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Mar 26

Tech Tuesday: Spark Plugs, more than you want to know

This week for Tech Tuesday we are talking about spark plugs. They start the fire inside your internal combustion engine. So hit the bump and find out more about this important part of your engine.

Spark plugs ignite the air fuel mixture inside the combustion chamber. So let me run down how it works and the terms involved with this part. If the terms get confusing, check out I-G’s super cool diagram debuting today.

I’ll start at the beginning of the plugs job with the ignition wire. From an earlier episode of tech talk, we know that the an electric current has been sent through the ignition wire and it meets the the terminal end of the spark plug. The knob end of the plug is designed to be a firm connection to the wire. It should fasten with a snap and you should definitely feel it connect.



So the current is now running through the center electrode and moves down through a resistor, that is not shown in the diagram. The resistor is there to hold up any loose current that may leak into the plug, through either EMF transmission or crossed over from another wire, in order to stop pre-ignition with a small spark. It makes sure that the spark is only sent when the big charge comes down the wire. Normally the resistor is located inside the insulator the next part of the spark plug.

The insulator is the, normally white, ceramic part. It has only one job, to not allow the current for the spark to cross the center electrode to the outer body. It makes sure the spark happens where it should, in the combustion chamber. Now it is made out of a ceramic material because it does not conduct electricity and the ceramic is resistant to the extreme temperatures in the combustion chamber. While you see the white part above the hex nut body, it continues all the way down to the tip of the center electrode. The top part is glazed so that the protective boot of the ignition wire doesn’t stick to the plug.

Now that the current has flowed through the resistor, it is directed through the center electrode. You will only see the tip poking out right under the ground strap. The center electrode normally has a copper core surrounded by a steel jacket to protect the copper from the heat in the combustion chamber. Copper is a very soft metal. While it is the third best conductor of electricity, it isn’t very durable, especially in the volatile heat and pressures inside a lit combustion chamber. Today we have alloy tipped electrodes. In the US, there is a law that all cars built for passenger use must have emission systems that last 100K miles. Spark plugs are key to getting a clean burn. So the manufacturers have resorted to using a platinum tipped center electrode. The platinum is much more resistant to the heat and last much longer. While they are rated to 100K miles, They are usually toast by 60K. Also on the market is an iridium tipped spark plug. The iridium is more resistant to heat than the platinum allowing for a finer tip on the center electrode. The finer tip allows the current to be much more focused, giving the plug a hotter spark. A hotter spark gives a cleaner burn and more power. My recommendation is that the platinums give the best life in a conventional engine and the iridiums are best saved for high compression and forced induction engines.

Now that the current has reached the end of the center electrode, its time to jump the gap. This is where the current now becomes a spark that ignites the compressed air/fuel mixture. The spark gap is where the business happens and the distance is critical. If it’s too close, you are losing power. If its too far, the compression could blow the spark out. There is more power to be had with a wider gap. Especially with a higher output ignition system. But the gap increase should be minimal at best.

Now that the spark has ignited the mixture, the current runs into the ground strap. In a traditional spark plug the ground strap is a copper core like the center electrode. The ground strap is easy to explain, but the variations on the market today are endless. It all started with the Splitfire brand. Where the ground strap is forked and the spark can see the middle of the combustion chamber. Its supposed to make more power. I’m not sold. By splitting the spark it makes it easier to blow out. It has less intensity so the fire starts slower. And there are more variations. Two prong, four prong, side strap, all of them are, in my opinion junk. The original design ground strap is still the best. It is much more durable and reliable. There are some tips about alignment that can see small gains. When you install the spark plug notice the location of the ground strap. You can mark it on your socket. The ground strap should face away from the heart of the combustion chamber, or on a centrally located plug, it should face away from the intake valves. Depending on the brand of spark plug, there is a gasket or crushable washer at the base of the hex nut. If the ground strap is not in the position you want it you can use a shim to take some of the threads and push the gasket down.

The current now travels up the outer body of the plug and travels into the block by the contact of the threads. The hex nut allows you to screw the plug into its hole and is in automotive applications a 5/8 SAE. Some of the more dedicated gearheads have heard of a hotter or colder plug. What the heat range refers to it the self cleaning temperature of the plug. Without a doubt, the inside of the combustion chamber is a hell on earth. With just cold cranking, no ignition, you will see pressures of 100 to 150 psi of pressure in there. That’s around ten times normal air pressure. Once you add the combustion effect, pressures jump about ten fold. You can also add in the the extreme temperatures due to the combustion event. Normally entering the exhaust manifold or header temps are around 1800 degs. In cylinder under compression, gas temps can reach 3500 degs. This is what the business end of the spark plug deals with many times a second. The amount of surface area that the insulator touches the spark plug body determines the heat range. The greater the area, the colder the plug. The less the hotter. Too cold of a plug and the electrodes get a build up and foul. Too hot and it will vaporize the metal. In general, you want to use the recommended plug for a stock or mildly modified engine. A hotter plug would be put in a cylinder with a bad ring or oiling problem. A colder plug would be used in an engine with significant compression increases like adding forced induction.

And that is the real deal about the spark plug. Any questions leave them in the comments.

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