Vanos..

[Land Shark <lndshrk@xxxxxxxxxxxx>]

OK.. here we go.. 

In normal overhead cam engines, the cams are connected to the crankshaft by either
a belt or chain and some sort of gears..

In M50, M52, and S50 motors , we have chain and sprockets (like on a bike)..

The crankshaft drives a sprocket on the exhaust cam.. the exhaust cam sprocket is BOLTED
to the exhaust cam... a second set of teeth on it are moving a second chain that
goes "across" to the intake cam.. here's where the magic is..

The big sprocket on the intake cam is NOT bolted to the cam.. it's got a big hole
in the middle of it.. in the inside of that hole is a helical set of teeth (helical
means they curve to the left or right as they go thru the metal.. not straight)

Now.. on the end of the CAM is a little gear that is also helical on the OUTSIDE
but it's too small to connect with 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.... now.. as the sprocket turns.. it turns
the cup thru one set of teeth, and the cup turns the little gear inside which, being
bolted to the cam.. turns it.. so now.. the chain from the turning exhaust cam is also
turning the intake cam.. although a bit indirectly :)

The V (Variable) in Vanos is due to the HELICAL nature of those teeth.. if you push
the cup in.. the relative sync of the cam to the sprocket is changed.. same if you
pull it out some.. in our BMW's, the full travel of that gear will cause the cam to
move a full 12.5 degrees while the outer sprocket doesnt move.. 

Now.. what moves this cup gear?? .. a hydraulic mechanism that works on oil pressure
controlled by the DME.. 

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 5000rpm, 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 complain about is usually tolerances that make the sprocket wiggle
a bit as the cup gear is moved in or out.. sounds like a bunch of marbles in
a box.. and it comes from the top DRIVERS side of the motor.. near the bump in the front
of the cylinder head where the Vanos system is.

Hope this helps..

Jim

PS: And ALL Vanos motors make that marbly, rattly, "gargling" noise.. some owners just
    are a bit more sensitive to noises like that..

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End of bmw-digest V9 #294
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