In Reply to: What does this diaghram do on 74s? posted by 12 11 1 358 847 on December 22, 1999 at 13:55:22:
Part number 12 11 1 358 847 is the BMW number for the distributor bellows only. A vacuum bellows can be either one which advances timing or retards timing. Tiis (74+) used a vacuum retard system. At wide open throttle, the advance curves for the mechanical advance distributor and the mechanical advance/vacuum retard distributor are exactly the same. There is no performance advantage to the mechanical (no vacuum) type distributor. The vacuum unit did help with emissions under partial throttle conditions.
Have a great New Year, Paul.