AS COPIED FROM Vacuum Advance Page
Ignition Advance System
Without some way to adjust the ignition for changing rpm or load, the engine wouldn't make much power, and it'd be pretty hard on a gallon of gas.
These are the reasons for the ignition advance system. Three types of ignition advance are used in most street ignition systems;
Static, vacuum and centrifugal. Together, they account for total ignition advance.
The static advance is the ignition timing that's set at the crankshaft. The vacuum and centrifugal advance systems help optimize ignition timing based on
engine load and speed. When the engine's idling, the spark plug must ignite the air/fuel mixture slightly before the piston reaches the top of its stroke.
This ensures the majority of fuel is ignited and will begin to expand when the piston reaches the top of its stroke, pushing the pistons down. This takes
full advantage of the leverage effect of the connecting rod journal on the crankshaft and is known as the static timing. As engine rpm increases,
piston speed goes up. But the burning and expansion rate of the fuel stays the same. To conpensate for this, ignition of the air/fuel mixture needs
to occur earlier so the fuel can begin expanding when the piston is at the top of its stroke. If ignition begins too late, the piston is already moving
down the cylinder and the pressure against the piston generated by the expanding fuel drops. This causes a drop in cylinder pressure with a resulting
decrease in engine torque. On the other hand, if the ignition timing becomes too far advanced, the fuel begins expanding before the piston reaches the top
of its stroke. The pressure caused by the expanding fuel pushes down violently on the piston as it's still moving to the top of the cylinder, causing the hammering
noise that is typical of detonation. It also slows the momentum of the crankshaft, resulting in a loss of power. As you can see, the ignition must advance at a
certain rate to take full advantage of the lerverage effect of the crankshaft and to prevent engine damage. The vacuum advance system is responsible for increasing
ignition advance based on engine load. The lighter the engine load, the greater the advance. The greater the engine load, the less the advance. The vacuum advance
consist of a diaphragm within a metal housing. Connected to one end of the housing is a source of manifold vacuum. On the other end, a plunger is connected to
the diaphragm, wich, in turn, is attached to the distributor breaker plate. When the engine is lightly loaded, such as at idle and when cruising, manifold vacuum is high
and ignigtion timing can be advanced without causing detonation. The high manifold vacuum pulls on the diaphragm. In turn, the diaphragm plunger pulls
on the breaker plate, causing it to rotate. Since the ignition trigger (breaker points, magnetic switch, and so on) is mounted on the breaker plate,
it begins to open earlier, relative to the distributor shaft. And this causes ignition timing to increase.This increase in ignition timing subsdtantially improves
fuel economy. If the engine is accelarated, manifold vacuum drops. This allows the vacuum diaphragm, and in turn, the breaker plate, to ease back
toward their neutral states, reducing ignition timing. Where the vacuum advance system adjusts ignition timing based on engine load, the
centrifugal advance system adjusts timing based on engine speed. The centrifugal advance system consists of two weights positioned at the
top of the distributor shaft. The weights are held to the distributor drive shaft with springs. As engine rpm increases, centrifugal force pulls the weights away from
the distriburtor shaft causing the springs to expand. The weights are connected to the breaker plate assembly. As the weights move outward, they rotate
the breaker plate within the distributor housing, causing the ignition timing to advance. The advance rate is controlled by the mass of the weights and the tension
of the centrifugal weight springs. The vacuum advance system works in concert with the centrifugal advance system in a type of see-saw routine.
Whenenver engine rpm increases, the vacuum advance system allows the ignition timing to ease back. But when engine rpm increases, the
certrifugal advance system increases ignition timing. So they tend to help balance one another. However, two things combine to make this an uneven trade-off.
First, if the centrifugal advance weights and springs are selected for maximum performance, they won't add any more ignition advance above 2500 rpm
or so. Second, the vacuum advance system is more sensitive than the centrifugal advance system; changes in throttle position that may change engine rpm
only slightly and therefore cause no real change in centrifugal advance rate, can cause more significant changes in the vacuum advance system.
Overall, the more ignition timing you can run without causing detonation, the better the performance and fuel economy.
And these are two good reasons for retaining the vacuum advance system for street operation.
AND ANOTHER PAGE Vacuum Advance Page
VACUUM ADVANCE AND WHY YOU WANT IT FOR YOUR CAR
An often-asked question from many callers relates to whether they really need a vacuum advance mechanism on their distributor. I think this question stems from their observation that many "high performance" distributors do not incorporate vacuum advance and the resulting implication is that it is not desirable or necessary for a "good" ignition. There are a handful of applications where vacuum advance is not of significant benefit:
1) Pure racing engines
2) Severe duty very large trucks
3) Constant speed and load applications (airplanes, generators, pumps)
Other than the above, for normal automotive applications the vacuum advance will benefit the engine as follows.
1) Improved idle cooling
2) Improved idle quality
3) Improved fuel economy
4) Improved throttle response
5) Improved drivability
6) Enables improved spark knock control under full throttle accelerations
7) Enables leaner fuel jetting at light load to further improve fuel economy.
The basic reason for all these improvements is that the vacuum advance mechanism allows the distributor to supply a more optimum spark timing proportional to the load and speed output. Without the vacuum advance the distributor can only vary spark timing in proportion to speed and ignores its need for approximately 20 additional degrees of spark timing ("advance") at light loads: (idle and cruise conditions)
The basic reason for the change in optimum timing at light loads is that when operating at light loads, the mixture is leaner for fuel economy and less dense because of light load. These conditions cause the charge to burn slower, and thus, to reach peak pressure at optimum point in the cycle, the spark must be initiated earlier. Failure to do this will result in "retarded" spark timing and all the aforementioned losses.
All engines are different, and have different spark timing requirements, but they are all the same in that as load is decreased, additional spark timing is required for optimum combustion.
Do yourself a favor - 1) make sure your distributor has a vacuum spark system and 2) experiment to find out what your engine "likes" for timing at idle, light load, and heavy load. Then change the vacuum can to achieve a result closer to the optimum.
[ March 10, 2004, 07:21 PM: Message edited by: PhilSine ]