The other threads I've done show what total advance the engine likes at various conditions- but you're right, building a map from that info was not specifically covered. The reason for that is that there are a few ways to derrive that value. For example - If you know through testing that your engine is making the best power at your normal cruise altitude, and lets just say that's 22 inches and 2500 RPM, perhaps the total advance is 31 degrees. Well, since the total advance is a composite of the RPM setting and the MP setting, there's plenty of options to add RPM and MP to equal timing. Personally, I like to play with MP as the real workhorse of the tuning so my RPM advance tops out at a conservative data plate value of 25 degrees, all in by 2000 RPM. With the RPM value established as 25 degrees at all realistic flight RPM settings, I can concentrate on the level of load the engine is under - and thats MP. So in our example above, I'd target the total advance value and "back into" the MP setting. Since I want 31, already have 25 (from the RPM), then my target MP setting is 6 degrees advanced at 22 inches. 25 degrees RPM + 6 degrees MAP = 31 degrees total. But one data point does not build an ignition curve, you say... Correct. So lets look at a few more points of interest. I know through testing that a max effort takeoff at sea level does not require even close to the data plate value of advance, so I target a negative (retard) value at 30 inches. Lets call it -3 for example. So on takeoff we are maxed out on RPM advance (25 degrees) AND have reached 30 inches (-3 degrees). 25-3= 22 degrees total. There's another data point on the curve. Another performance goal is a smooth idle, so lets hit that one up for another data point. With the engine running at idle and warm, its easy to punch up the advance (I'd use the LOP setting for this test) until the engine RPM peaks and otherwise "seems" happy. Theres a pretty good chance that you will be able to move the RPM several hundred just by advance alone. Pick a condition you like, note the MP and total advance, then derrive the MP setting by subtracting the known RPM value from the total. Lets say this MP value is 14 inches and the advance is 15 degrees. There's one more point on your MP curve. So far we have:
14 inches = 15 degrees advance
22 inches = 6 degrees advance
30 inches = 3 degrees retard
These are 3 "hard" points on my curve, and the rest I simply filled in with a natural slope. The more data points you have, the more accurate the curve, obviously. But I'm really only concerned about these few conditions since this is where the engine spends 99% of its life. As I build time in other regions, I'll go back and refine the curve a bit.
In the interst of full disclosure, this is contrary to Ross' recommendations and there are some good reasons for that. If you have 25 degrees available with RPM as well as 15 degrees with MP, then if you find yourself where both of those conditions are met, then you are going to see 25 + 15 = 40 degrees total. And that does happen in my airplane durring a descent into the pattern where the throttle is closed enough to pull the MP down to the max value and RPM is above 2000 (my "all in" value). There's no detonation danger in such a condition and its not going to hurt anything, but 40 degrees is completely unnecessary. Its a harmless, but annoying byproduct of my tuning scheme. However, it does show that you need to think things through before plugging in these values.