W116

The last Mercedes-Benz eight-cylinder engine to be reviewed in detail by Autocar - indeed the last Mercedes engine of any kind - was the split, in-line straight eight which re-established the three pointed star reputation in post war racing. With its desmodromic valve gear and middle power take-off it was a complex engine. Indeed the recollection of it pre-war Mercedes engines has left the impression in a good many people's minds that all Mercedes engines are complex.

A look at the latest small vee-8 from Unterturkheim quickly dispels this illusion. It is a copybook example of how experience plus careful design can create a simple-to-make high output engine of considerable refinement. With a bore and stroke of 9smm x 65.8 the 3,499c.c. engine produces 200bhp DIN at 6,500 rpm. Maximum torque is 211 lb.ft. at a relatively high 4,000 rpm.

M116 3.5L V8

This cross-section shows the generous water passages and port layout.

A number of factors influenced the choice of vee-8 configuration when design was started in 1963. A primary requirement was to provide a larger capacity power unit for the important US market. Also, although West German and European fiscal laws penalized heavily any size bigger 2.7 litres, imported cars like the 3.4 and 3.8 litre Jaguars were selling well to enthusiasts. Late in the development period it became known that German motor tax laws were likely to be changed to allow for bigger engines. A 3.5-litre was therefore a sure bet for the home market.

Having got around to considering cylinder arrangements it was felt that a 3.5-litre six-cylinder engine with modern over-square cylinder dimensions would be unacceptably long. Also Daimler Benz were becoming safety-conscious and realised the need to build-in a crush area ahead of the engine which was either empty of full of "soft" components. A long engine would have entailed a very long bonnet.

A vee engine was the obvious choice. Its shape is easier to fit between front suspensions than a flat engine and does not compromise steering lock. Furthermore there is room between the cylinders and under them for exhaust systems and ancillaries. Six-cylinder and flat-crank vee-8s were studied on paper and on computer, but were discarded. The first because of its whirling shake and the other because of its own variety of secondary cross shake. A main consideration was that the engine should run for 50,000 miles without major attention. Sections of the US clean air law require an engine to maintain its emission characteristics for this milage.

At the same time the unit had to be economical to produce, for Mercedes sell in Germany for half the price they are sold here in the UK, had to develop 200 bhp and should be capable of running at full power indefinitely.

Constant high speed running calls for low piston speeds to minimise wear. The 65.8mm (2.59in.) stroke chosen results in a mean maximum piston speed of 2,800 feet per minute, well within the capabilities of modern piston ring materials. For example the Mercedes 250 engine is operating at a mean piston speed of 3,100ft/min, a road speed of 105mph. The 3.5 vee-8 300SEL coupe is geared for 126.2mph at maximum engine revs.

Should cast iron seem an old fashioned material for the cylinder block of an advanced engine from one of the most respected companies in Europe, it should be borne in mind that the engine was designed at the period of maximum disillusionment with light alloy in the USA, where the Mercedes was intended to be sold. Mercedes also had their good reasons for the choice. Cast iron is economical to buy, predictable and has good wearing and sound damping properties. The weight penalty was 30kg (66lb) compared with aluminium. But it was reckoned that at least that weight of sound damping material would have been needed to subdue the increased noise transmission from a light alloy block. Also West German noise regulations call for a noise measurement from the side of the car. It is uneconomical to sound damp the sides of the engine bay just for this purpose.

Another desirable attribute in an engine is rigidity. Cast iron is better in this respect than aluminium. The short stroke also makes for a compact and therefore stiff cylinder block and crankcase unit. Every effort has been made to keep size down, and therefore weight. The result is a casting which measures only 10.5in high and 16in wide in its machined state. These dimensions include the crankcase walls which are extended 2.6in below the crankshaft centreline almost to the bottom of the crank swing. The object is to increase the beam stiffness of the unit with the gearbox. Further to increase stiffness, the five malleable-iron main-bearing caps are each retained by four bolts in line. They do double duty as ties between the crankcase walls. The bearing caps fit into mortise recesses in the crankcase partitions for accurate side location. The recesses supplement the clamping loaps the four fixing-bolts in preventing shuffle.

Ample water jackets are provided right round the bores and the full depth of the cylinders. These are essential to get as much heat as possible away from the engine when it is working at full power inside a hot engine compartment on a hot day.

Five 64mm main bearings support the forged and nitrided two-plane, four-throw crankshaft. With the 90 deg cylinder angle this arrangement is free from primary and secondary out-of-balance forces and gives equal firing intervals. However, irrespective of firing order, the exhaust pulses on each bank are irregular and cross-over exhaust pipes are called for if the familiar vee-8 beat is to be eliminated. Large balance weights formed on the end webs are drilled for final balance. Thrust is taken at the centre main bearing and the high speed garter spring lip seal, working on the flywheel boss, retains oil at the flywheel end. A nice point is that the boss is ground with a spiral pattern which acts as a micro-groove Archimedean pump pushing stray oil back into the sump. In order that the main-bearing cap faces and recesses can be machined at one pass of a milling cutter, the seal is carried in a separate, bolt on diaphragm.

The compact dimensions of the engine have not been achieved at the expense of an unfavourable connecting-rod to stroke ratio. The 135mm rods (between centres) gives a ratio of better than 2 to 1. Apart from reducing piston side thrust there is room to use a longer stroke crank in conjunction with a taller block, using the same rods. The rods themselves are steel forgings, carefully proportioned to avoid stress raisers, with balancing pads at both ends. The cap dowel-bolts have knurled heads which are an interference fit in circular recesses in the shoulders of the rods. This avoids the stress raising notch that is usually milled across the shoulder to go with a D head bolt.

M116 3.5L V8

There is a viscous coupling for the fan and twin vee belts drive the power steering pump.

Big and small end sizes are 52mm and 26mm respectively. Lubrication to the fully floating gudgeon pins is via a drilling through the connecting rods and from oil traps in the faces of the pistons. These are light alloy forgings with cast-in steel anti-expansion rings. Bearing in mind that customers are likely to drive the engine flat-out from the word go, all the rings have molybdenum sulphide inserts. Two compression and one scraper ring per piston are fitted. The second compression ring in interesting in having a recessed "nose" ground in the lower face and a backing spring.

Going against the trend towards bowl-in-head pistons, the M116 has been given cross-flow wedge heads with large squish areas. Mercedes have found that the large quench areas so formed keep hotter than small ones and reduce hydrocarbon emissions. The size of the vestigial combustion chamber is virtually defined by the 44mm inlet valves which are inclined at 20 deg from the vertical and work on cast-iron inserts in the light alloy heads. Inlet throat diameter is 40mm with 38mm tracts. This diameter suggests a mean gas speed of 280 feet per second at full speed which is relatively slow and is the probable reason for the high speed, 4,000rpm, at which maximum torque is produced. However, the torque curve is flat, with 170 lb ft developed at as low as 1,000rpm and 211 lb ft at 4,000rpm. Exhaust valves are sodium cooled and special attention is paid to getting water flow round the guides.

Simple tappet adjustment

Mercedes prefer to operate the valves of their overhead camshaft engines by means of rockers, as they have always done, rather than following the trend to bucket tappets. Their reasons are that they are dealing with a known quantity and that the valve clearances can be adjusted quickly by most engine mechanics. While they agree that shimmed bucket tappets will usually go the Federal-required 50,000 miles without attention, they might needs it before that and most assuredly will shortly afterwards. With reason they point out that not every workshop in the USA will take on the job.

Rocker layout is similar to that of the Pontiac Tempest, with the rockers pivoting on spherical-headed, adjustable posts. This ensures that line contact with the cams is maintained and leaves the door open to make a simple conversion to hydraulic adjustment at a later date by substituting hydraulic adjusters for screwed posts. Each rocker weighs 80 grams of which 28 grams is reciprocating weight.

It is interesting that Mercedes like Jaguar have chosen chain drive instead of belts for their camshafts. The main reasons for the choice were the known long life of chains and the fact that belts would have to be replaced at approximately 25,000 mile intervals. The layout is also the same as that of the Jaguar V-12. A single run, duplex chain is driven by an 18 tooth sprocket on the crankshaft and passes round 36-tooth sprockets on the camshafts and under a 36-tooth distributor drive sprocket located in the vee of the engine. Long, rubber covered spring steel guides check lash on the drive side of the run and a hydraulically backed spring tensioner controls the slack side. The whole assembly is contained by a simple, die-cast alloy cover in which are formed the water pump and distributor drive housings.

Lubrication is provided by a gear pump slung beneath the front main-bearing cap and driven by a chain reduction gear from the front of the crank. Mercedes prefer chain for this application because of the high loadings that go with skew gears. They only tolerate them for the distributor because of the very light loadings. Oil is picked up from the wall of the stepped sump by a collector fitted with a specially shaped diaphragm-pickup. It maintains suction even when the oil is surging under 1 G cornering forces. The rather shallow sump well is dictated by the need to keep the engine overall height to a minimum (27.25in) to help their stylists achieve a low bonnet line.

Oil is pumped to an oil radiator usually in front of the main radiator, before being filtered and passed to the crankshaft. Separate drillings from the main gallery, located in the angle of the vee, are taken to copper pipes running the length of the cam boxes and feeding the camshaft bearings. The camshaft chain is lubricated by oil mist.

Cooling is straightforward by means of an involute pump cast into the front face of the timing cover. It is driven, along with the steering pump, by twin-belts from a multi-groove pulley on the crankshaft nose. The fan, with a thermal clutch, is mounted on the outer end of the water pump pulley. Water is pumped first into the jackets with the aim of getting a good scouring action in this area before it passes into the inlet.

Fuel injection and ignition

With Bosch only just around the corner it is only natural that Mercedes go to them for all their electrical and petrol injection equipment. The Bosch transistorized ignition and electric fuel injection is triggered by the ignition distributor driven by the camshaft chain.

Induction air is ingested through a pancake air-cleaner into a vertical trunk, where the throttle butterfly is located. The trunk in turn feeds a horizontal manifold located in the vee, from which four swan neck pipes each feed a pair of inlet ports. Fuel is injected through vertically disposed solenoid controlled injectors immediately upstream of the valves. In common with the Bosch mechanical system, the injection pulses are not exactly timed to valve opening. There are four pulses to two turns of the crankshaft and each pulse injects fuel into two tracts paired in the order 1 and 5, 4 and 8, 6 and 3, 7 and 2; which is in phase with the firing order. For example, group one injection takes place after the inlet valve of number one cylinder has been open for 30 degrees of crank rotation and the inlet valve of number five is due to open when the crank has rotated a further 60 degrees. The injections "computer" takes into account inlet ambient temperature, barometric pressure and cooling water temperature. The last named sensor is constantly variable and actuates a slow-running bleed behind the butterfly valve.

The virtue of this injection system is that it does the work of an eight choice carburettor system that should be viewed in this light when considering cost. Although the power output is no better than if carburettors were used, hydrocarbon emissions are lower and more easily controlled.