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Understanding Pre-Ignition

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This article will explain pre-ignition as it occurs in the combustion chamber and goes hand in hand with the article entitled 'Understanding Detonation'. Pre-ignition, sometimes referred to as a ?ping? is caused by the air fuel mixture igniting in the combustion chamber too soon, unlike detonation, which occurs late in the combustion sequence. In the modern days of computer controlled engines, severe detrimental pre-ignition is becoming rather rare. To know the answer to this is to know the causes of pre-ignition, which I will explain now. Pre-ignition is caused when the air fuel mixture comes in contact with a glowing hot surface within the combustion chamber and ignites before the spark takes place. Most of the time this is caused by hot exhaust valves, over heated spark plug electrode or super hot combustion chamber deposits. In the old days of poorly tuned carburetors (look it up if you don?t know that word) deposits were probably the number one cause of pre-ignition in the engine. Since most of the cars we are dealing with here are computer controlled, if tuned properly, the build up of deposits will not be enough to cause pre-ignition.

This leaves us with two causes, hot exhaust valves and hot spark plugs. Unless you had a 3-angle valve job done by someone who didn?t know what they were doing it is very unlikely that valves would be your problem either. The reason being is that most of the cooling of the exhaust valves is done when the valve comes in contact with the valve seat. While the valve is closed (which it is over ? the time) heat is being transferred from the valve to the head, this keeping the valve cool. So, unless the valve was somehow altered so that the seat was not wide enough to effectively transfer heat, the valve will not heat up. Sometimes people attempt to make the engine breath better by removing valve material. The fact is that the factory valve specs already have as much material removed as they can and still properly cool. Those pesky engineers knew what they were doing after all. Healthy valves in the proper environment simply will not heat up enough to cause pre-ignition. This does not rule out burned or cracked exhaust valves. A burned or cracked valve even one with a large seat area can heat up due to escaping exhaust gases. If your engine is pinging, and your showing rising leak-down or lowering compression numbers, this may be your problem. Oh, you may be wondering why I only mentioned exhaust valves. The intake valves, other then being cooled by seat contact also get a blast of cold refreshing fuel laden air every time they open. They have the AC on them compared to the exhaust valves, which only see superheated exhaust gases.

Ok so we went through two out of three, the only thing left are the spark plugs, and unless ?someone? put the wrong heat range in for the setup, that won't cause pre-ignition either. When the plug runs too hot, it ignites the fuel as soon as it comes in contact with it, or before the compression stroke has traveled very far. This is what can cause the most severe form of pre-ignition. Since the air fuel mixture has already been ignited the piston is trying to compress a burning mixture.  This can heat the top of the piston extremely rapidly. The heat can soften the top of the piston to a point that combustion chamber pressures will burst through the center of the piston and create a hole straight into the crankcase. When that happens, your day is done.

There is a fine line between a plug running too hot and just hot enough to keep it clean. I recommend checking your plugs often in a highly modified motor. I have compiled a chart (with a little help from NGK) that may help you with this task. You can find it here.

Spark Plug Reference Chart

By choosing the correct heat range you can match plugs to your combustion chamber conditions, and avoid very costly repairs, not to mention the embarrassment of blowing your own smoke screen halfway down the track.

If you hear a ?ping? back off the throttle and check your plugs! Sometimes the problem can be taken care of by simply running higher-octane fuel, though if your running Nitrous or some kind of forced induction you should already be doing this.

If you have any further questions about this article please feel free to post them in this web sites forum.