Post by Garrett on Apr 25, 2005 14:58:33 GMT -5
UNDERSTANDING IGNITION TIMING
Q: Why does a Non-Restricted engine seem to operate well with 28 degrees Ignition Timing and a .052" jet at 6200 rpm, while a Restrictor Plate engine runs well with 30 degrees Ignition Timing and a .062" jet at 5600 rpm?
Theory #1: You could assume as a rule that the higher the rpm band you operate in, the less Ignition Timing you need to use.
Theory #2: You could assume as a rule that since the Restricted engine is utilizing a larger jet that you would need increased Ignition Timing in order to burn all of the fuel.
So we have:
Non-Restricted= Low Ignition/High Fuel
Restricted= High Ignition/Low Fuel
Is either theory true? No. The hole in both theories is that a Limited Modified engine will operate properly in excess of 9200 rpm while using High Fuel and High Ignition Timing.
What about the Restricted engine using a larger jet? Even though the jet size is larger on a Restricted engine, it has to be larger because of the reduced fuel signal in the carb venture. So, the Restricted engine will rarely use more fuel than a Non-Restricted engine, even though the jet size would make a person think so.
So, if tested within a predefined test time, we now have:
a Restricted engine uses 4oz of fuel
a Non-Restricted engine uses 6 oz of fuel
and a Limited Modified uses 9 oz of fuel
Does more fuel use require more Ignition Timing? No. The final clue to my puzzle came in building an Open class engine. Larger than all of the aforementioned engines, this baby was a Fuel Hog, yet it operated best at 28 degrees of Ignition Timing.
So, we have:
Open= Low Ignition/Very High Fuel
How can that be? The answer is found in VE (Volumetric Efficiency). Volumetric Efficiency is simply how well an engine cylinder fills and evacuates (empties) within a given amount of time.
In the case of our Restricted engine, any engine when restricted by a plate forms turbulence inside the port, which in turn impedes flow. The end result is reduced VE. Turbulence creates a poorly mixed fuel/air charge which resists efficient combustion in the cylinder, thus Increased Ignition Timing is required in order for the engine to produce peak combustion pressure at the most opportune time, transferring the resultant forces to the piston, connecting rod, and eventually the crankshaft.
What about the Limited engine? The Limited worked well with Increased Ignition Timing because there was a lot more fuel to burn, but the Open worked well at 28 degrees with even more fuel. Why? Because of flow. The Open had optimized larger ports, Big valves, and a harder "pull" to fill the cylinder because of increased cubic inches (bore and/or stroke are larger). In short, because the Open filled the cylinder more efficiently (VE) than any of the other engine designs.
Q: Why does a Non-Restricted engine seem to operate well with 28 degrees Ignition Timing and a .052" jet at 6200 rpm, while a Restrictor Plate engine runs well with 30 degrees Ignition Timing and a .062" jet at 5600 rpm?
Theory #1: You could assume as a rule that the higher the rpm band you operate in, the less Ignition Timing you need to use.
Theory #2: You could assume as a rule that since the Restricted engine is utilizing a larger jet that you would need increased Ignition Timing in order to burn all of the fuel.
So we have:
Non-Restricted= Low Ignition/High Fuel
Restricted= High Ignition/Low Fuel
Is either theory true? No. The hole in both theories is that a Limited Modified engine will operate properly in excess of 9200 rpm while using High Fuel and High Ignition Timing.
What about the Restricted engine using a larger jet? Even though the jet size is larger on a Restricted engine, it has to be larger because of the reduced fuel signal in the carb venture. So, the Restricted engine will rarely use more fuel than a Non-Restricted engine, even though the jet size would make a person think so.
So, if tested within a predefined test time, we now have:
a Restricted engine uses 4oz of fuel
a Non-Restricted engine uses 6 oz of fuel
and a Limited Modified uses 9 oz of fuel
Does more fuel use require more Ignition Timing? No. The final clue to my puzzle came in building an Open class engine. Larger than all of the aforementioned engines, this baby was a Fuel Hog, yet it operated best at 28 degrees of Ignition Timing.
So, we have:
Open= Low Ignition/Very High Fuel
How can that be? The answer is found in VE (Volumetric Efficiency). Volumetric Efficiency is simply how well an engine cylinder fills and evacuates (empties) within a given amount of time.
In the case of our Restricted engine, any engine when restricted by a plate forms turbulence inside the port, which in turn impedes flow. The end result is reduced VE. Turbulence creates a poorly mixed fuel/air charge which resists efficient combustion in the cylinder, thus Increased Ignition Timing is required in order for the engine to produce peak combustion pressure at the most opportune time, transferring the resultant forces to the piston, connecting rod, and eventually the crankshaft.
What about the Limited engine? The Limited worked well with Increased Ignition Timing because there was a lot more fuel to burn, but the Open worked well at 28 degrees with even more fuel. Why? Because of flow. The Open had optimized larger ports, Big valves, and a harder "pull" to fill the cylinder because of increased cubic inches (bore and/or stroke are larger). In short, because the Open filled the cylinder more efficiently (VE) than any of the other engine designs.