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So I have over 13,000 miles on my Slingshot, with 2,000 miles hauling ass with the @jonv turbo. I thought it was a good time to check the spark plugs, since I had not changed them since around the 2,000 mile mark. I have never wanted to be a spark plug expert, but have used them as an engine health indicator in the past, and since I read so much about them, I'll share some very Cro-Magnon (that is "caveman" for the Cro-Magnon out there) versions of all of the stuff I read.
I think most people would pretty easily identify a plug with problems (oily, black, burnt, melted, crusty, etc) so I'll concentrate on some more specifics of a seemingly "good" plug that can tell you a little bit about your engine.
We will start with a picture of mine for a point of reference to compare and judge. For the record, I believe these are the stock AC Delco plugs, and with nearly 10,000 miles on them these are looking good, but they are speaking to me like the plug whisperer about some things I should know or improve, especially since I'm running a turbo.
I'm only going to touch on two indicators: heat and air fuel ratio. The big deal with turbos, is whether you are running too rich (too much fuel) or too lean (not enough fuel). The leaner you are, the more power you make at the risk of detonation or pre-ignition (burning the fuel when the cylinder is not in the proper position). This leads to broken rods, bad times, and sad faces. High heat is associated with lean conditions (gas actually cools the process) so you want a perfect balance of heat and the right amount of fuel. After the pic, is a basic quick analysis of your plugs that can help you "listen" to your engine.
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Heat:
There's two main indicators for heat: The ground strap (the "hook" bending around the end) and the center electrode (needle doodad in the middle). Every plug should have a color change happening on the ground strap hook, and this is pretty much your mark for temperature. The closer it is to the end, the colder it's running. Closer to the threads and you are running hotter. Most people agree that the ideal "temperature" is to have this fuzzy color change right at the bend of the ground strap. Having this color change at the elbow is also an indicator of good engine timing, but is outside the scope of this post. As you can see mine are slightly closer to the thread, so I could be running a little hot. Not so much a problem as it is a sign that I can improve the temperatures with a colder plug (spark plugs can be purchased colder or hotter, and you should consult someone in the know on what is best for your engine).
Moving on to the center electrode, it should have a mark that is as close to the tip as possible (but still showing). You can see that I have a very stark line (more on that below) that is pretty close to the tip, so its a secondary indicator that we are likely closer to "in range" temperatures.
A third, and less reliable, indicator is how far up the threads you have color. The farther up the threads that there is color, the hotter the engine is running. Ideal heat indicator here is about 2-3 threads. You can see I have a shiny band after the 2-3 threads and then more color, which could be from undertightening the plugs. For this reason, this indicator can be an unreliable source, but could help if the other two aren't talking to you.
Lean/Rich Air Fuel Ratio (AFR):
So the darker your plug is, the richer it is running. Again, the ground strap, electrode, are good indicators if they are discolored, gunked, or have other obvious defects, but there are two other indicators that are better at giving you a high level look at AFR: the porcelain insulator (white in the pic above, around the electrode), and the base ring (last edge immediately following the last threads, and to which the ground strap "hook" is welded).
The porcelain insulator by most experienced mechanics agree that the color should be a dull white, gray, or slightly tan color. Too dark is too rich. Too white and shiny and it's too lean. Mine are a dull white, which again, is a possible indicator of my engine running hotter than ideal conditions, and could mean it's running on the leaner end of things, but coupled with the base ring color it more likey to be a hot and rich situation. Another small thing to note, that if the insulator is overly shiny and covered in metal and dark spots, this means you have had detonation or pre-ignition and should have your engine looked at by an experienced tuner or mechanic.
Moving on to the base ring, it should have color on what is called a "full turn." This means that the entire ring is covered in something. The color indicates whether you are rich or lean. In my case it is a dark black color, which indicates that I'm running rich. Here is a really informative excerpt from one of the websites that I read:
EDIT DUE TO POPULAR DEMAND: Spark Plug Heat Rating (Range):
Since there was a lot of curiosity about the heat rating for the plugs I am running in comments, I thought I would elaborate on what I mentioned several times, and below in my Final Thoughts about "colder" and "hotter" plugs.
The heat rating of the stock ACDelco 41-103 spark plug is 5. For those that aren't familiar with spark plug heat ratings, the lower the number, the hotter the plug. In effect your spark plug has the secondary job of pulling or keeping heat in the combustion chamber. Spark plugs range between 2-11, however modern cars are nearly always between 5-7. Here is a graphic from NGK illustrating this point and what happens when you run too hot or too cold:
![]()
The stock 2.0L Turbo of the Solstice and Cobalt both run a plug with a heat rating of 5 and that engine makes around 263hp to the crank, though compression is slightly lower at 9.2:1 vs our 10.4:1. Higher compression in and of itself does not raise engine heat, but power does. For example the above engine compression ratios do not mean that the 10.4:1 compression runs hotter than the turbo's compression, however higher horsepower is a byproduct of stronger combustion, ergo hotter temperatures. Turbos by definition are already creating hotter air and higher compression due to the turbo transferring heat into it and condensing the air, and intercoolers are one of a few methods available to counteract this increase in temperature. Changing your engines spark plugs with a different heat rating is another. Having done the "hotter and colder" debate in my days as a WRX modder, the real tuning of spark plugs is to pull them out, read the signs, and decide if you need to run hotter or colder (one of the main discussion points of this article). Automatically switching to a colder plug for your application is only necessary if you know that the plug is running some weird heat rating. There are too many variables to automatically assume that you are running plugs that are too hot. Air temps, altitude, humidity, intercooler size, air fuel (AFR, rich runs cooler) and they all contribute to the heat of the combustion chamber.
Final Thoughts:
It appears that my engine is running in a rich and "plenty safe" AFR, with a possible trend towards hotter temperatures, though the different indicators could mean that the temperature is fine, but the signs are giving a better indicator of AFR than they are of engine temperature. I may buy a colder set and run them for 100 miles or so and see what those plugs tell me. Spark plugs can be a great indicator and assistant in listening to your engine's health as well as helping tune your engine to perfection. I hope this has been helpful and saved someone many hours of reading. As always your mileage may vary, and you should do your due diligence to your own situation as every engine can be different due to the variables of weather, humidity, altitude, etc.
UPDATE (9/24/2015): LTR6IX-11 Heat Range:6
As mentioned before, I wanted to see what going a step colder would do. I was a bit confused by the results, as you would expect the colder plug to leave me with a richer fuel mixture, but according to the signs, you would think that isn't the case. As you can see below the base ring is lighter and the ground hook was white, which usually indicates a leaner fuel mixture. What's wrong here? So I checked the older plugs, and sure enough the gap was a bit wider (0.040") whereas I had gapped the new plugs at 0.036". This would explain a hot center fire, but rich appearance since the gap would result in an inefficient fuel burn. I will be running the colder plugs for another 100 miles or so to give them time to cycle a bit more fuel. I double checked my AFR/Boost gauge logs to see if anything out of the ordinary was going on, and the AFRs are still at the appropriate range for the amount of boost I am seeing. Here is a picture of the plug:
![]()
UPDATE (9/26/2015): 200 mile spark plug reading of the NGK LTR6IX-11
So here's the plugs after a little over 200 miles.
![]()
You can see they are mostly resembling the plugs I had before, except with the correct gap, they seem to be burning at the most ideal temperature, timing (you can see that band is right on the apex of the hook), and safety (running rich) according to all of the signs that I can read. Now all that's left is to wait for the two bar tune, and hopefully improve the efficiency without any more danger. I can say that I have gotten 30mpg+ on every tank that I've had since installing the JonV turbo. This was calculated mpg, not dash relayed.
My last fill up was 7.7 gallons at 241 miles (31.3 mpg).
I think most people would pretty easily identify a plug with problems (oily, black, burnt, melted, crusty, etc) so I'll concentrate on some more specifics of a seemingly "good" plug that can tell you a little bit about your engine.
We will start with a picture of mine for a point of reference to compare and judge. For the record, I believe these are the stock AC Delco plugs, and with nearly 10,000 miles on them these are looking good, but they are speaking to me like the plug whisperer about some things I should know or improve, especially since I'm running a turbo.
I'm only going to touch on two indicators: heat and air fuel ratio. The big deal with turbos, is whether you are running too rich (too much fuel) or too lean (not enough fuel). The leaner you are, the more power you make at the risk of detonation or pre-ignition (burning the fuel when the cylinder is not in the proper position). This leads to broken rods, bad times, and sad faces. High heat is associated with lean conditions (gas actually cools the process) so you want a perfect balance of heat and the right amount of fuel. After the pic, is a basic quick analysis of your plugs that can help you "listen" to your engine.
Heat:
There's two main indicators for heat: The ground strap (the "hook" bending around the end) and the center electrode (needle doodad in the middle). Every plug should have a color change happening on the ground strap hook, and this is pretty much your mark for temperature. The closer it is to the end, the colder it's running. Closer to the threads and you are running hotter. Most people agree that the ideal "temperature" is to have this fuzzy color change right at the bend of the ground strap. Having this color change at the elbow is also an indicator of good engine timing, but is outside the scope of this post. As you can see mine are slightly closer to the thread, so I could be running a little hot. Not so much a problem as it is a sign that I can improve the temperatures with a colder plug (spark plugs can be purchased colder or hotter, and you should consult someone in the know on what is best for your engine).
Moving on to the center electrode, it should have a mark that is as close to the tip as possible (but still showing). You can see that I have a very stark line (more on that below) that is pretty close to the tip, so its a secondary indicator that we are likely closer to "in range" temperatures.
A third, and less reliable, indicator is how far up the threads you have color. The farther up the threads that there is color, the hotter the engine is running. Ideal heat indicator here is about 2-3 threads. You can see I have a shiny band after the 2-3 threads and then more color, which could be from undertightening the plugs. For this reason, this indicator can be an unreliable source, but could help if the other two aren't talking to you.
Lean/Rich Air Fuel Ratio (AFR):
So the darker your plug is, the richer it is running. Again, the ground strap, electrode, are good indicators if they are discolored, gunked, or have other obvious defects, but there are two other indicators that are better at giving you a high level look at AFR: the porcelain insulator (white in the pic above, around the electrode), and the base ring (last edge immediately following the last threads, and to which the ground strap "hook" is welded).
The porcelain insulator by most experienced mechanics agree that the color should be a dull white, gray, or slightly tan color. Too dark is too rich. Too white and shiny and it's too lean. Mine are a dull white, which again, is a possible indicator of my engine running hotter than ideal conditions, and could mean it's running on the leaner end of things, but coupled with the base ring color it more likey to be a hot and rich situation. Another small thing to note, that if the insulator is overly shiny and covered in metal and dark spots, this means you have had detonation or pre-ignition and should have your engine looked at by an experienced tuner or mechanic.
Moving on to the base ring, it should have color on what is called a "full turn." This means that the entire ring is covered in something. The color indicates whether you are rich or lean. In my case it is a dark black color, which indicates that I'm running rich. Here is a really informative excerpt from one of the websites that I read:
Based on this information, it looks like I'm running a "plenty safe, maybe too rich" running range, but also counter intuitively to our heat diagnosis, seems to indicate that maybe the plugs are not running too hot as previously thought. The previously mentioned darker color on my ground strap hook is another indicator of my engine running on the rich side of things.
EDIT DUE TO POPULAR DEMAND: Spark Plug Heat Rating (Range):
Since there was a lot of curiosity about the heat rating for the plugs I am running in comments, I thought I would elaborate on what I mentioned several times, and below in my Final Thoughts about "colder" and "hotter" plugs.
The heat rating of the stock ACDelco 41-103 spark plug is 5. For those that aren't familiar with spark plug heat ratings, the lower the number, the hotter the plug. In effect your spark plug has the secondary job of pulling or keeping heat in the combustion chamber. Spark plugs range between 2-11, however modern cars are nearly always between 5-7. Here is a graphic from NGK illustrating this point and what happens when you run too hot or too cold:
The stock 2.0L Turbo of the Solstice and Cobalt both run a plug with a heat rating of 5 and that engine makes around 263hp to the crank, though compression is slightly lower at 9.2:1 vs our 10.4:1. Higher compression in and of itself does not raise engine heat, but power does. For example the above engine compression ratios do not mean that the 10.4:1 compression runs hotter than the turbo's compression, however higher horsepower is a byproduct of stronger combustion, ergo hotter temperatures. Turbos by definition are already creating hotter air and higher compression due to the turbo transferring heat into it and condensing the air, and intercoolers are one of a few methods available to counteract this increase in temperature. Changing your engines spark plugs with a different heat rating is another. Having done the "hotter and colder" debate in my days as a WRX modder, the real tuning of spark plugs is to pull them out, read the signs, and decide if you need to run hotter or colder (one of the main discussion points of this article). Automatically switching to a colder plug for your application is only necessary if you know that the plug is running some weird heat rating. There are too many variables to automatically assume that you are running plugs that are too hot. Air temps, altitude, humidity, intercooler size, air fuel (AFR, rich runs cooler) and they all contribute to the heat of the combustion chamber.
Final Thoughts:
It appears that my engine is running in a rich and "plenty safe" AFR, with a possible trend towards hotter temperatures, though the different indicators could mean that the temperature is fine, but the signs are giving a better indicator of AFR than they are of engine temperature. I may buy a colder set and run them for 100 miles or so and see what those plugs tell me. Spark plugs can be a great indicator and assistant in listening to your engine's health as well as helping tune your engine to perfection. I hope this has been helpful and saved someone many hours of reading. As always your mileage may vary, and you should do your due diligence to your own situation as every engine can be different due to the variables of weather, humidity, altitude, etc.
UPDATE (9/24/2015): LTR6IX-11 Heat Range:6
As mentioned before, I wanted to see what going a step colder would do. I was a bit confused by the results, as you would expect the colder plug to leave me with a richer fuel mixture, but according to the signs, you would think that isn't the case. As you can see below the base ring is lighter and the ground hook was white, which usually indicates a leaner fuel mixture. What's wrong here? So I checked the older plugs, and sure enough the gap was a bit wider (0.040") whereas I had gapped the new plugs at 0.036". This would explain a hot center fire, but rich appearance since the gap would result in an inefficient fuel burn. I will be running the colder plugs for another 100 miles or so to give them time to cycle a bit more fuel. I double checked my AFR/Boost gauge logs to see if anything out of the ordinary was going on, and the AFRs are still at the appropriate range for the amount of boost I am seeing. Here is a picture of the plug:
UPDATE (9/26/2015): 200 mile spark plug reading of the NGK LTR6IX-11
So here's the plugs after a little over 200 miles.
You can see they are mostly resembling the plugs I had before, except with the correct gap, they seem to be burning at the most ideal temperature, timing (you can see that band is right on the apex of the hook), and safety (running rich) according to all of the signs that I can read. Now all that's left is to wait for the two bar tune, and hopefully improve the efficiency without any more danger. I can say that I have gotten 30mpg+ on every tank that I've had since installing the JonV turbo. This was calculated mpg, not dash relayed.
My last fill up was 7.7 gallons at 241 miles (31.3 mpg).
