"Pulling Timing"????
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Joined: Sep 2001
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From: Lake Elsinore, CA
Re: "Pulling Timing"???? (4N EATER)
Normally, a certain amount of advance is dialed into the firing of the spark plug before the piston reaches top-dead-center. To a point, the more advance you can dial in, the more completely the fuel is burned and the more energy/power you can extract out of each cylinder charge.
You actually don't want fuel to instantaneously "explode" in the combustion chamber as that is an inefficient and damaging occurance. The fuel burn should be a propagating flame front that develops evenly across the top of the piston, increasing pressure as the gas burns and expands and pushes the piston back down creating the power stroke. Too much advance in certain situations can cause the fuel to burn unevenly and cause detonation - knock or ping. Hot spots in the chamber, high compression, low octane fuel, nitrous, etc. can pre-dispose the engine to detonation. The timing is retarded or "pulled" to combat detonation and uneven fuel burn - in other words, the firing of the spark plug to ignite the fuel-air charge is delayed so that more optimal conditions exist.
A good explanation from How Stuff Works:
"There is a small delay from the time of the spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder reaches its maximum. If the spark occurs right when the piston reaches the top of the compression stroke, the piston will have already moved down part of the way into its power stroke before the gases in the cylinder have reached their highest pressures.
To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke, so by the time the piston starts down into its power stroke the pressures are high enough to start producing useful work.
Work = Force * Distance
In a cylinder:
Force = Pressure * Area of the piston
Distance = Stroke length
So when we're talking about a cylinder, work = pressure * piston area * stroke length. And because the length of the stroke and the area of the piston are fixed, the only way to maximize work is by increasing pressure.
The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions. The time that the fuel takes to burn is roughly constant. But the speed of the pistons increases as the engine speed increases. This means that the faster the engine goes, the earlier the spark has to occur. This is called spark advance: The faster the engine speed, the more advance is required.
Other goals, like minimizing emissions, take priority when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant. Retarding the timing may also eliminate knocking; some cars that have knock sensors will do this automatically."
Another phenomenon to be aware of is that there is overlap between the opening and closing of the intake and exhaust valves. This creates a "scavenging" effect where the vacuum created by the receding piston draws in a certain amount of the unburned exhaust gases on the intake stroke to more completely burn the fuel and raise the efficiency of the engine. This can cause it's own problems because those hot exhaust gases can cause the intake charge to pre-ignite.
Software tools are available that allow you to monitor in real time the factors that affect timing and to optimize it for a particular setup. AutoTap/EFI Live, Ease, HP Tuners are examples of monitoring tools and while LS1 Edit and HP Tuners are software packages that allow you to change those parameters.
[Modified by Patches, 8:13 AM 5/5/2004]
You actually don't want fuel to instantaneously "explode" in the combustion chamber as that is an inefficient and damaging occurance. The fuel burn should be a propagating flame front that develops evenly across the top of the piston, increasing pressure as the gas burns and expands and pushes the piston back down creating the power stroke. Too much advance in certain situations can cause the fuel to burn unevenly and cause detonation - knock or ping. Hot spots in the chamber, high compression, low octane fuel, nitrous, etc. can pre-dispose the engine to detonation. The timing is retarded or "pulled" to combat detonation and uneven fuel burn - in other words, the firing of the spark plug to ignite the fuel-air charge is delayed so that more optimal conditions exist.
A good explanation from How Stuff Works:
"There is a small delay from the time of the spark to the time when the fuel/air mixture is all burning and the pressure in the cylinder reaches its maximum. If the spark occurs right when the piston reaches the top of the compression stroke, the piston will have already moved down part of the way into its power stroke before the gases in the cylinder have reached their highest pressures.
To make the best use of the fuel, the spark should occur before the piston reaches the top of the compression stroke, so by the time the piston starts down into its power stroke the pressures are high enough to start producing useful work.
Work = Force * Distance
In a cylinder:
Force = Pressure * Area of the piston
Distance = Stroke length
So when we're talking about a cylinder, work = pressure * piston area * stroke length. And because the length of the stroke and the area of the piston are fixed, the only way to maximize work is by increasing pressure.
The timing of the spark is important, and the timing can either be advanced or retarded depending on conditions. The time that the fuel takes to burn is roughly constant. But the speed of the pistons increases as the engine speed increases. This means that the faster the engine goes, the earlier the spark has to occur. This is called spark advance: The faster the engine speed, the more advance is required.
Other goals, like minimizing emissions, take priority when maximum power is not required. For instance, by retarding the spark timing (moving the spark closer to the top of the compression stroke), maximum cylinder pressures and temperatures can be reduced. Lowering temperatures helps reduce the formation of nitrogen oxides (NOx), which are a regulated pollutant. Retarding the timing may also eliminate knocking; some cars that have knock sensors will do this automatically."
Another phenomenon to be aware of is that there is overlap between the opening and closing of the intake and exhaust valves. This creates a "scavenging" effect where the vacuum created by the receding piston draws in a certain amount of the unburned exhaust gases on the intake stroke to more completely burn the fuel and raise the efficiency of the engine. This can cause it's own problems because those hot exhaust gases can cause the intake charge to pre-ignite.
Software tools are available that allow you to monitor in real time the factors that affect timing and to optimize it for a particular setup. AutoTap/EFI Live, Ease, HP Tuners are examples of monitoring tools and while LS1 Edit and HP Tuners are software packages that allow you to change those parameters.
[Modified by Patches, 8:13 AM 5/5/2004]
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Joined: Jan 2004
Posts: 156
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From: Chesapeake VA
Re: "Pulling Timing"???? (4N EATER)
DANG THAT WAS ALOT OF INFO!!! :eek: So does it hurt the car in any way to do it???? Like after a while a boosted motor will prolly go, does this prolong it? If so, why dont people do it in all applications for boost????
