Listener Stuart Watson sent us this email discussing dry sump engines, and their benefits and design details. Stuart pointed us at this link for more information.
The majority of road bikes use wet sump lubrication, the ‘sump’ being a pan beneath the crankshaft where the oil is stored. A pump picks it up from here and forces it around the engine into the various bearings, spray jets and so on. After the oil has passed through the high pressure part of the system it drains back down into the sump purely under the influence of gravity.
It’s simple and inexpensive, but there are disadvantages. The first is the size of the sump. Usually this will have to hold around 4 litres, maybe more. This is quite large – look at a 5 litre oil can to get an idea – and clearly it has to be at the lowest point, so the engine has to sit higher than might be ideal. Under hard acceleration or braking, or when the going is very bumpy, the oil in the sump can slosh around. In extreme cases this can mean the oil pump’s pick up tube becomes open to the air, and air bubbles are passed around the lubrication system, causing a lot of wear and damage. But it also means the oil can wash up against the crankshaft, which usually spins just above the surface of the sump oil. This causes a lot of drag, reducing engine performance as well as causing the oil to become foamy, which degrades its lubrication abilities.
Some bikes though use a semi-dry sump system, including many off-road machines as well as the BMW. What this really means is that the system is to all intents and purposes a dry sump design, with two oil pumps, but the oil tank is still incorporated inside the engine cases. In the F800’s case it’s still stored beneath the engine, but not directly beneath the crankshaft.
It’s a little more complex but by doing it this way the designers have more scope for lowering the engine and making it more compact.