I don't think the stock system has fixed timing, at least not on my '87 16V, unless we mean something different by fixed timing. I believe the Triumph Adler knock box has some kind of electronic ignition advance map. So while it's "fixed" in that I don't believe it can be changed via calibration or EEPROM or chip-tuning, it's not fixed in the sense that there is the same advance at idle as at 7000rpm. The only change that can be made is to rotate the distributor, which will make a global offset to the ignition map.

Correct me if I'm wrong, but the way I think the ignition system works on the VW's with KE-Jetronic, (KE-Jetronic is only for fueling control) is that the knock box contains some kind of ignition advance map. And said map can be retarded in case knock is detected, but said map can never be advanced electronically, only by rotating the distributor. I believe what happens is that the hall effect sensor is in effect a trigger wheel. A pulse from the hall effect sensor goes to the knock box. The knock box knows the engine rpm, and depending on the engine rpm, sends a trigger to the TCI-h ignition coil, which in turn sparks the correct cylinder through the distributor. So there is a delay time between the signal from the hall effect sensor and to the ignition coil. This delay time would be greater at lower engine rpms, and at higher engine rpm's, with more advance, the delay time would be reduced, according to the map.

Now I'm not an engine calibration engineer, but I'm told by someone who is, that there will be a lot of scatter in the spark when the input signal from a four vane hall effect sensor. The four window hall effect sensor simply doesn't provide enough resolution to accurately control the spark timing, and there will be a lot of variance relative to say a 36 or 60 tooth crank trigger. But even a 12 tooth camshaft trigger would be better than a four window/tooth. I'm also told that the spark advance map has a greater effect on engine output than the fueling, that 1-2* difference in timing can make a difference, especially in the all important area under the curve.

So, the way I understand the rules is that whatever will fit inside the stock distributor is fair game. But the stock method of spark distribution must be retained, so the cap and rotor must still be used. There will be some hardware limitations to advance based on the length of the rotor edge, but I would think the four window hall effect sensor is more of a handicap. So if someone has designed a 12tooth wheel to fit the stock 16V distributor they should offer it for sale.

Now about the comment about belt stretch/variance between the cam and crank. I suppose this is why all the OEM's use a crank trigger wheel for timing, the camshaft is only used for synchronization. But still, a 12 tooth trigger wheel on the camshaft is better than a four window vane sensor, and maybe 12 teeth is the resolution limit, above which the effects of the timing belt are noticeable. I don't know, it's a good question.