Quick86
06-26-2006, 03:35 PM
Source: http://www.carbdford.com/viewtopic.php?t=5543
This is an article I have put together from many sources.
I hope it answers all of your Duraspark Timing and Tuning questions.
Lets start at the beginning......
Maybe you've just built up a brand new engine, or upgraded to new heads and a cam, perhaps you're simply trying to dial-in an existing combination. In either scenario, one area of tuning that is highly overlooked and greatly misunderstood is timing. All too often we see people dropping in their distributor, making a quick adjustment with their timing light, and setting off to make another pass.
Timing is everything, and without a proper timing curve, every thing else goes out the window. Jetting changes, fuel pressure adjustments, are all useless if first the timing is not set correctly.
So what is timing? In a nutshell, timing or 'ignition timing' relates to when the sparkplug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to melted pistons.
Why an engine needs more advance as its speed increases.
When the compressed mixture inside a cylinder is ignited it takes time for the flame front to reach the piston and for the expanding gases to start pushing it down. The time that this takes changes according to a number of variables such as mixture strength, how well the cylinder has filled (dependent on volumetric efficiency and throttle opening), compression ratio and combustion chamber shape. Given the same circumstances of mixture strength, cylinder filling and CR, the time taken for the mixture to fully ignite and burn is the same regardless of engine speed. At increasingly higher RPM however, the time available for this burn to take place is correspondingly less, so it follows that you have to start burning the mixture earlier in order for it to push on the piston at the right time. This is the basis for increasing ignition advance.
Too much of this and the burning mixture hits the piston as it rises (pinking or pinging), too little and the flame front reaches the piston far too late and does not do a good job of pushing the piston down and the engine behaves like a herd of turtles. One of the reasons a diesel engine does not perform at higher RPM is that it has compression only ignition, so there is no way to increase the effective ignition advance.
How this is achieved
The distributor as fitted to conventional ignition systems does not just distribute the spark amongst the cylinders and switch the coil; it also contains a centrifugal mechanism that advances the ignition timing automatically as engine RPM rises. Normally there are a pair of weights within the distributor which under the affects of centrifugal force tend to be thrown outwards, this tendency is greater as RPM increases. The weights are shackled by two small springs that restrain them progressively. As the weights move outwards they exert a turning force on the top of the distributor shaft relative to the driven part of the shaft, this moves in the same direction as the distributors rotation thereby causing the points/electronic trigger to actuate earlier and advancing the ignition timing. As engine speed increases the weights overcome more of the spring's tension and advance the timing still more. There is normally a stop of some kind that limits the amount of advance that the distributor can supply. This centrifugal mechanism is usually hidden away underneath the baseplate of the distributor.
In this article we're going to focus primarily on carburated, non-computer controlled, engines which have fully adjustable distributors. The EEC-IV computer controlled Fords allow for setting initial timing, but the rest is adjusted by the computer. The newer modular engine Fords have distributor-less ignitions which offer no adjustability from the factory, although companies like Steeda have recently developed timing adjusters for these engines. Some Fords, particularly in the 70's and early 80's, had distributors where timing was fixed due to emissions reasons.
When it comes to timing the most common myth is that adjusting the timing simply means moving the distributor clockwise or counterclockwise. While this does affect the timing, it is not the correct way to adjust the timing curve. To explain why, we first we need to define some terms.
Advancing and retarding timing refers to increasing or decreasing the 'time' at which spark is delivered to the cylinders. This 'time' is measured in crankshaft degrees, signified by marks on the harmonic balancer, and a reference pointer on the block or timing chain cover. When the piston is at Top Dead Center (TDC), this is synonymous with zero degrees on the balancer. Ten degrees before that point would mean the piston is ten degrees of rotation from being at TDC.
So how does the crank position relate to the distributor?
The distributor shaft on Ford engines is driven by the camshaft gear, which is turned at half-crank speed by the timing chain connected to the crankshaft. Thus there is a direct correlation between the position of the crank and the position of the distributor. Remember, the distributor is a switch. Regardless of the type of distributor you have, there is a fundamental design common to all of them; the shaft is in a fixed position, spinning in direct relation to the crankshaft. On the shaft sits the trigger which activates the switch. On electronic distributors the trigger may be a magnetic sleeve with eight openings, or in the case of points, its simply an arm that open and closes the points. The distributor housing does not spin and it contains the actual switch, such as the Pertronix box, which is mounted on a breaker plate. By rotating the housing you in effect move the position of the switch, changing when it triggers a spark. When you rotate the distributor to "adjust the timing" you are moving the switch on the housing side in relation to the trigger on the shaft.
Rotating the distributor housing clockwise on a Ford advances the timing (i.e. spark is being fired a greater number of degrees before the piston reaches TDC), and counterclockwise decreases the timing.
When referring to timing, there are really four terms that must be considered; initial timing, mechanical (or centrifugal) timing, total timing, and vacuum advance. There is also cam timing which is more appropriately termed valve timing, since it deals with when the valves open and close in relation to crank position. We won't talk about this since it has no dynamic bearing on ignition timing.
Initial: This is the most common adjustment that people associate with timing. At idle, with the vacuum advance hose disconnected and plugged, this is the timing that you would see if you flashed timing light on the timing marks. On typical stock engines you'd see as low as 0 to as high as 15 degrees. Most Ford shop manuals specify around 6-8 degrees initial timing advance for the 289-351 motors.
Mechanical/Centrifugal: Most V8 distributors contain an internal advance mechanism consisting of two each of weights, springs, and slotted 'reluctor' arms. There is also a stop tab for the arms. On Fords this assembly can only be seen by removing the cap, rotor, and breaker plate; we'll get to removal a bit later. As the distributor shaft spins with increasing rpms, the centrifugal force acts on the weights, which begin to force outwards against the springs. This movement rotates the shaft and thus advances the timing. The slotted arm controls how much the weights can move the assembly, and the springs control how fast the assembly reaches that limit. The reluctor arm on a Ford has two slotted sides, only one side contributes to the timing, the arm can be flipped around if more advance is needed (see pictures.) On Fords each side is stamped with a number, usually 10L and 13L; or some have 15L and 18L. These numbers refer to 1/2 of the total degrees of timing that will be obtained when using that arm. So for example a 15L arm would contribute 15 x 2= 30 degrees of timing when full against the stop.
This is an article I have put together from many sources.
I hope it answers all of your Duraspark Timing and Tuning questions.
Lets start at the beginning......
Maybe you've just built up a brand new engine, or upgraded to new heads and a cam, perhaps you're simply trying to dial-in an existing combination. In either scenario, one area of tuning that is highly overlooked and greatly misunderstood is timing. All too often we see people dropping in their distributor, making a quick adjustment with their timing light, and setting off to make another pass.
Timing is everything, and without a proper timing curve, every thing else goes out the window. Jetting changes, fuel pressure adjustments, are all useless if first the timing is not set correctly.
So what is timing? In a nutshell, timing or 'ignition timing' relates to when the sparkplug is fired in relation to piston position. At idle, when engine speeds are the lowest, the plug fires just before the piston reaches the top of its stroke. As engine speeds increase, the time between piston strokes is less, and therefore the plug must fire sooner. In all cases the plug is fired in advance of the piston reaching top dead center. There is a small window of time in which the combustion need to take place in order to produce peak power. Too late and power is lost, too soon and detonation occurs, which can lead to melted pistons.
Why an engine needs more advance as its speed increases.
When the compressed mixture inside a cylinder is ignited it takes time for the flame front to reach the piston and for the expanding gases to start pushing it down. The time that this takes changes according to a number of variables such as mixture strength, how well the cylinder has filled (dependent on volumetric efficiency and throttle opening), compression ratio and combustion chamber shape. Given the same circumstances of mixture strength, cylinder filling and CR, the time taken for the mixture to fully ignite and burn is the same regardless of engine speed. At increasingly higher RPM however, the time available for this burn to take place is correspondingly less, so it follows that you have to start burning the mixture earlier in order for it to push on the piston at the right time. This is the basis for increasing ignition advance.
Too much of this and the burning mixture hits the piston as it rises (pinking or pinging), too little and the flame front reaches the piston far too late and does not do a good job of pushing the piston down and the engine behaves like a herd of turtles. One of the reasons a diesel engine does not perform at higher RPM is that it has compression only ignition, so there is no way to increase the effective ignition advance.
How this is achieved
The distributor as fitted to conventional ignition systems does not just distribute the spark amongst the cylinders and switch the coil; it also contains a centrifugal mechanism that advances the ignition timing automatically as engine RPM rises. Normally there are a pair of weights within the distributor which under the affects of centrifugal force tend to be thrown outwards, this tendency is greater as RPM increases. The weights are shackled by two small springs that restrain them progressively. As the weights move outwards they exert a turning force on the top of the distributor shaft relative to the driven part of the shaft, this moves in the same direction as the distributors rotation thereby causing the points/electronic trigger to actuate earlier and advancing the ignition timing. As engine speed increases the weights overcome more of the spring's tension and advance the timing still more. There is normally a stop of some kind that limits the amount of advance that the distributor can supply. This centrifugal mechanism is usually hidden away underneath the baseplate of the distributor.
In this article we're going to focus primarily on carburated, non-computer controlled, engines which have fully adjustable distributors. The EEC-IV computer controlled Fords allow for setting initial timing, but the rest is adjusted by the computer. The newer modular engine Fords have distributor-less ignitions which offer no adjustability from the factory, although companies like Steeda have recently developed timing adjusters for these engines. Some Fords, particularly in the 70's and early 80's, had distributors where timing was fixed due to emissions reasons.
When it comes to timing the most common myth is that adjusting the timing simply means moving the distributor clockwise or counterclockwise. While this does affect the timing, it is not the correct way to adjust the timing curve. To explain why, we first we need to define some terms.
Advancing and retarding timing refers to increasing or decreasing the 'time' at which spark is delivered to the cylinders. This 'time' is measured in crankshaft degrees, signified by marks on the harmonic balancer, and a reference pointer on the block or timing chain cover. When the piston is at Top Dead Center (TDC), this is synonymous with zero degrees on the balancer. Ten degrees before that point would mean the piston is ten degrees of rotation from being at TDC.
So how does the crank position relate to the distributor?
The distributor shaft on Ford engines is driven by the camshaft gear, which is turned at half-crank speed by the timing chain connected to the crankshaft. Thus there is a direct correlation between the position of the crank and the position of the distributor. Remember, the distributor is a switch. Regardless of the type of distributor you have, there is a fundamental design common to all of them; the shaft is in a fixed position, spinning in direct relation to the crankshaft. On the shaft sits the trigger which activates the switch. On electronic distributors the trigger may be a magnetic sleeve with eight openings, or in the case of points, its simply an arm that open and closes the points. The distributor housing does not spin and it contains the actual switch, such as the Pertronix box, which is mounted on a breaker plate. By rotating the housing you in effect move the position of the switch, changing when it triggers a spark. When you rotate the distributor to "adjust the timing" you are moving the switch on the housing side in relation to the trigger on the shaft.
Rotating the distributor housing clockwise on a Ford advances the timing (i.e. spark is being fired a greater number of degrees before the piston reaches TDC), and counterclockwise decreases the timing.
When referring to timing, there are really four terms that must be considered; initial timing, mechanical (or centrifugal) timing, total timing, and vacuum advance. There is also cam timing which is more appropriately termed valve timing, since it deals with when the valves open and close in relation to crank position. We won't talk about this since it has no dynamic bearing on ignition timing.
Initial: This is the most common adjustment that people associate with timing. At idle, with the vacuum advance hose disconnected and plugged, this is the timing that you would see if you flashed timing light on the timing marks. On typical stock engines you'd see as low as 0 to as high as 15 degrees. Most Ford shop manuals specify around 6-8 degrees initial timing advance for the 289-351 motors.
Mechanical/Centrifugal: Most V8 distributors contain an internal advance mechanism consisting of two each of weights, springs, and slotted 'reluctor' arms. There is also a stop tab for the arms. On Fords this assembly can only be seen by removing the cap, rotor, and breaker plate; we'll get to removal a bit later. As the distributor shaft spins with increasing rpms, the centrifugal force acts on the weights, which begin to force outwards against the springs. This movement rotates the shaft and thus advances the timing. The slotted arm controls how much the weights can move the assembly, and the springs control how fast the assembly reaches that limit. The reluctor arm on a Ford has two slotted sides, only one side contributes to the timing, the arm can be flipped around if more advance is needed (see pictures.) On Fords each side is stamped with a number, usually 10L and 13L; or some have 15L and 18L. These numbers refer to 1/2 of the total degrees of timing that will be obtained when using that arm. So for example a 15L arm would contribute 15 x 2= 30 degrees of timing when full against the stop.