VANOS is a combined hydraulic and mechanical camshaft control device managed by the car's
DME engine management system.
The VANOS system is based on an adjustment mechanism that can modify the position of the
intake camshaft versus the crankshaft. Double-VANOS adds an adjustment of the intake
and outlet camshafts.
VANOS operates on the
intake camshaft in accordance with engine speed and accelerator pedal position. At the lower end of the engine-speed scale, the
intake valves are opened later, which improves idling quality and smoothness. At moderate engine speeds, the
intake valves open much earlier, which boosts torque and permits exhaust gas
re-circulation inside the combustion chambers, reducing fuel consumption and exhaust emissions. Finally, at high engine speeds,
intake valve opening is once again delayed, so that full power can be developed.
VANOS significantly enhances emission management, increases output and torque, and offers better idling quality and fuel economy. The
latest version of VANOS is double-VANOS, used in the new
VANOS was first introduced in 1992 on
the BMW M50 engine used in the 5
|Here's how it works:
In overhead cam engines, the cams are connected to the crankshaft by either a belt or chain and
gears. In BMW VANOS motors there is a chain and some sprockets.
The crankshaft drives a sprocket on the exhaust
cam, and the exhaust cam sprocket is bolted to the exhaust cam. A second set of teeth
moves a second chain that goes across to the intake cam. The big sprocket on the intake cam is
not bolted to the cam, for it has a big hole in the middle. Inside the hole is a helical set of teeth.
On the end of the
cam is a gear that is also helical on the outside, but it's too small to connect
to the teeth on the inside of the big sprocket.
There is a little cup of metal with helical teeth to match the cam on the inside and to match the sprocket on the
outside. The V (Variable) in VANOS is due to the
helical nature of the teeth. The cup gear is moved by a hydraulic mechanism that works on oil pressure controlled by the
At idle, the cam timing is retarded. Just off idle, the DME energizes a solenoid which allows oil pressure to move that cup gear to advance the cam 12.5 degrees at midrange, and then at about
5000 rpm, it allows it to come back to the original position.
The greater advance causes better cylinder fill at mid rpms for better
torque. The noise some people hear is the result of tolerances that make the sprocket wiggle a bit as the cup gear is moved in or
Double-VANOS (double-variable camshaft control) significantly improves torque
since valve timing on both the intake and outlet camshafts are adjusted to the power required from the engine as a function of gas pedal position
and engine speed.
On most BMW engines that use a single VANOS, the timing of the intake cam is only changed at two distinct rpm
points, while on the double-VANOS system, the timing of the intake and exhaust cams are continuously variable throughout the majority of the rpm range.
the opening period of the intake valves are extended by 12 degrees
with an increase in valve lift by 0.9 mm.
Double-VANOS requires very high oil pressure in order to adjust the camshafts very quickly and accurately, ensuring
better torque at low engine speeds and better power at high speeds. With the amount of
un-burnt residual gases being reduced, engine idle is improved. Special engine management control maps for the warm-up phase
help the catalytic converter reach operating temperature sooner.
Double-VANOS improves low rpm power, flattens the torque curve, and widens the powerband for a given set of camshafts. The double-VANOS engine has a 450 rpm lower
torque peak and a 200 rpm higher horsepower peak than single-VANOS, and the torque curve is improved between 1500 - 3800 rpm.
At the same time, the torque does not fall off as fast past the
of double-VANOS is that the system controls the flow of hot exhaust gases into the intake manifold individually for all operating conditions. This is referred to as "internal" exhaust gas re-circulation, allowing very fine dosage of the amount of exhaust gas recycled.
While the engine is warming up, VANOS improves the fuel/air mixture and helps to quickly warm up the catalytic converter to its normal operating temperature. When the engine is idling, the system keeps idle speeds smooth and consistent thanks to the reduction of exhaust gas re-circulation to a minimum. Under part load, exhaust gas re-circulation is increased to a much higher level, allowing the engine to run on a wider opening angle of the throttle butterfly in the interest of greater fuel economy.
Under full load, the system switches back to a low re-circulation volume providing the cylinders with as much oxygen as possible.
E34 525i M50 (91-95) Engine
upgrades from Korman. See what Korman can do to a VANOS engine
with a new chip and camshafts. Similar
results for the E36.
describes valve timing, which manufacturers have it, with a paragraph
manual repair page shows how to install a VANOS control unit.
review of BMW's 3.0-litre inline six has a description of double-VANOS.