Thanks for the support guys!
Here is how you read one of these things. For any given engine condition you are able to determine the ignition timing in degrees BTDC. Start by finding the RPM on the X axis. Then find the engine load on the Y axis. Then find the height of the intersection of the X and Y points on the Z axis (red lines). The height of the Z axis defines the ignition timing (green line). Its a simple lookup problem determined by the interesction of an X and Y coordinate value. I have red-lined the graph to illustrate how it is done. I hope it makes sense. To answer your question Jim, the Z cooridinate is the result of X and Y and not a measurement.
Now, I have to say this graph is useless to me because the engine load units is in milliseconds and I dont understand how engine load can be expressed in those units. But I am sure the Mercedes engineers do. The Z axis is in degrees BTDC which is what I would expect and the X axis is RPM. You cannot use this graph to get any precise numbers because it is not possible to arrive at the Z axis number accurately. One would prefer to have a spreadsheet form of the data represented by the 3D curve so a precise value could be looked up and read. The microprocessor in the ignition module is doing exactly the lookup function described. Its a simple and commonly used technique to control a process that is not linear or easily modeled or calculated using mathematical formulas. Its called a 3D lookup table.
I also know that the engine temperature is an input to the ignition module and if the engine temperature is to high a different map is used to avoid overheating the engine. Its a very smart module and the timing is very precisely controlled to protect the engine and deliver optimum performance under a wide set of operating conditions.
So back to original mission when I first replyed to this thread. That answer is a big "it depends on load, temp and many other things.....". This map represents the complexity of the answer and in fact is probably not the right one for the engine in question.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k
Last edited by dpetryk; 10-25-2002 at 12:36 PM.