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Re: NON VANOS ROUGH IDLE SOLUTION (archive)

[ Follow Ups ] [ 5-series (E34) Message Board ] [ Msg. Board FAQ ]

Posted by Jack Sweet on February 22, 2000 at 17:29:11:

In Reply to: Re: NON VANOS ROUGH IDLE SOLUTION posted by moots on February 22, 2000 at 16:49:49:

Advancing and retarding the cam means changing the rotational position of the camshaft(s) themselves and, therefore, the positions of the lobes in relation to a given point of crankshaft rotation. This can be done via offset sprocket bushings or an adjustable camshaft sprocket. You can even get custom-ground camshafts with the "advance" or "retard" ground in.

If I understand correctly, there are two things going on here. First, you'll be opening and closing the exhaust valves "sooner" in relation to a given point of crankshaft ratation and opening and closing the intake valves "later" in relation to a given point of crankshaft rotation. This will, I think, widen (?) the overlap of the intake valve opening/exhaust valve closing events. This can result in a smoother idle.

Second, closing the exhaust valves "sooner" and opening the intake valves "later" can increase cylinder pressure a tad. This can result in a couple of more h.p.

(The same thing can be accomplished in a pushrod engine like a small Chevy, for example, by installing a camshaft with wider lobe centers and installing it 3 or 4 degreed advanced. You'll smooth out the idle and pick up some top end, all other camshaft specs. being equal, but you'll kill some low-end grunt).

I'd suggest that the biggest gains from this mod would probably be seen on a full-tilt race engine, though. Might not be worth the effort on a street driven car. I dunno, though. Any BMW engine gurus out there?


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