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What you call "predetonation" doesn't occur that way. Mixture is still ignited by a sparkplug. What happends is that mixture doesn't burn in controllable fashion (trough so called "flame-front") but detonates, which causes dramatic pressure rise in the cylinder that damages engine.
With other words, i you compress the mixture too much and ignite it with a spark plug, it will explode and not burn upp nice and clean. If you compress it even more it will explode on itself, but then we are talking 22:1 compression and diesel fuel (this doesn't happend in gasoline engines).
What ignition retardation does is to retard the timing so that spark ignites the mixture later than "Upper Dead Point" , when the pistn is on it's way down. This means that mixture gets ignited when it's less compressed so it burns cleanly and doesn't detonate. Unfortunately, this is to no use beacuse you will get reduced power from late ignition.
With other words:
It's better to have engine operating with as early ignition as possible than to boost lots of mixture and then retard the ignition. If you had theese two scenaris providing same amoun of power, the one with early ignition would use much less gasoline.
As a matter of fact, Trionic 5 actually advances ignition until it hears pinging, then it backs a little just to start creeping up again.
With other words: newer SAAB ECU modifies timing indeed.
Lesson learned:
There is no point winding up boost if you need to retard the timing too much. You might even get less power wth ths approach.
There is always a sweet spot on this curve between boost and ignition retard, and you'll need a dyno to find it. Hint: it's not so far away from SAAB's original design, they ain't stupid , you know ;) They did all this stuff you are going trough before they build the engine....
, Thu, 12 Apr 2001 01:01:20
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