With all the talk about whether or not to keep the vacuum advance units
on distributors, and the lack of understanding of what exactly the
function of these are to some individuals, I thought I might try to
enlighten a few of you as to what the function of the vacuum unit is.
This is no means definitive and I don't purport to be an expert, but I
have learned a few things over the years (ahem, your kidding, Paul
McCartney had a band before wings?) and thought I would share this
information for those interested. Also for the purposes of this
discussion the following only applies to maximum efficiently or power,
and not the control of harmful emissions. That said, and all
disclaimers applied, a little simplified internal combustion basics -
Engines, to operate efficiently, require the spark to fire at some point
BEFORE the piston reaches TDC. This is to allow the explosion to build
enough pressure (push) on the top of the piston, at just the right time,
to provide optimum power. If it is started too soon (advanced) then
this explosion reaches piston while it's still traveling upward and you
lose power, (trying to push the piston the wrong way) waste energy, and
create heat in the combustion chamber area (and usually knocking or
detonation from an explosion instead of a nice smooth flame traveling
from the upper cylinder to the piston top). If started too late
(retarded) then you loose power because the piston is already traveling
downward, before the flame explosion can "push" it. This also creates
heat in the surrounding combustion chamber because remember, heat is
energy. This energy, if not used to push the piston, is released either
into the surrounding water jacket or the exhaust manifold instead of
powering your vehicle. Both are inefficient as far as maximum power is
concerned, but it makes an effective heater! As the engine RPM's
increase, given that the flame propagation speed remains the SAME, then
the combustion cycle needs to be started earlier to achieve the desired
"push" on the top of the piston. Also, as the pressure (more fuel/air)
inside the cylinder increases, then the less advance the engine can
handle at a lower RPM (bigger explosion). So as you can see it depends
upon the speed (RPM) of the engine, AND the amount of air/fuel mixture
(throttle position) that the engine is operating at. OK, internal
combustion 101 out of the way -
Now that we understand (I hope) why we advance our timing it should be
clear that as the engine speed (RPM), and combustion pressure (amount of
air/fuel mixture, or volumetric efficiently) increases then the timing
has to advance or retard accordingly. (there is a maximum amount but
that depends on many variables, that we needn't go into for the purpose
of this discussion). So, say the engine is idling. Very low cylinder
pressure (load), very low speed. Since there is relatively little
fuel/air mixture inside the cylinder, then we need to advance the timing
quite a bit (say 30 deg before TDC for this discussion) to start the
flame at the proper time. As the RPM's increase and more fuel/air is
introduced BUT, no or very little load is applied, then the cylinder
PRESSURE remains fairly constant and therefore we can use the same
amount of advance (30 deg). A purely mechanical advance wouldn't
achieve the same advance at idle as it would at 3000 or 4000 RPM (unless
of course it had almost no spring pressure retarding the centrifugal
weights controlling the advance mechanism). However, the intake
manifold pressure is very low (high vacuum) so we can use this to
advance the timing, via one side of a diaphragm connected to this
source, and the other to the breaker, or sensor plate in the
distributor. Now say your cruising at 2000 RPM little load, again low
cylinder pressure, optimum advance (30 deg) engines happy. Suddenly you
snap open the throttle. Now you have maximum cylinder pressure, low
engine speed and advance needs to be at say 12 deg to prevent
detonation. If the advance were purely mechanical again, and set for
optimum advance (30) at the no/low load condition, then we would have
too much advance for this high load condition, and one unhappy engine
because of detonation. However, during high load conditions, the intake
manifold pressure drops to zero (equals outside manifold pressure or no
vacuum). IF the mechanical timing were now optimized for high load, low
speed conditions (12 deg@2000 RPM), then the vacuum unit can optimize
the timing at light or no load conditions (30 deg) because it is in
effect not operating at high load conditions, and the mechanical advance
can be optimized for high cylinder pressure or maximum load conditions.
So in this case, when you stomp on the pedal, the timing (at 30 deg
light load, relatively high vacuum) would drop back to 12 deg, because
the vacuum is now not operating, as stated before, the manifold pressure
increased (vacuum dropped to zero) and the diaphragm returned to it's no
vacuum position. In this way, timing can be optimized for all engine
conditions. For racing, and max power applications, you don't really
need a system for controlling advance at low or no load conditions
because these engine are operating at maximum power most if not all the
time. (and is one reason why some tend to overheat at idle) Also,
another reason that early emission systems with idle retard, or advance
cutouts have a provision that during extended idle periods, when the
engine begins to overheat, it restores PROPER advance to prevent that
overheating!
|