw whilst being both antiquated and enjoyable. An early 350SL feels markedly different to the cars that replaced it in the R107 lineage.

From 1971 to 1976 the M116 engine had a 9.5:1 compression ratio with Bosch D-Jetronic injection. These engines delivered around 200hp at 5,800rpm, whilst low compression versions (with compression ratios of 7.5:1 for certain markets where fuel quality was an issue) produced 180hp.

In 1976 the compression ratio was lowered to 9.0:1 and injection changed to a Bosch K-Jetronic constant flow system, along with hydraulically operated valves. This variant of the M116 produced 195hp at 5,500rpm, with 202 lb ft torque at 4,000 rpm until 1978, then torque increased to 210 lb ft.

1980 – The 350SL becomes the 380SL

The 350SL was replaced by the 380SL in 1980, powered by two new variants of the M116 engine. The 380SL arrived in European markets 5 months before the USA, because the early version of the 380SL engine didn’t meet US emissions regulations.

Between 1980 and 1981 all markets except the USA had a 3,818cc engine, after which all engines (including the USA) would be 3,839cc with different bore and stroke dimensions – redesigned to meet USA market regulations. Where the 3.5 litre 350SL had a cast iron cylinder block, the 380SL used a lightweight alloy unit.

In Europe the early versions of the 380SL had a compression ratio of 9.0:1 producing 215hp with 224 lb ft torque at 4,000 rpm, with power dropping to 201hp from 1981 onwards.

The USA market 380SL with the same 3,939cc produced 155hp at 4,750 rpm, thanks to restrictive catalytic converters in the exhaust system. Unlike the European 380SL, the USA cars from 1981 to 1984 used a single row timing chain, which was prone to stretching. Many of these were replaced (under warranty) to dual timing chains with twin sprockets, which became standard for 1984.

For most classic cars, rust-free American cars from sunny states are popular to import, the restrictive emissions equipment and lower compression engines have always been slightly less desirable than European specification SLs.

The 380SL refined the format, giving the R107 SL a completely different character to the 350SL that preceded it. It’s not just the engine, but the changes to the cabin and interior mean these two strikingly similar cars give a very different driving experience. Hope from a 350SL into a 380SL and you’ll note that the 350SL feels like a muscle car of the past, left over from the heydey of motoring, whilst the 380SL feels like a step into a bold new future… a trend that the 420SL would develop.

1986 – The 380SL becomes the 420SL

In 1986 Mercedes-Benz overhauled the R107 range, replacing the 280SL with the 300SL and the 380SL V8 with the new 4.2 litre Mercedes 420 SL. This would be the final roll of the dice for the M116 engine, with displacement now bored out from 3,839cc to 4,196cc. It would be used in the S Class as well as the SL.

With 215 hp, the 420SL sat above the 185 hp of the 300SL and below the 241hp of the larger 5.0 litre 500SL. (420SL engines fitted with catalytic convertors were rated at 201 hp, and had electronically controlled mechanical fuel injection)

The European ‘non cat’ Mercedes 420 SL utilised a 9.0:1 compression ratio, producing more torque than the 380SL. It used a Bosch KE-Jetronic injection system with EZL breakerless ignition.

The Mercedes 420 SL arrived in 1986, the year the R107 SL underwent a significant transformation. If the 380SL was a departure from the 350SL, then the 420SL was an entirely different beast. Mercedes-Benz further refined the interior and driver comforts, whilst improved handling and braking were complimented by larger, wider wheels and a lower front apron. A grown up SL to take the model into the tail end of the 1980s.

Should I buy a Mercedes-Benz 420SL?

Of all the variants the Mercedes-Benz R107 SL was offered in, the 420SL is the least numerous. Some 13,500 300SLs were built, with 20,000 500SLs and almost 50,000 560SLs, whilst 420SL build numbers totalled just 2,148 worldwide.

We estimate that around 600 of these cars remain in the United Kingdom, a third of the number of 300SLs in the country.

A 420SL is distinctly different to drive when compared to either a 300SL or 500SL. A V8 propelled car will always stand apart from a six-cylinder, with that traditional V8 burble and the heavier feel of the larger powerplant. Compared to the raw and somewhat raucous 500SL, the 4.2 litre 420SL feels refined and subdued without lacking any of the driving potential.

This presents a chance to own the rarest of all SLs, fitted with one of the finest engines.

The Mercedes 420 SL is more than deserving of your attention. Find yours here.