A few days ago I was following a few posts about the way a vacuum advance
works. Some of what was said, just didn't sound right. Vacuum advance is
one of those things I have to sit down and figure out each time, so, several
days later, I found a past post to help educate myself and others. Here is
a nice long description of how and why vacuum advance does what it does.
It's written by the list's own Barry Schwartz almost two years ago.
Later,
Jack Brooks
1960 TR3A
1974 Norton 850 Commando
Hillsdale, NJ
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Date: Thu, 22 Jan 1998 16:47:29 -0800
From: Barry <bschwart@xxx>
Subject: Vacuum or not? A synopses (kinda long)
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!
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