Quote:
Originally Posted by rookie
I bought a scangauge for my 2001 Chevy Truck and it will display what your
timing doing while you are driving and interestingly at 60 mph and about 2000
rpm my timing is at 47 to 48 degrees and when I stab the throttle it drops to
low 30's and then climbs back up to high 30's as rpm increases.
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Rookie, there’s two keys in you post that changes all of the rules in regards
to ignition advance, those are throttling and load.
Lets look at throttling first, basically what we have is a method of controlling
volumetric efficiency of the engine, open the throttle and the VE goes up,
close it and VE goes down. What does this have to do with ignition advance?
The more air/fuel we let into the cylinder the higher the cylinder pressure
gets, the higher the pressure gets the faster the burn requiring less ignition
advance.
Secondly we must factor in engine load, we know that load increases can
bring on detonation. Years ago this was evident when we would climb a hill
and the engine would rattle like crazy. I have given this a lot of thought and
what I come up with is as engine load increases, the resistance to push the
piston down the bore at the beginning of the power stroke increases. This
resistance causes a rise in cylinder pressure by demanding more force from
the expansion of the combustion gasses to push the piston down.
So the engineers at GM programmed the engine management system to
sense the increase in VE and engine load and respond accordingly to reduce
ignition advance. How does turbulence and burn rate factor into all of this?
Your turn.
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There's a third factor I totally forgot, that’s RPM. As RPM goes up , the time
for flame travel goes down, twice the RPM means 1/2 the amount of time for
combustion to complete.