M103 spark advance map
 

Yesterday I ran some tests to characterize the spark advance - engine revs curve with a dial back timing light using the OE US model 750 ohm resistor and zero (shorted the plug) and infinite (removed the resistor) resistance.

The shorted plug case is supposed to represent the least rate of advance and least total advance, and the open circuit case the highest advance rate and most total advance.

For all cases the initial timing was 9 and the vacuum line to the EZL module was removed and plugged, but prior tests indicated maximum vacuum advance of 14 degrees.

Figure reading error is plus/minus one degree and 100 revs, but might be a little more at high revs since the mark tended to jump around a little more.

750 ohm OE resistor:
17@1500
21@2000
27@2500 (maximum)
27@3000
27@3500

Zero ohm - plug shorted
09@1500
13@2000
18@2500
19@3500
21@4000 (maximum)

Infinite ohm - open circuit
16@1500
21@2000
27@2500
28@3500 (maximum)

It's possible that once the bulk of timing is in the advance continues to increase at a very low rate to higher revs, but it is difficult to discern.

The resistor is still out, but I haven't noticed a SOTP improvement in performance that has been reported in other discussions.

If anyone else can run a similar test, it would be nice to have another data set as a sanity check.

Duke

Duke,

I realise you replied to one of the threads I posted regarding this subject some time ago. I posted the same thread in different forums so you may be interested to read the various other replies. They can be found at:
http://www.peachparts.com/shopforum/showthread.php?t=67772
http://www.peachparts.com/shopforum/showthread.php?t=67773
http://www.benzworld.org/forums/forums/thread-view.asp?tid=242608
http://www.benzworld.org/forums/forums/thread-view.asp?tid=555023
http://forums.mbworld.org/forums/showthread.php?t=42712
http://forums.mbworld.org/forums/showthread.php?t=42715

I have since been running both my M102 and M103 with the different value R16 as described in those threads. Note that in my case the greatest ignition advance was not obtained with R16 removed but with a higher value (and a different value for each engine) to that fitted originally. The original values for my Aussie delivered cars were lower than those fitted to the equivalent European or US models due to the low octane (91RON) unleaded fuel available here at the time they were sold new. We now have higher octane fuel available (95 and 98RON). I have found that my M103 does require the higher octane fuel to avoid pinging with the more advanced timing, however since my original threads were posted I have found that the M102 is quite happy running regular fuel even with the more advanced timing. As for noticing any difference in the seat of the pants, I recently had cause to set the M103 back to the original setting due to the unavailability of the higher octane fuel. In doing this I did perceive a loss of mid range torque at wider throttle openings compared to when running the more advanced timing. Back when I changed from the original setting to the more advanced setting the difference was not so obvious.

Greg

I've been thinking about puting in a 1K pot with a calibrated dial. I could experiment.

As I've been doing a little research on retarding my timing in order to improve emissions, I stumbled upon this thread and find it interesting that the shorted plug condition retards timing by almost 1/2 at 1500RPMs.

But what's more astounding is that the open circuit condition, that I've read many rave review about, seems to have virtually no impact on timing advancement, and therefore engine performance. This is quite a discrepancy. What could be the explanation for this?