While building the "Dragon" motor for my Z8, optimizing the VANOS system to take advantage of the upgrades has been of particular interest to me, and I thought some of you might enjoy a little refresher course on this remarkable system.

VANOS stands for VAriable NOckenwellen Steuerung (variable cam control) and is BMW's nomenclature for a system to adjust valve timing to optimize an engine's performance under varying conditions.

There is a VANOS system to control intake valve timing and another VANOS system to control exhaust valve timing. When both are used on an engine, the system is called DOUBLE VANOS. The Z8's engine incorporates DOUBLE VANOS.

The following explanation is over-simplified but hopefully adequate for discussion purposes. The purpose of the intake system on an engine is to combine air with fuel and deliver the mixture to the cylinders where it can be compressed, ignited by a spark to create power, then sent to the exhaust system for elimination. In order for the air/fuel mixture to enter and leave the cylinders, it must pass through openings in the cylinder heads. These openings have valves in them to control whether they are open or closed. Whether the valves are open or closed is determined by the camshafts which have lobes on
them to force the valves into the open position, while springs force the valves to close.

The important thing to understand is that on most engines (without VANOS) the rotation of the camshafts, thus the timing of the openings and closings, is fixed in relationship to the rotation of the crankshaft and therefore the positions of the pistons in the cylinders. The trouble with a fixed relationship is that as the engine runs at different speeds, the optimum timing of the delivery of the air/fuel mixture and its subsequent release into the exhaust system varies. This means that a fixed valve timing system can only deliver optimum conditions for combustion at one specific rpm; every other rpm will be a compromise.

Enter DOUBLE VANOS! By developing a way to vary the relationship of the camshafts' rotation to the crankshaft's rotation, BMW can optimise the timing of the delivery of the fuel/air mixture and its release into the exhaust system. This is accomplished by a moveable camshaft drive gear with a helical timing gear. As the camshaft drive gear moves in or out, the helical cut of the timing gear forces the camshaft gear to rotate forward or backward slightly. This changes the timing of the camshafts' rotation relative to the fixed rotation of the crankshaft, thus allowing the valve timing to be advanced or retarded. The onboard engine computer is put in charge of deciding what that timing should be to optimise engine performance and it uses hydraulics to move the shaft in or out. The Z8's DOUBLE VANOS system can adjust the intake valve timing by 54 degrees while the exhaust timing can be adjusted by 39 degrees. The adjustments take less than 250 milliseconds.

The benefits of having DOUBLE VANOS on the Z8 include high torque at low and medium speeds, reduced emissions, better fuel consumption and smoother idling. When combined with the Z8 engine's individually controlled throttle butterflys, engine performance can be controlled to an unprecedented degree, resulting in what many consider to be the finest production engine BMW has ever built.




Grease Monkey