So an update on the process since my 123\TUNE+ (Bluetooth) unit has arrived last Friday but not installed yet. My car is a 280 SL with emissions control in working order (Vacuum valve operating ok and controlled by speed relay (and 123 ignition pre-programmed at program E). I intend to do the following and would like feedback from the forum on my plan forward.
1. I will install the new 123\tune+ at 9 deg BTDC as per instructions using the green LED inside the unit to confirm static timing.
2. I will download the attached curves for centrifugal advance and vacuum advance, yes advance in this case. If you look carefully at the attached curves I use the ported vacuum - which is always there because of the design of my throttle body- to advance the ignition timing by 10 degrees -not 20 deg as per original Bosch 062- (starting at cranking speed) until the vacuum valve would switch this vacuum off at 2400 and the vacuum advance would disappear. Looking at attachment 1 at the centrifugal advance curve, the centrifugal advance goes up by 10 deg to offset the shut off vacuum. So at 2300rpm I program the centrifugal advance to increase by 10 deg since that will soon be offset by a 10 deg retard from vacuum valve vacuum shutoff. Ideally I would have programmed for 2375rpm + 10 deg centrifugal advance but 123 software only accepts multiples of 100rpm so I went for 2300prm.
3. I hope that this way I will have 9 deg Static + 10 vac advance + plus the centrifugal advance according to rpm up to 2300rpm (seen on my curves) and from 2400rpm onwards 9 deg static plus up to 29 deg centrifugal advance ie a total of 38 deg ignition advance.
4. I am doing this to keep the 2way vacuum valve operating as well as the rest of my emissions control system. In the process my ignition system would become a vacuum advance system - since this can be achieved using the 123 built in software and mobile device App-.
5. Of course I could bypass the vacuum valve and connect directly the 123 distributor to the throttle body ported vacuum in which case I wouldn’t have to increase centrifugal advance by 10 deg at 2400 rpm and at the same time I would limit my max centrifugal advance to 19 deg so my total advance would be again 38 deg (9 static+10 vac +19 centrifugal).
6 A final option would be to ignore the ported vacuum and switching valve all together, still connect vacuum to the 123 for originality but program the 123 to have only centrifugal advance and ignore any vacuum inputs. Something like what is shown in the second attachment.
Sorry for the long post, I just wanted to be as clear as possible. Maybe in the end I will try all three scenarios and see which would work best