Ignition Advance (Part 2)

[nmauduit]

I was referring to the official lotus Workshop manual Page 15.

The Wilkins Book! I had not thought of looking there.
P168 , Half way down, right of page, under All Big Valve ,

12 static up to 2000 then straight line up to 26 maximum.
(S/E Weber is exactly the same line except for 2 degrees less static)
Same as the manual.

Most curves I can source are like the 40953 (Top of the graph).
A steep advance (primary spring) until about 2000 then the secondary adding to flatten the curve.
Giving a sharp kink in the line - hence "primary to secondary transition"

There isn't any with the 41189.

My post was about the effect of 2 different springs rather than actual values. When both springs are fully extended (i.e. travel mechanically blocked) then the advance curve becomes flat (does not increase even if rpm goes up). If the 2 springs are identical, or if one is very weak, or very stiff, then that would translate into a single slope within the operating range.

I consider the advance curve as a tuning parameter, and for my weber head street engine would rather have a 2 slope curve similar as the 40953 curve as shown in the book : advance rising quickly from idle to about 2500rpm (which is later than in the manual), then more slowly, before getting to a maximum around 4000 (which is earlier than in the manual). Of course, tuning for optimal performance at all revs for a given engine should be done meticulously (a rolling road or dyno would help), depending on CR ratio, mix richness, cam profile etc...

[MarkDa]

Its not matter of the springs becoming fully extended to halt the advance there's a stop on the base plate that stops the weights going any further.
The max advance is primarily about how fast petrol burns. The object of the exercise is to develop maximum pressure when pistons gets to tdc.
Too much advance and you get pinking, not something we hear in our modern cars because they have knock sensors to retard the ignition when it happens.