VANOS Variable Valve Timing: Evolution, Troubles, and Service

March 25, 2013 | Reply More

If you’ve ever built an old-fashioned pushrod V8 engine for high performance, you might be familiar with a little item called an “offset cam drive key.”  This is basically just a Woodruff with a step in it that indexes the sprocket to one side or the other of its slot in the nose of the camshaft, allowing you to advance or retard valve timing as you see fit to shift the power curve up or down the rpm scale. A variation on this is the offset bushing used for pin-drive‑type sprockets. Having the valves open and close earlier gives better low‑end and mid‑range output, while the opposite contributes to increased power as you near redline. But, obviously, you have to choose one or the other, and, with single-cam designs, intakes and exhausts are equally affected, which isn’t anything like optimal.

How about if you could make this dynamic with valve timing changes occurring when appropriate?  Combined with separate adjustments for the intake and exhaust cams of a DOHC engine, the potential improvements in power output, efficiency, and idle smoothness become very dramatic indeed.  This situation was the impetus for the introduction of VANOS (a somewhat implausible abbreviation of the German variable Nockenwellensteuerung), BMW’s name for its highly-accurate variable valve timing system.  The original version, introduced in 1992 on the M50, operated on the intake camshaft only, and had just two phase-change points.  This evolved into the much more sophisticated Double Vanos, which not only adjusts the timing of both the intake and exhaust cams, but does so in an infinitely variable manner.

NOTE: Valvetronic is BMW’s variable valve lift system.  It uses a fast-acting (300 milliseconds for full range!) mechanism controlled by a dedicated module to dynamically alter valve lift profoundly enough to allow for the elimination of the traditional throttle and its attendant pumping losses.  In many models beginning in 2001, it’s used in conjunction with VANOS for the most efficiency and power imaginable. This is a separate system that we’ll cover in a future issue of the bimmer pub.

Spiral action

The first VANOS unit, which debuted in ’92, has one helical-gear mechanism for advancing the intake cam. It only featured two phase changes, but allowed for a combination of smooth idle and improved mid-range and top-end power.

The basic mechanical principle at work here begins with a cam-drive sprocket that engages the camshaft itself through a helical or spiral gear.  When engine oil pressure is applied to this mechanism (as controlled by the DME through a one-wire solenoid valve), it moves the gear axially against a spring, changing the positional relationship between the sprocket and the shaft.  In other words, the cam is twisted slightly from its static position with regard to the crankshaft.

Cam timing is retarded with the engine off, at idle, and just above, which contributes to smoothness and helps get the cat hot quickly.  As rpm rises, the DME applies the hydraulic pressure that pushes the gear, twisting the camshaft and providing

 

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