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Discussion Starter #1 (Edited)
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.

20150916_233508.jpg


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:
  • full-turn = richer , safer, almost all air is used
  • full-turn thats darker= richer , plenty safe or maybe too rich
    darker means not enough heat to burn deposits off and/or the soot of rich combustion is leaving its color deposited onto base-ring. Thicker layer of darker soot all the way around the base-ring
  • 3/4 turn= leaner than full-turn, leanest you should ever be, thinner soot deposits
  • 1/2 turn= leanest ... edge of preignition / detonation, almost all fuel is burned
    excessive heat is burning off thinner soot deposits
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:
20150924_001033.jpg


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.
20150927_000439.jpg


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).
 

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WOW!! Awesome write-up jookyone!! I've printed your instructional post , and will be sticking it in my service manual. (once it gets delivered).
Thanks for taking the time to create this well written and informative post....Very Useful stuff!!!!
 

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Why in the blue blazes would you have changed them at 2,000 miles??? That is tomfoolery.
 

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Why in the blue blazes would you have changed them at 2,000 miles??? That is tomfoolery.
Not often I concur with Aufgeblassen but I have to agree with him on this. It is my understanding you should have received new (colder) spark plugs when you had your turbo installed to deal with the added heat. It has only been 2000 miles and you should have plenty of miles left on them. Then again, spark plugs are not that expensive, so do what you think is best.
 

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I got the distinct impression that jookyone was simply taking the time to check his spark plugs after racking up around 2,000 miles on them. He goes on to describe his readings and how to use what the spark plugs are telling him to decide whether or not he needs to try a colder or hotter plug. Maybe I'm misinterpreting his post, but that's what I get from it.
 

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Not often I concur with Aufgeblassen but I have to agree with him on this. It is my understanding you should have received new (colder) spark plugs when you had your turbo installed to deal with the added heat. It has only been 2000 miles and you should have plenty of miles left on them. Then again, spark plugs are not that expensive, so do what you think is best.
It was 11000 miles on the plugs he changed. we are better off not to pay attention to some who don't pay attention to details.

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
 

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What I gathered from reading Jooks post was that he pulled the plugs that were installed when his turbo was installed to do a reading. But he used his original plugs that had 10,000 miles on them as an example for his post.

I know it's kinda hard to keep things straight after riding around in the Slingshot. It makes your head spin.
 

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One thing that will discolor plugs and give you false color readings is running octane boosters. It will darken the inside area of the plug.

I never run it unless I can't find 93 at the pump. And even then I only use a few ounces per full tank.
 

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Great, detailed write up @jookyone . Thank you for taking the time (day or so by my calculations) to do it . It surely gives myself and others something to look for.
 

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Discussion Starter #11 (Edited)
For clarity, I did change my spark plugs at 2,000 original miles on the vehicle. I do this with pretty much all of my vehicles because I want a fresh baseline after what I would consider a good wear-in period. It's a personal choice and to someone else's point, doesn't cost very much and it gives me a good time interval to check on the engine's health. After that I check around 10,000 miles for the same reason, it's a good time interval to check on the engine's health. I should note that I typically drive around 30,000 miles a year across a few vehicles. In this case it's been about 5 months since I changed them.

I have edited the above post to include information about spark plug heat ratings. Here is the cliff note version: the stock plug has a heat rating of 5 which matches the stock heat rating of my WRX, the turbo 2.0L Solstice and Cobalt and many other force inducted engines. If this was a 350hp+ modification, I might automatically go to a colder plug, but given the power levels that we are experiencing, it isn't automatically necessary. One of the main points of this article, is to determine if we are running hotter or colder, so hopefully the additional information I added above helps in the understanding.

Cheers

EDIT: For those following along, and have a turbo, all of these tunes are rich for safety. They aren't ideal, but better to run rich than blow an engine. Based on what we are seeing above, it is evident that I am running rich (all of the black) but not hot enough to burn off all of the extra fuel. Running colder plugs would increase the chance that I foul the plugs, especially given how rich these tunes are running at the moment, and I'm happy with the color and cleanliness of the ground strap, electrode, and insulator.
 

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Discussion Starter #12
WOW!! Awesome write-up jookyone!! I've printed your instructional post , and will be sticking it in my service manual. (once it gets delivered).
Thanks for taking the time to create this well written and informative post....Very Useful stuff!!!!
You may want to print again with the added section on heat ratings :)
 

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Discussion Starter #14
Not often I concur with Aufgeblassen but I have to agree with him on this. It is my understanding you should have received new (colder) spark plugs when you had your turbo installed to deal with the added heat. It has only been 2000 miles and you should have plenty of miles left on them. Then again, spark plugs are not that expensive, so do what you think is best.
@Aufgeblassen
You can choose to save $28 on spark plugs and run the extended life out of all of your car parts, often with little to no worry. I fall into the camp of having my car parts in as peak a shape as possible, since I tend to drive really hard, and usually have modified my vehicles in some way. The early plug change is just a personal quirk and while technically unnecessary, I do it for my own peace of mind and as a precaution to the way I drive and play with the engine.
 

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Discussion Starter #15
.
Still using OEM recommended plugs with a Turbo ?
I never run it unless I can't find 93 at the pump
I trust the knowledge of my past and try to learn in the present. In this case a spark plug with a heat rating of 5 seems to be doing its job well enough, and I have never just automatically gone colder because "thats what turbo guys do."
Bill Hahn is very forthcoming with a wealth of information, and he very informatively posted boost and power numbers for both 91 and 93 Octane fuels, and their safe application to this motor. I am not a huge fan of Octane boosters or fuel additives unless I plan on being at the track all of the time and want to make the most power, since higher Octane runs hotter and as you indicated additives can gum up the combustion chamber. I only have 91 Octane available to me here and throughout most of the country, so my engine is going to be running cooler than those using 93 Octane. Bill also happens to live in Florida (Rab in Alabama) where it is hotter and muggier than where I live. The point of this article was to explore my spark plugs and determine what my engine was doing on the current setup to then make an informed decision about whether to change them or not. I was not having any engine issues, problems, or otherwise, it has been 2,000 miles and as mentioned above, that is when I take a look at the plugs to see how the ol' girl is doing (my cars are always girls). Though with the jonv exhaust this girl would have a really really deep voice.
 

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Discussion Starter #16
I should also mention that since this forum is helpful and we have a slew of experienced car guys here, that this thread could be a place to post pictures of your own spark plugs for the discerning analysis of what they look like and what your engine might be doing, or how it could be improved. I'm sure there will also be the "your plugs suck! and you suck at plugs!" comments but it comes with the territory when you put yourself out there for judgement on the internet :)

Cheers!
 

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I trust the knowledge of my past and try to learn in the present. In this case a spark plug with a heat rating of 5 seems to be doing its job well enough, and I have never just automatically gone colder because "thats what turbo guys do.".
For clarification referencing a heat range number is only relevant when discussing a specific manufacturers plugs. Different manufacturers use different methods and ranges to indicate heat range. For some a higher number is "hotter" where others it is "colder".

Some examples :

Capture.JPG

Spark Plug Cross Reference HEAT RANGE CHART
 

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Discussion Starter #18
For clarification referencing a heat range number is only relevant when discussing a specific manufacturers plugs. Different manufacturers use different methods and ranges to indicate heat range. For some a higher number is "hotter" where others it is "colder".

Some examples :

View attachment 27737
Spark Plug Cross Reference HEAT RANGE CHART
Great information!
As my first experience with spark plug heat ratings was with my WRX and specifically conversations revolving around NGKs plugs, that is the scale I'm most familiar with. NGK also seems to be at forefront of informing consumers about heat ratings. They even have a plug lineup called "One Step Colder" which is wildly popular in the turbo import arena. It seems that turbo applications are really driving the understanding of plug heat ratings and NGK seems to be at the forefront of both usage and distributing information to customers.

Cheers.
 

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@Aufgeblassen
You can choose to save $28 on spark plugs and run the extended life out of all of your car parts, often with little to no worry. I fall into the camp of having my car parts in as peak a shape as possible, since I tend to drive really hard, and usually have modified my vehicles in some way. The early plug change is just a personal quirk and while technically unnecessary, I do it for my own peace of mind and as a precaution to the way I drive and play with the engine.
Change spark plugs? I never keep anything long enough for that. With the lengthy oil change intervals these days I even trade some vehicles before the first oil change. :D
 

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Discussion Starter #20
For those worrying about my engine (thanks!) the Hahn kits are running a recommended plug with a heat rating of 5 also.
NGK LZTR5A-13 (In the NGK naming convention the "5" is often their heat range)

Cheers!
 
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