A
Anonymous
Guest
I wrote this to help clarify the differences on the various vacuum ports on a carburetor and how they may affect the vacuum advance mechanism on the Load-a-matic distributor. It seems that a lot of folks have misconceptions about the differences between ported and manifold vacuum.
Starting with the basics....As the piston falls and the intake valve opens, the mass of air in the manifold is exposed to an additional volume in the cylinder. As the air expands, filling the cylinder, the pressure in the manifold falls relative to the pressure outside the carburetor. I think many folks get confused about vacuum because the fall victim to thinking of it as negative pressure. There is no negative pressure. Vacuum is just the difference between one positive pressure that is less than the surrounding positive pressure. In the case of a carburetor, it is the difference in atmospheric pressure and the pressure at some point in the carburetor. Typically we may see or read about manifold vacuum, ported vacuum or venturi vacuum. These are vacuum readings at three different points in the carburetor.
Manifold Vac and Ported Vac are nearly identical for most of the range of throttle position. The significant difference between the two is that manifold vacuum is measuring the pressure below the throttle plates and ported vacuum is measured from a point above the throttle plate. They do not act opposite of each other. If manifold vacuum is low, then ported vacuum is low.
Vacuum readings on an engine at elevated rpm’s in the driveway are not the same as vacuum readings under loaded driving conditions. With very little load on the engine (in neutral or park in the driveway), it does not take very much movement in the throttle plates to provide enough additional air and fuel to accelerate the engine. The engine will speed up with relatively little throttle movement. I’ll bet few of us rarely run a car at wide open throttle in the driveway for more than a fraction of a second because it is so easy to redline an engine with no load on it. Since the throttle plates are not wide open and are restricting the air flow into the engine, the vacuum continues to increase or be elevated when the engine is rev’d in the driveway. The throttle plates choke the air flow and regulate the pressure differential across the carburetor.
With a load on the engine, it takes much more air & fuel to accelerate the engine and the car. So you can punch the throttle wide open and minimize the restriction of air flow into the engine. The pressure differential across the carburetor nearly equalizes. The plates may be wide open for several seconds, allowing manifold vacuum to drop for a significant period of time.
On the other hand, venturi vacuum does operate opposite of manifold vacuum. Venturi vacuum is measured at a point very near a tapered reduction in the cross-sectional area of a pipe. The venturi is a small choke in the pipe. The velocity of the air increases as it is forced through the smaller opening of the venturi. Due to the conservation of energy, some of the energy that the air had at a higher pressure and lower velocity is now converted to an airstream moving at a faster velocity and lower pressure. A port located in the venturi senses the reduction in pressure or the venturi vacuum. Venturi vacuum is a localized phenomenon restricted to the immediate area around the venturi. Once past the venturi restriction, the inside diameter of the carb increases again and the velocity of the air slows down and the pressure increases again to that pressure seen by the ported vacuum source.
The venturi on a carburetor is located in the air horn body of the carb well above the throttle plates. So at idle when the throttle plates are closed and air flow through the venturi is low, the venturi vacuum or the difference in pressure between the surrounding atmospheric pressure and a point downstream of the venturi is little or nothing. As the throttle plates open and air flow increases, the venturi vacuum increases. This is where the Load-a-matic can mimic the centrifugal advance of a dual advance or centrifugal advance distributor. As engine rpm’s increase, air flow increases and the increasing venturi vacuum advances the distributor’s spark.
Venturi vac will usually be much less in magnitude than the manifold vac. So, distributors using venturi vac are likely to use lighter springs or larger diaphragms. The spring rates are also going to be calibrated according to the size of the venturi on the carburetor. A carb with a larger venturi will generate less restriction to flow and therefore a smaller vacuum signal will be supplied to the distributor. So even if your replacement carb has a venturi vacuum source, unless the venturi size is the same, the advance curve provided by a Load-a-matic distributor probably will not be optimum.
When a manifold or even a ported vacuum source is connected to a distributor that is calibrated to operate on venturi vacuum, you will get the incorrect amount of vacuum advance at the wrong time. A Load-a-matic distributor calibrated to operate with a SCV (spark control valve) and hooked to a manifold vacuum source will deliver maximum ignition advance at idle and light throttle cruising conditions. But it may fall to nearly no vacuum advance when you plant your foot in the gas pedal. Acceleration will be poor because as engine speed increases, the timing will not advance until the throttle plates close sufficiently to generate a manifold vacuum signal.
Sorry for the lengthy dissertation.
Doug
Starting with the basics....As the piston falls and the intake valve opens, the mass of air in the manifold is exposed to an additional volume in the cylinder. As the air expands, filling the cylinder, the pressure in the manifold falls relative to the pressure outside the carburetor. I think many folks get confused about vacuum because the fall victim to thinking of it as negative pressure. There is no negative pressure. Vacuum is just the difference between one positive pressure that is less than the surrounding positive pressure. In the case of a carburetor, it is the difference in atmospheric pressure and the pressure at some point in the carburetor. Typically we may see or read about manifold vacuum, ported vacuum or venturi vacuum. These are vacuum readings at three different points in the carburetor.
Manifold Vac and Ported Vac are nearly identical for most of the range of throttle position. The significant difference between the two is that manifold vacuum is measuring the pressure below the throttle plates and ported vacuum is measured from a point above the throttle plate. They do not act opposite of each other. If manifold vacuum is low, then ported vacuum is low.
Vacuum readings on an engine at elevated rpm’s in the driveway are not the same as vacuum readings under loaded driving conditions. With very little load on the engine (in neutral or park in the driveway), it does not take very much movement in the throttle plates to provide enough additional air and fuel to accelerate the engine. The engine will speed up with relatively little throttle movement. I’ll bet few of us rarely run a car at wide open throttle in the driveway for more than a fraction of a second because it is so easy to redline an engine with no load on it. Since the throttle plates are not wide open and are restricting the air flow into the engine, the vacuum continues to increase or be elevated when the engine is rev’d in the driveway. The throttle plates choke the air flow and regulate the pressure differential across the carburetor.
With a load on the engine, it takes much more air & fuel to accelerate the engine and the car. So you can punch the throttle wide open and minimize the restriction of air flow into the engine. The pressure differential across the carburetor nearly equalizes. The plates may be wide open for several seconds, allowing manifold vacuum to drop for a significant period of time.
On the other hand, venturi vacuum does operate opposite of manifold vacuum. Venturi vacuum is measured at a point very near a tapered reduction in the cross-sectional area of a pipe. The venturi is a small choke in the pipe. The velocity of the air increases as it is forced through the smaller opening of the venturi. Due to the conservation of energy, some of the energy that the air had at a higher pressure and lower velocity is now converted to an airstream moving at a faster velocity and lower pressure. A port located in the venturi senses the reduction in pressure or the venturi vacuum. Venturi vacuum is a localized phenomenon restricted to the immediate area around the venturi. Once past the venturi restriction, the inside diameter of the carb increases again and the velocity of the air slows down and the pressure increases again to that pressure seen by the ported vacuum source.
The venturi on a carburetor is located in the air horn body of the carb well above the throttle plates. So at idle when the throttle plates are closed and air flow through the venturi is low, the venturi vacuum or the difference in pressure between the surrounding atmospheric pressure and a point downstream of the venturi is little or nothing. As the throttle plates open and air flow increases, the venturi vacuum increases. This is where the Load-a-matic can mimic the centrifugal advance of a dual advance or centrifugal advance distributor. As engine rpm’s increase, air flow increases and the increasing venturi vacuum advances the distributor’s spark.
Venturi vac will usually be much less in magnitude than the manifold vac. So, distributors using venturi vac are likely to use lighter springs or larger diaphragms. The spring rates are also going to be calibrated according to the size of the venturi on the carburetor. A carb with a larger venturi will generate less restriction to flow and therefore a smaller vacuum signal will be supplied to the distributor. So even if your replacement carb has a venturi vacuum source, unless the venturi size is the same, the advance curve provided by a Load-a-matic distributor probably will not be optimum.
When a manifold or even a ported vacuum source is connected to a distributor that is calibrated to operate on venturi vacuum, you will get the incorrect amount of vacuum advance at the wrong time. A Load-a-matic distributor calibrated to operate with a SCV (spark control valve) and hooked to a manifold vacuum source will deliver maximum ignition advance at idle and light throttle cruising conditions. But it may fall to nearly no vacuum advance when you plant your foot in the gas pedal. Acceleration will be poor because as engine speed increases, the timing will not advance until the throttle plates close sufficiently to generate a manifold vacuum signal.
Sorry for the lengthy dissertation.
Doug
