At 11:25 PM 1/24/2000 -0600, you wrote:
>In my old autoshop class, (a lot of years ago - highschool) we ran a test on
>an old 350. Advanced the timing and detonation appeared. Overly retarded
>the timing and detonation appeared.
>
>Retarding the timing moves the heat to the outside of the combustion chamber
>(exhaust manifolds) which helps clean up the emissions (unburnt fuel)...but
>can leave "hot spots" near the edge of the combustion chamber causing
>preignition. Advancing the timing does just the opposite with the heat
>factor, but as we all know, causes detonation. Both sound almost alike.
>
>- Bernd
Ok, I follow. But that is not the optimum running of an engine. You'll
burn up the exhaust valves in no time. Random ramblings...something for
everyone on this one...when the piston reaches the top of the piston bore
on the compression stroke, the piston stays stationary as the connecting
rod swings over the top of the crank throw and then starts the downward
power stroke. That dwell time - the point where the piston remains
stationary when the rod rotates on the rod journal as the crank swings from
before it's highest part then over as the piston and rod push down on the
power stroke (try to picture it, hard to describe), that dwell time is very
important. That piston location is called top dead center (TDC). That is
halfway between when the piston reaches it's highest point and right before
it starts to move down - in degrees of rotation. Ignition base timing is
anywhere from 8 to 15 degrees BTDC (before top dead center) - depending on
application.
When the piston reaches, let's say 12 degrees BTDC, the spark plug
fires. The piston is stationary as the crank and rod swings to change
direction. That dwell time allows the mixture to burn hot and burn as
completely as possible. The mixture was heated due to the compression of
the gases, which aids in a full burn. The resistance of the piston creates
an extremely hot burn which completely burns all fuel. So when the piston
and rod are lined up on the downward swing on the crank, full force is
applied to the piston from the expanding gases which produces max power output.
Retarding the timing means to decrease the timing advance. So instead of
firing at 12 degrees, it's reduced to a lower number bringing the spark
plug firing closer to TDC. That gives the mixture less time to burn and it
will still be burning as the piston moves down on the power stroke. That
doesn't yield max pressure on the piston since not enough pressure was held
during the burning process due to the increase in volume from the piston
moving down. If it is retarded too much, the mixture will be burning when
the exhaust valve opens and the fire will blow out into the manifolds or
headers. Which like Bernd said will heat up the manifolds. That doesn't
make optimum power - the power is wasted as heat and not pressure.
Advancing the timing moves the plug firing farther from TDC, giving the
mixture more time to fully burn. This creates more power under certain
circumstances. The main factor in that is fuel octane. The lower the
octane the easier it is for the gas to burn. Light a fire (Bar-B-Q) with
87 octane and watch it ignite and flare up real quick with a whoosh. Try
it with 93 and observe the difference, it's a little slower. Ohh this gets
in depth :) Little background- Compressing a gas will create heat. How
much it is compressed and how fast is directly proportional to the amount
of heat generated Advancing the timing too much with low octane gas will
cause the mixture to combust on it's own without the help of the spark
plug. The result is the pressure created slams into the piston that can't
move since the crank is still moving around and the rod hasn't swung over
enough to be on the downward swing. That pressure slamming on the piston
creates an audible noise - called pinging. That is the sound of the gasses
expanding and slapping the piston that can't move down yet. That decreases
your power output and also, under severe conditions and heavy load will
melt the tops of the pistons. Sometimes blowing a hole in the piston.
If you have higher octane, you can get away from the preignition because
the fuel is harder to ignite. It will not combust on it's own with the
increase in heat and pressure. That is ideal as it gives time for the
mixture to distribute itself around the combustion chamber and to fully
atomize the air and fuel - so you get a good solid burn when the plug
fires. That's why racers use high octane fuels - 100, 101, 103 and higher
octane fuels. That allows them to advance their timing to create more
power and get away with high compression ratios- 12 to 14:1 for gas.
Yer keepin' me on my toes Bernd
So as you can see ignition timing is very critical. That little window of
dwell time is very important for a proper burn. Speed changes require the
spark to fire even earlier to keep up with the increase of engine
RPM. Then the advance must decease as the engine slows down or you will
get ping from the mixture being fired too early. That is why I feel it is
a problem with the computer on the Dakota. Either the ignition timing is
not getting a high enough priority on the processor or the processor is two
slow to keep up with all the tasks it must do. Or it could be a design
flaw, maybe the combustion chamber shape causes hot spots or the spark plug
placement in the chamber makes it too hot. Try the 2-3 colder plug idea,
prove it's not the plugs or prove that it is. Every heat range colder
removes between 70 and 100 degrees from the plug to the head. Maybe they
changed the design of the water passages in the head - so less heat is
removed from the head, causing the plugs to stay too hot. Dunno till
someone tries the 2-3 step colder plug idea. All that it's gonna do with a
colder plug is not get the plug hot enough to clean itself - so it will
foul out prematurely. Just grab some cheap plugs - send me the bill, I'll
pay for the experiment. Get some regular cheap Champions with at least 3
heat ranges colder than stock for a no questions yes or no on the plug.
Anyone?
Mike
__________________________
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