F 800 GS Design
The parallel twin familiar from the F 800 S/ST models forms the basis for the GS drive. For the new usage, however, some aspects of the engine have been modified. In order to allow long spring travel with a wheel spacing that remains moderate, plus an ideal distribution of weight, the cylinders are now only angled forward by 8.3 degrees instead of the 30 degrees with the F 800 S/ST.
This solution has been made possible by a new design for the lower engine housing section made from die-cast aluminum. It contains application points for the engine protection plate and provides the ideal working conditions for the semi-dry sump lubrication. In addition, the cylinder head has been reinforced in the area of the frame connection at the front on the right.
The GS engine has a modified clutch cover which creates more space in the footrest area and more room for a new oil dipstick and a changed clutch release shaft. Finally, the water pump housing and cooling hose connections have been adapted to the new position of the engine.The parallel twin familiar from the F 800 S/ST models forms the basis for the GS drive. For the new usage, however, some aspects of the engine have been modified. In order to allow long spring travel with a wheel spacing that remains moderate, plus an ideal distribution of weight, the cylinders are now only angled forward by 8.3 degrees instead of the 30 degrees with the F 800 S/ST.
This solution has been made possible by a new design for the lower engine housing section made from die-cast aluminum. It contains application points for the engine protection plate and provides the ideal working conditions for the semi-dry sump lubrication. In addition, the cylinder head has been reinforced in the area of the frame connection at the front on the right.
The side-effect of these modifications is that the engine is a kilogram lighter than the unit used in the S/ST model series.
Like the famous F 800 engine, the transverse parallel twin works with an even firing order (360° ignition offset) without crank offset on the crankshaft. Because there is one firing cycle for each crankshaft rotation, the sound is deliberately like that of boxer engines which work with an identical firing offset!
Most importantly, however, the even firing sequence creates the best conditions for a balance load change with high torque yield.
Unique Engine Balancer
The mass forces are offset by a balancing mechanism that is unique in series engine design. Instead of having conventional counterweight shafts, the oscillating mass forces are balanced by a joint system on the center of the crankshaft with deliberately positioned counterweight masses: an eccentric on the crankshaft offset against the crank pins by 180° carries what is known as a balancing connecting rod.
This connecting rod is hinged to an approximately horizontal balancing rocker. The kinematics are designed in such a way that the balancing connecting rod moves contrary to the two engine connecting rods. Because it is guided through the relatively long rocker, an almost linear swivel movement of the connecting rod head is achieved -- that is, the small connecting rod eye describes a slightly curving path.
The mass distribution at the connecting road head and rocker is chosen in such a way that the mass forces resulting from the swinging movement combat, in every crankshaft position, the oscillating mass forces from the crank drive (piston and con-rod proportion). This means that first and second order mass forces are almost completely eliminated, resulting in low-vibration engine running.
Engine Lubrication System
The oil circulation system of the F 800 GS engine also includes a number of sophisticated details. To prevent punch losses, a semi-dry sump lubrication system has been designed which works without any separate engine oil tank.
Lubricant coming from the main bearings collects in a tray which also holds the balancing system and which is sealed off from the actual oil tray. Oil in this area is constantly vacuumed up by an oil pump and transported to the transmission housing, before running from there, under no pressure, via openings in the crank housing into the oil tray. The oil pressure pump supplies the lubricating oil system from this reservoir.
Cam Shafts and Timing
The cylinders in the GS engine are fitted with a high-tech cylinder head. As in the new engines in the K model range, here also, two upper camshafts driven by a toothed chain rotate and control four valves per cylinder via cam followers.
The F 800 GS, unlike the S/ST models, uses slightly modified camshafts which help the engine develop its power in a way that is ideal for enduros: with excellent torque, even and easily controllable. The valve drive via cam followers is low-wear, only produces minimal friction losses and is particularly rev-proof. The valve play therefore only needs to be checked after 20,000 km at the earliest.
Valve timing BMW F 800 GS
- Intake opens 14° after OT
- Intake closes 18° after UT
- Outlet opens 18° after UT
- Outlet closes 14° before OT
- Valve lift 9.64 mm
Fuel Injection
Other features that are typical of BMW include the fuel mixture preparation, which is via a manifold injection with BMS-KP engine control and two 46 mm throttle valves. The injection quantity is determined by the specially tuned engine control, not only via the injection period but also via the pressure that the fuel pump provides, depending on the power requirement.
The fuel system operates without return and only carries the quantity that the engine actually needs. Because of this economical, patented regulation system, the fuel pressure can be modified within a wide range so that the mixture is always ideal.
To measure the fuel quantity supplied, not only the well-known parameters such as load, engine speed and temperature are used, but also the residual oxygen content in the exhaust gas. The corresponding information is provided by a lambda probe positioned at the point where the manifolds join. This is followed immediately by the three-way catalytic converter, provided as standard, which quickly warms up after the cold start and can thus start its conversion work quickly.
The air necessary to form the mixture reaches the new suction silencers via suction snorkels positioned in the cool air stream. The positioning well above the engine, which is good for both on-road and off-road riding, and the
large volume, which supports torque were made possible by the positioning of the tank under the seat. The suction snorkels of the F 800 GS are visible as a design element.
Exhaust System
The completely new, weight-optimized exhaust gas system is made entirely from high-grade steel and weighs in at just 8.5 kg. The single-wall manifold system is connected with the silencer via a plug connection with tension spring.
The attachment of a slip-on silencer is therefore easy and less costly, especially since the catalytic converter is integrated into the manifold. The end silencer provided as standard is constructed as a two-chamber silencer in a combined absorption/reflection design, offers a gas flow volume of 8 liters and is mounted on the left side of the motorcycle, half way up.
The F 800 GS is fitted with a secondary air system which, in combination with the regulated catalytic converter, reduces the emission of pollutants to EU3 requirements.
High-Revving Engine With Great Torque
The modern fuel injection system isn't the only thing that ensures a prompt response from the F 800 GS twin; its low centrifugal mass also contributes to its agile power development. Even at the bottom of its rev range, the four-valver, which, with a bore-stroke ratio of 82 to 75.6 mm is not particularly short-stroked, soon speeds up and releases a good 90% of the maximum torque in the broad range between 4,000 and 7500 RPM.
Between 5,000 and 8,000 RPM, the engine develops its power dynamically, accompanied by a unique "Boxer" sound. The nominal performance data of the 798 cc twin of 85 HP at 7,500 RPM and 61 lb-ft of torque at 5,750 RPM therefore gives an incomplete picture of the potential that is available in practice.
In combination with the low total weight and closely stepped six-speed trans-mission, the F 800 GS accelerates from 0–100 km/h in about 4 seconds, and the top speed is over 200 km/h.
But it is not only the acceleration of the new GS model that is impressive; its traction is also remarkable. The BMW development engineers have focused quite deliberately on the smooth, confident development of power in the mid engine speed range rather than on absolute peak performance.
Sporting riders will be bowled over by the engine’s acceleration, while touring riders should enjoy the twin’s strong traction with minimal shifting. The parallel twin, which has been optimized for Enduro operation, not only manages to walk the tightrope between powerful torque and dynamic acceleration, it also proves that good performance does not necessarily involve high fuel consumption.
Ridden over country roads, a consumption of well below 5 liters of premium grade fuel per 100 km is possible. If required, the F 800 GS can also be fitted for use with normal octane fuel, which, however, reduces the peak performance by 2 HP and increases fuel consumption slightly. This modification is carried out by calling up a characteristic map in the control software, and can be cancelled again at any time.
The sophisticated engine concept shows additional technical finesse in its peripherals. The water pump, for example, is on the right of the cylinder head and is driven by a cog wheel unit on the camshaft. The advantageous positioning of the pump -- directly behind the radiator with integrated thermostat -- means that only short hose connections are needed. The engine therefore looks particularly compact and tidy.
An oil-water heat exchanger next to the easily accessible oil filter ensures that the engine warms up particularly quickly after a cold start. In addition, the heat exchanger also limits the engine oil temperatures.
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