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Ground strap timing mark

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I just have some thoughts about spark plug inspection, and how ignition timing can effect the "blue line" position on the ground strap. I always tune with a knock sensor, but I also inspect the spark plugs to gain more insight on how the engine is operating. There is a lot of information regarding this topic, and the general idea is that the position of the blue line on the ground strap can be an indication of how much heat is being produced, and the position of the mark should generally be midway between the base and the tip of the ground strap. The other idea is that the amount of ignition advance can change the position of the mark, and advancing the ignition timing can move the mark towards the threads as the temperature of the combustion/combustion chamber can increase.

One thing that bothers me about this is, that the combustion/combustion chamber temperature will increase depending on the amount of load that is placed on the engine. For instance, an N/A engine will produce much lower combustion temperatures than an engine that has been turbocharged. All things being equal, it would make sense that the position of the mark would be more towards the base/threads of the plug for a turbocharged engine, regardless of ignition advance. Therefore it may appear that the ignition timing is too far advanced, but in reality it is the increased combustion temperature causing the line to move further towards the base of the plug. In practice however, it seems that the position of the line remains consistent for a properly tuned N/A, turbo, supercharged engine.

Perhaps there has been more importance attached to this topic and there are many other factors that can influence the mark on the ground strap.

One thing to realize about most plug reading information -- they intend for this to be the result of an ignition cut at full power. Basically it came from drag guys cutting the ignition at the end of a WOT drag run, then reading the plugs to determine the combustion conditions. Road racers will replicate this by cutting the ignition at the end of a straight, then coasting into the pit lane. And of course you can do it on the dyno.

It is not very useful for cars simply used on the road.

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