Here's the approach I'm taking to calculating my base ignition table. That is, my starting point to get the car driveable enough to whizz it around the block a few times and eventually to a tuner.
I need a sanity check, because it doesn't seem to be working out for me. Please check my logic and let me know if I'm completely off base or if there's an easier way.
I'm using the centrifugal advance and vacuum retard curves published in my workshop manual for Euro '78 and later 930s.
Its basic freshman year algebra 1 stuff to calculate the formulae for these (f=mx+b). For example the advance is
- Below 600rpm, 1.25 degrees
- From 600-1,600rpm advance=1.25+0.0055*(rpm-600)
- From 1,600rpm-3,400rpm advance=6.75-0.000833*(rpm-1,600)
- Above 3,400rpm advance=5.25 degrees
Similar sort of thing for the vacuum retard.
So my plan was to build an Excel table with the same rpm and load cells matching what is in Tunerstudio, calculate the cell values in Excel, then transfer to the ignition table in TS.
The Euro ignition timing is set at 29 degrees BTDT @ 4,000 rpm with the vacuum hose disconnected. So that is, with centrifugal advance maxed out and no vacuum retard.
With that fixed point, and then adding in advance, and subtracting out retard, I should be able to calculate the actual advance for each cell.
Am I interpreting these charts correctly? For example, the advance chart. I interpret that to mean ADDITIONAL advance on top of whatever static timing is set, not ABSOLUTE advance.
In other words, my base ignition timing is 29 BTDC minus what the centrifugal advance is at 4,000 rpm which is ~5.25 according to chart, or 23.75 degrees BTDC at 600 rpm. Using the above formula I'd get 3.175 degrees advance at 950 rpm + 23.75 = 26.925 at idle.
I can't see where my logic is wrong, but the numbers I end up with don't sound right to me. Other ignition tables I've seen show around 18-20 BTDC at idle.