Vacuum Advance Distributors

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!