The gasoline engine is actually a variable compression engine, unlike the diesel. Every time the intake valve opens and air pressure forces in the air fuel mixture, a specific volume of gas (as in "gas, liquid, and solid") is pushed into the cylinder by atmospheric pressure. (We all know that there is no such thing as “suction”.) The amount of gas ingested is dependent on the pressure differential between the atmosphere and the cylinder, and also the displacement of the engine. So, for naturally aspirated engines, the amount of gas that is ingested is normally dependent mainly on the displacement. (Exhaust tuning, or "5 cycle" principle, and also ram tuning are an exception to this. They contribute a supercharging effect to the filling of the cylinder.) Supercharging will vary the amount of gas ingested according to the pressure differential, and can therefore "make" a smaller engine larger, by forcing the smaller engine to ingest what a larger one would normally pump on the intake stroke.
Even though the amount of gas that is drawn in is the same, the amount of air/fuel mixture is not. Remember that a solid has a fixed volume and a fixed shape, a liquid has a fixed volume but no fixed shape, and a gas has neither a fixed volume or a fixed shape. This means that a pound of a gas may fill a small peanut butter jar, or a whole room.
So, your Y Block cylinder ingests the same amount of gas on each intake stroke, but the amount of air/fuel mixture varies according to the manifold vacuum, which in turn is controlled by the throttle plates in the carburetor. When the throttles are nearly closed, as in cruising at 40 miles an hour, there are only a few oxygen and a few gasoline molecules in the 36.5 cu. inches of air your 292 cylinder ingests. But, when you are going up a hill in second at 30 mph, pulling a heavy load with the throttle wide open, there are many oxygen and many gasoline molecules in the 36.5 cu. inches of air your 292 ingests.
So, the effective compression at light load is very low, and the effective compression at full load is high, and highest when your engine is running at the torque peak at full load when the throttle is wide open. That's the point at which your engine is ingesting the maximum amount of air/fuel mixture per intake stroke, and when the compression actually approaches the rated figure of your engine. (This is called "volumetric efficiency", which occurs at the peak torque rpm value.)
When your motor is running at light load, there are only a few molecules of fuel and oxygen in the cylinder and they are relatively far apart. So, in order to get them to burn at the right time, you have to start the "fire" (spark) earlier or the piston will be descending before much pressure is generated and power will be wasted. Think of a forest fire. If the forest only has a few trees, it will take a longer time before the forest fire jumps between individual trees and a longer time before the fire reaches the far end of the forest. On the other hand, if the forest is dense with trees, the fire will spread rapidly from one end to the other. This is similar to the case when the cylinder is being completely filled because the throttle is wide open -- each intake stroke contains many, many fuel and oxygen molecules.
The spark needs to also occur earlier if the engine is running faster. For a given air/fuel density, there is a specific amount of time that the mixture will take to burn. But, as the engine turns faster, the mixture needs to be ignited earlier so that the burning gasses will make the pressure rise at the right time, giving maximum "push" on the piston. The faster the engine runs, the shorter amount of time the mixture will have to do its work. So, it must be "set off" earlier when the motor is spinning at higher rpm.
This is why your engine needs more advance during light load conditions, and less advance when it is pulling under a heavily loaded condition.
So, to provide the right spark timing for an engine, two things must be known: the speed of the engine, and the load of the engine. Compression ratio (how closely packed the molecules are in the cylinder) and fuel quality (the ability of the fuel to resist pinging) are factors that distributor tuning must take into account when the distributor is set up.
This page was last modified on 21 July 2004