The ultimate 308 ignition curve

A couple comments. I always work in kPa (absolute) so i struggle a little bit but what you have looks about right. The I recently learned that the cam timing on the carb cars is just plain bizarre which is why it only pulls 10in at idle....if you retime the cams to around 106/106 - 104/110 lobe centers you will see more like 18in vacuum and it will be better in everyway with a LOT more low end and pull better almost everywhere. Bunch of cam info here https://www.ferrarichat.com/forum/threads/any-new-camshafts-worthwhile-for-308-carb-cars.685183/page-2

But that's not you question. The higher vacuum timing for sure help the engine run a bit cooler and improve fuel mileage and will I guess make it feel a bit more responsive when your at higher vacuum. Once you open the throttle your using the lower vacuum settings so it won't do much if anything once to actually open the throttle. Motorcycles have great throttle response and none of the older bikes had any sort vacuum advance that I can remember???

Last 7500 looks like typos at the higher vacuum, there usually shouldn't be a bump in the curve like that. Some engine do like a little less timing at high rpm but then 8k would be less than 7500 not more. and 3 degree over 500 rpm is a lot.
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Yes, that 42 was a typo should be 45 at 7500 rpm and -10 psi vacuum.

Sorry for using PSI vs kPa...my ECU setting is using PSI so was easier for my limited brainpower. The manual uses mmHg and < logic so I hoped to not get it reversed (more negative PSI = less absolute kPa = less mmHg, etc).

You confirmed to me that at more vacuum (=less throttle) more timing is good.

However, some interesting points which is clear in the graph-version is that:

1. the "high vacuum" setting has a curve that is generally lower (than WOT) at <2500 RPM and then crosses over to higher timing than WOT at >3000 RPM, and reaches max timing at 5000 RPM
2. the "mid vacuum" is like above but in general less timing
3. the "low vacuum" curve is more gradually reaching max timing at 5500 RPM
3. I changed all timings at idle (<1000 RPM) to be 6 degrees -- the manual seemed to indicate it being 3 to 10 degrees at idle depending on vacuum -- my engine seems to idle well around 6-7 degrees, so I just used that for all vacuum settings.

I guess the above means

A. when I am at idle and "floor it" it will probably quickly jump to the low-vacuum curve.
B. when I accelerate at mid-throttle it will probably use the mid-vacuum curve reaching max timing at 5k and I guess typically at that point a drivers action would be press the pedal more (or less) and if you press the pedal more I guess it will go to the low-vacuum curve.
C. if case "B" but I keep the throttle mid pressed, then it will grow to 42-degrees advance but under medium load and I guess that much advance will not be too much (why???)
D. the bottom-right of the table is only encountered if I raise RPMs high when in neutral -- so its only good for revving at idle (which is pointless, unless you are 18 ).

Thoughts?

 

Correct ignition timing is related to cylinder pressure, which is related to manifold pressure, which is a function of rpm and throttle position. When there is higher pressure (less vacuum) the mixture burns hotter and faster and when the pressure is lower (high vacuum) it burns cooler and slower. So less timing is needed at high pressure (WOT) than at lower pressure partial throttle conditions. Its not really about tuning to try to get it to feel the right way, its just about getting the engine to run properly. At WOT you tune on the dyno and it is what it is. At lower throttle it honestly doesn't much matter very much because you are purposely restricting HP with the throttle so if the engine isn't perfect for those conditions you'll never know but will maybe use a touch more fuel than if it was perfect. if you have EGT available you tune part throttle to the lowest EGT and call it good, if not the factory numbers are probably fine

Idle is different.....you want it to idle nice and then come off idle smoothly. The old mechanical systems work, but you have more flexibility with tables in an electronic setup like you now have. You want to make sure timing is stable at idle rpm and then blends smoothly into driving timing as the throttle opens and rpm rises ..... if not idle can "hunt" or move around and it can want to jump or surge coming off idle which makes it harder and less pleasant to drive. I don't recall your setup but big ITBs make coming off idle....I have idle timing connected to my throttle pedal position to help going in/out of idle smoothly.