Spark Burn Time Duration

     The length of time the Spark arc is maintained across the spark plug electrodes is called Burn Time. The Spark Burn Time is measured in milliseconds (ms) that's how short really is. 1 sec = 1000 ms.  In the following diagram, the Burn Time is the period from the peak of Firing Voltage, through the current flow portion of the spark arc ( Spark Burn Voltage Line ), to the point where the coil energy can no longer sustain the spark. The Spark Plug Burn Time duration is used to determine the condition of secondary ignition components ( i.e. anything after the secondary windings of the ignition coil- spark plugs, high tension leads, distributor cap, rotor, central HT lead, coil tower.) If the duration of one cylinder is much shorter or longer than the others, a problem with that cylinder is indicated.

    The length of the spark burn time is directly affected by the amount of secondary coil voltage available, the amount of voltage required to overcome secondary circuit resistance and create the spark, and the amount of voltage remaining to sustain the spark ( in relation to the conditions within  the combustion chamber). Factors that have direct effect on Spark Burn Time Duration include:

  • Coil output ( energy )

  • Secondary circuit resistance

  • Air/Fuel charge quality and quantity

  • Secondary short circuits that cause the spark energy to be shunted elsewhere

  • Low primary circuit current

    Below are some common guide lines for factors that affect the spark time burn duration:

Spark Burn Time Duration
Factor Short Normal Long
High secondary resistance X    
Open secondary circuit X   X
Grounded secondary circuit     X
Low resistance in secondary circuit     X
Spark Plug conditions:      
Wide gap X    
Narrow or no gap, foiled     X
Round or worn electrode   X  
Sharp electrode   X  
Too hot   X  
Too cold   X  
Ignition Timing Conditions:      
Retarded ( too late ) X    
Advanced ( too early )     X
Air/Fuel Mixture Conditions:      
Lean X    
Rich     X
Cylinder Compression:      
High X    

     Spark Burn Time duration is affected by the same factors that affect Spark Firing Voltage, however the effect is inverted - if a factor causes the Firing Voltage line to go higher, the Duration of the spark is shorter, and vice versa. Lower Firing Voltage requirements lengthens the Burn Time Duration and vice versa.

Perform this test procedure to determine if the Burn Times are within specifications:

     With the engine at normal idle, observe the Burn Time duration and note the readings. Then increase the engine speed to about 2000 RPM and note the readings. The spark Burn Time durations for each cylinder should be about equal. Here are some typical guide lines:

  • 1.0 to 2.0 ms is considered normal

  • 0.8 to 1.0 ms is shorter than normal, but depending on engine and ignition system design may be acceptable.

  • Less than 0.8 ms is too short and is indicating a problem

  • 2.0 to 2.4 ms is longer than normal but depending on engine and ignition system design may be acceptable.

  • 2.4 ms or more is too long and is indicating a problem

     Spark Burn Time Duration changes during acceleration and deceleration, that is why we need to know and compare them under all three conditions. Under acceleration the Spark Burn Time increases to it's Maximum to accommodate for the richer mixture. This is known as Spark Burn Time Maximum. Under deceleration it decreases since there is not enough fuel to burn. This is known as Spark Burn Time Minimum. The Minimum burn times and the Maximum burn times are just as important as the "normal" Spark Burn Time Duration at idle.

    An commonly used method to simulate acceleration and deceleration conditions is the so called Snap Throttle Test. With the engine idling, simply snap momentarily the accelerator pedal right down to the floor and take you foot out immediately. This will rev up the engine momentarily, loading the cylinders (and the ignition system) with fuel. Now, there is more time needed to burn all this fuel so the spark will run for longer, and hence produce the highest burn times it can. Vice versa, when you take you foot off sharply you'll "choke" the engine leaving it with no fuel or air to burn. It's like going downhill in gear with no throttle depression. This will shorten the spark burn times duration as there isn't much to burn.

Some examples of Spark Burn Time Durations indicating a problem:

Spark Burn Time below 0.8 ms Min. Burn Time below 0.8 ms Observed Under Snap Throttle Test Max. Burn Time over 2.4 ms Observed Under Snap Throttle Test  
Plug gaps Worn spark plugs Plug lead/s shorting to ground  
Rotor gap ( if all cylinders the same) Faulty suppressors Closed plug gap/s  
Broken HT Leads Lean mixture ( no enrichment ) HT tracking from coil tower to ground  
Broken coil HT Lead Low coil energy Dirt & dust in distributor cap  
Burnt coil or distributor tower   Hairline cracks in distr. cap  
Rotor to cap carbon brush or spring   Very high primary energy  
Worn spark plugs ( rounded electrodes)   Incorrect coil ( low primary resistance)  
Incorrect heat range plugs   Incorrect heat range plugs  
Low coil primary supply (+15) volts   Engine flooding - rich mixture  
Low coil primary energy      
Faulty ballast, immobiliser, etc.      
Excessively slow timing      
Very low or very high compression      
Lean mixture, Air leaks, faulty E.G.R. valve      
Incorrect Engine Mechanical Timing      


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