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Spark Plug FAQ


Aug 19, 2005
Midland, Michigan
The Spark Plug FAQ: or "What spark plugs should I use in my DSM?"

Every few weeks, sometimes even every few days, there comes a thread where a new owner of a DSM asks what the best spark plugs are for his or her car. More rare, but still seen every once in a while is the question posed where the owner is having problems with the car stumbling, hesitating, losing power, and otherwise not running quite right. In some cases, it is due to not running the correct spark plug or plug type. In this FAQ, I'll try to depict a few different types of spark plug and the pro's and con's of each. There's a fairly definite answer to the question you may have: "What spark plug should I use in my DSM," but we'll get to that later.

First, a few pictures of some various spark plugs you may be currently using, have used in the past, or have considered using. Note: The following applies mostly to turbo DSM's and may not reflect usage in a non-turbo application.

#1. The NGK BPR6ES. This is what the majority of DSM'ers who don't have too many mods or are running fairly low boost will tend to use.

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#2. This is the NGK BPR6EKN. This is what you'll most likely be offered if you walk into an auto parts store or dealership and ask for plugs for your turbo DSM, because this is the plug called for in the Owner's Manual and the shop manual. It was the standard factory plug for turbo DSMs. Notice the dual electrode. It's fairly pointless, since the spark will only jump to one of them, but these are an option for our cars, although not the best option. Unless part of your shop's income is generated by selling spark plugs, of course.

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#3. This is a Bosch Super plug for the DSM 4g63t. It's a copper plug, fairly similar to the NGK, more or less. You may be offered this plug when you go to an auto parts store.

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#4. This is a Bosch Platinum plug. This is another option you may be given when you go to your local auto parts store. It has been the experience of nearly ever DSM'er that you should avoid platinum plugs at all costs in turbocharged applications. The salesperson will most likely tell you that platinum plugs last a long time, or maybe they're on sale. It doesn't matter. They simply are not the best, nor worth the cost, for our cars.

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A note about the pic: Do you notice anything missing that was visible in the previous pictures? That's right. The center electrode is amazingly small, nearly invisible. Below are two internal diagrams of the Bosch plugs (copper and platinum) from the exterior of their respective boxes.

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#5. Here is a Bosch Platinum +4. It has four electrodes and a small platinum center electrode.

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I don't have any pictures of any iridium plugs because they're special order where I work, and we didn't have any in stock. This isn't suprising because of their cost. For the price of a single iridium plug, you could have an entire set of standard copper plugs with money left over for a gapping tool and a frosty beverage.

One of the most frequently asked questions that crops up often here on DSMtuners is: "What kind of spark plug should I use in my (turbo) DSM?"
Use NGKs. They are the best for our cars. For some unknown reason, our cars just 'prefer' them.


An important feature of spark plugs that often goes unnoticed, or is often misunderstood, is what is known as the "heat range" of a spark plug. The spark plug must dissipate heat. Different heat ranges of spark plugs dissipate heat at different rates, which allows people to use different plugs for different applications. The plugs do not create heat, but instead remove it. The heat is transferred through the metal shell of the plug, to the head, where it is removed by the oil and water passages in the head. The way it does so is best shown by this diagram from NGK's website:

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Different companies sometimes use different methods for determining the heat ranges of their plugs. NGK, for example, uses lower numbers for hotter plugs and higher numbers for colder plugs; i.e. BPR6ES plugs are hotter than BPR7ES plugs. Using too cold of a plug in your car will lead to fouling, but using too hot of a plug may lead to a hot-spot developing on the plug surface, which may result in pre-ignition/detonation. The ideal situation is to use the coldest plug possible without fouling.

Here's a comparison of three spark plugs made by NGK. At first glance, you may not notice a difference. Upon closer inspection, you may find that the shape and thickness of the white insulator around the center electrode thickens as the plug's heat range goes colder. In addition, the insulator is in contact with more of the outer shell where the threads of the plug are as the plug's range goes colder. On the left is a BPR5ES, in the middle is a BPR7ES, and on the right is a BPR9ES.

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The more contact the insulator has with the outer shell, the more heat can be transferred out of the plug and into the head. Heat doesn't travel through air as well, so in a plug with less contact with the outer shell, the core of the plug stays hotter.

Another frequently asked question seen here (now that you know you need NGKs) is: "What heat range of NGK do I need for my (turbo) DSM?"
There are no set rules, but there are guidelines:
-For stock to near-stock cars, BPR6ES.
-For mildly modified to - heavily modified or high-boost, use BPR7ES.
-For heavily modified, high boost applications, use BPR8ES.
If plug fouling occurs, go one step hotter and monitor performance and results.

Stock car, T25/14b, 12-15 psi, upgraded intake/exhaust: 6ES
16g, 20 psi, water/meth injection : 7ES-8ES
GT35R, nitrous, the works: 8ES-9ES-10ES


Copper, Platinum, Iridium. (What's next, Adamantium?) What's the right one for you? As mentioned earlier, the vast majority of DSM'ers will swear by standard NGK copper plugs. Platinum is not as good a conductor as copper, but it's harder so it lasts longer. Iridium is also very hard, but it's also very rare, which makes it expensive. The consensus regarding iridium plugs is that while they work, they're not worth the price when standard $2/each copper plugs work more or less the same. Even though platinum plugs are closer in price to copper plugs, it's been my experience along with many other members, that running platinum plugs caused fouling, stumbling, hesitation, a loss of power, a decrease in gas mileage, and poor idle. Switching to standard NGK copper plugs solved the problems immediately.

Here's the answer to one of those common questions: "My car stumbles/hesitates/has no power/idles oddly/has lost gas mileage/does not perform well..."
If you are running platinum spark plugs and notice that your car isn't running right.... take them out and put in NGK BPR6ES spark plugs and see if your situation improves.


Another common question is in regards to the proper gap of a spark plug. This refers to the space between the center electrode and the side electrode. The average auto-parts store computer will usually suggest a gap for each spark plug that they have in their computers. However, in most cases, the best gap to use is the one specified by the manufacturer of your car. In the case of turbo DSM's, many people choose to gap their plugs to .028". DSM'ers have had good experiences with slightly larger gaps (.030-.032") as well as with smaller gaps (.026"). The computer where I work suggests a gap of .032" for the spark plugs for our cars.

Spark plugs do not come from the factory pre-gapped. You may open a spark plug up and find that it meets your gap needs, but this does not mean that the next identical spark plug will have the exact same gap. To be sure, manually gap each spark plug you install. There are several tools to help you measure a spark plug's gap. Here are three of the most common: a gapping disc ($.99), a blade measurer ($3) and a wire-gapper ($3). What you use is your preference. Many people say that the ramp-style gappers are not as accurate. They'll work in a pinch, but the wire and blade gapping tools are preferred.

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Here's an answer to another commonly asked question: "What should I gap my spark plugs to?"
Gap your plugs to .028"

My good friend Anthony (DSMunknown) has brought to my attention that there has been some discussion in past years regarding the gap of spark plugs opening up over time, possibly due to long projected tip or high exhaust gas temperatures. If you race or dyno your car regularly, checking your spark plug gaps on a regular basis (once ever 5 dyno pulls or once every 3-5 1/4 mile runs, or so) and monitoring whether they are opening up or not. If they are, you'll need to replace or re-gap more often than drivers who daily-drive their cars. More info on this will be forthcoming as research is conducted and reported.

To sum up: In general the BEST spark plug for our cars is the NGK BPR6ES gapped to .028", varying heat range depending on modifications.

For more information:
NGK Spark Plug Information
Decoding NGK Spark Plug part numbers.
Reading spark plugs
An in-depth look at spark plugs.
A VERY in-depth look at plug, brought to you by our cousins, the Stealth/3kGT.
A conglomeration of threads discussing different experiences with spark plugs.
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