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How Does a High Output Ignition Coil Help Performance?
A high performance ignition coil helps engine performance four important ways. First, the higher voltage allows for a larger spark plug gap, which results in a more robust initial flame kernal at the start of combustion. The result is a real-world engine torque increase. Second, having more voltage on tap means the voltage required to bridge the spark plug gap gets there faster, leaving less time for voltage diversion through the spark plug’s inevitable carbon deposits. Third, the higher voltage potential creates a stronger “push” on the electrical stream to the plug, resulting in increased electrical current, i.e. more energy, more snap. Fourth, with more voltage available, there is more in reserve for non-standard situations such as two-up riding while going uphill on poor fuel on a hot day with too low tire pressures and a 20 mph headwind.
So Much Voltage?
This last thought brings us to the subject of how much voltage is commonly used by the spark plug. The fact is the voltage needed to bridge the plug’s gap is not constant but always changing, and is nowhere near the level of the ignition coil’s voltage output potential. That is, a 60,000 volt ignition coil only fires its spark plug at 60,000 volts when it has high demand(load) increased rpm and cylinder pressure , but more often at 7,000-20,000 volts at idle and light demand. How can this be, and why the extra then? When a spark plug is about to fire, what happens is the air inside its gap is of course not conductive and must be made so. It is actually temporarily made conductive, and this is called by a fancy name that has to do with atoms and such, “ionization.”. It simply means that the air is prepared to flow electricity. Think of it as the air molecules getting so heated and excited by the rapidly building plug voltage that the result is voltage can flow through this agitated air to jump the plug’s gap. How much voltage is required to make ionization happen depends on several things ranging from the amount of cylinder compression (pressure) to how worn the plug’s electrodes are. But in round numbers you will be safe to think 5,000-7,000 volts at idle. As soon as the throttle is used however this goes up, and if the transmission is put into gear then load comes into the picture and the requirement goes up even more. So let’s settle on about 15,000-20,000 volts for a bike in a state of cruise. Now go uphill and the requirement increases. Be in too high a gear for conditions and it increases. Yank the throttle open hard and it increases. Go downhill and it decreases, downshift to a lower gear and it decreases. So the actual voltage is all over the place while the bike is being ridden, and higher voltage ensures there is always enough and that it gets there fast.
Forced Induction - Turbo/Nitrous
It is applications like forced induction and/or nitrous oxide that overwhelm the stock ignition system. Supercharging and turbocharging ups cylinder pressure through boost while a nitrous oxide system uses a chemical reaction to deliver the additional oxygen that results in higher cylinder pressures. Either way, lighting off the entire air/fuel charge efficiently is the key to realizing the optimum power output of your engine combination. Cylinder pressures can also be increased by upping the compression of a naturally-aspirated engine. Increasing the rev limit reduces the margin of error when it comes to the ignition system producing the voltage needed. Precise timing and sufficient energy are of prime importance as the time between firing cycles is shortened. The amount of boost, nitrous, compression and increase of rev limit will dictate when a performance coil will need to be added to the mix.
As the combustion chamber pressure rises, it gets more difficult to get the spark to jump across the plug gap. The reason is the gas in the gap is an insulator. If you stuff more molecules in there (higher pressure) you need a higher voltage to jump the gap due to pressure resistance. It isn’t resistance like in a resistor – it’s ionization resistance. The voltage has to ionize the gas in the gap to jump it.
So, if you raise the boost you are also raising the pressure in the combustion chamber and the requirement for higher voltage to jump the spark plug gap goes up. You can reduce the gap but this is just a compromise – the shorter spark will have less chance of igniting the mixture, so very quickly you will run into the same problem – flame front blowing out and your engine starts missing.
Coils do wear out. They will often show the proper resistance, but they will start to “break down” internally. What this means is at some voltage level the energy will flow through the internal insulating material, effectively acting like a short and absorbing the energy you need to produce a spark. When the combustion chamber is at low pressure the spark will jump the spark plug gap at a voltage that is not high enough to flow inside the coil. The voltage appears to rise immediately to its’ max. value, but it actually takes up to a few milliseconds, and halfway to the voltage levels you need something starts to break down.
Bottom line is, if you want to run high boost/nitrous combinations you need an ignition system in good working order – that is, capable of generating very high voltages without breaking down.
Some may argue that output voltage of an ignition system does not have to exceed 35,000 to 40,000 volts as normal demand doesn’t exceed this. The voltage required to initiate a spark across a spark plug gap depends on a number of conditions including, but not limited to, cylinder pressure, fuel mixture, and spark plug gap. Once the voltage to jump the gap is reached, a spark is formed and current flows creating the mass of the spark. Increases in cylinder pressure and spark plug gap, as well as less than ideal mixture ratios (either too rich or too lean) will increase the voltage required to create a spark. In more simple terms, when you twist the throttle, requirement goes up. If you increase your engine’s load while pulling a hill i.e. more fuel volume enters the cylinder (upping pressure), requirement goes up. If your plugs wear and the gap burns larger, requirement also goes up. As well, any changes that you voluntarily make such as exhaust modifications, bigger fuel injectors, camshaft changes,increased compression ratio or increasing your plug gap to get a bigger spark, all increase the firing voltage requirement.
A number of misconceptions have been made over the years. Let’s take a minute to review a few popular ones. ” Too hot a spark will burn holes in your pistons ” – not true. Pinging or detonation caused by poor fuel grade, incorrect fuel mixture, or incorrect ignition timing cause this kind of damage. ” Too hot a spark will cause your engine to run too lean and run hot ” – not true either. Fuel mixture alone dictates rich or lean. Too rich and combustion temperature goes down, too lean and it goes up. Too weak a spark can cause a mixture not to burn well leaving behind too much unburned fuel. However, having close to complete combustion of a proper mixture is not harmful. ” Any spark will do to run an engine ” – partly true. If the spark can initiate combustion, the engine will run. Beyond that it is a matter of efficiency. The fact is that there is a finite amount of time for a mixture to burn before the exhaust valves open. As well, the higher the RPM the shorter this time becomes. Here is an analogy to demonstrate this. If you were to take a sheet of paper and light it with a match, the paper will burn in ” x ” amount of time. If you were to use a larger flame to ignite the same sheet of paper, the subsequent larger flame front would consume the paper in a shorter period of time. Both methods will burn the paper no question. But like an engine, if you only have a finite amount of time to complete a burn, you would choose the larger flame. This is not to imply that fuel burns slowly in an engine. Fuel actually burns very quickly, however we are talking about only milliseconds of available time to have a complete efficient burn in a cylinder between valve openings.