If you search the archives you will find detailed data on the spark advance vs. RPM (without vacuum advance) for both open circuit, shorted, and OE 750 ohm R16/1 resistor that I documented for my engine with a dial back timing light. It's also in the Mercedes service literature. Shorting is the laziest curve, and open circuit is the most aggressive.

You can always run your own test with a dial back timing light, too.

The initial timing is fixed by the EZL and cannot be changed. On my M103 it's 9 deg. BTDC.

On my M103 there is a vacuum line running from the inlet manifold to the EZL module consisting of 3 mm nylon tubing with short molded rubber hose sections at the manifold and EZL.

I cut a short piece of 1/8" generic vacuum hose, plugged it with silicone, and substitute it for the molded piece at the manifold for emission testing, but you can plug this line at any point in the circuit that you find most convenient. The point is to block the vacuum signal to the EZL, so it will not advance the timing at less than full load.

The combination of the least aggressive centrifugal spark advance curve with no vacuum advance yields lower spark advance for all speed/load conditions during the ASM and IM240 emission test procedures, which reduces peak combustion temperature (lower engine out NOx) and increases EGT to keep the catalyst bed hotter, which will result in more oxidation and reduction reactions inside the cat.

Fuel additives that claim better emission test performance are a complete waste of money. The modifications that I discussed cost about a dollar and a few minutes of time to configure.

Duke