static ignition timing

[budgerian]

I'm about to set the timing on my 3.0 Si D-jetronic after changing rotor, condensor, points, plugs and cables and vacuum lines, but I can not comprehend why the static timing for these engines is at TDC according to the manuals. How is this supposed to work? Logic dictates that there should always be at least some degree of advance.
But since the BMW guys are no fools either I must be misunderstanding something here. Who can enlighten me?


Roger

 

[jhjacobs]

Static timing is normally used to get close enough to start the engine. It is also hard to get better than 5-10 degrees of accuracy. Also, the normal timing is set to xx degrees BTDC with the engine running and vacuum lines off. This means that the centrifugal advance is advancing and the who knows what the difference in vacuum will be doing. Additionally, there is a small dynamic effect from the points when stopped vs. running. I suspect that with engine off, TDC is a very close approximation of the equivalent timing and it will almost always be close enough to start the engine.

I don't know how many degrees of advance the centrifugal gives but I would bet that you would also get close by setting the engine to XX degrees BTDC (based on your engine spec) and then manually advancing the rotor while doing static timing; this will miss the vacuum contribution at the desired engine speed but probably will it will be in the ball park.

I"ve used static TDC timing on every engine I've every rebuilt, I think it is a tried and true technique widely used. I learned this from old school guys; they also taught me to use a beer can pull tab to set the point gap. Most of these guys had less than a full mouth full of teeth so you can form you own judgement.

Good luck.