Do I need colder spark plugs? Will colder spark plugs give me more power?
These are common questions and they unfortunately get answered incorrectly most of the time or incite a flame war. Spark plugs in general are always a controversial topic – copper vs iridium, hotter vs colder, single or multiple electrodes, brand vs brand and plenty more. In this article, we’re just going to discuss heat range.
Spark plugs have 2 jobs. The first is obvious, to create the spark necessary to ignite the air/fuel mixture in the cylinder. The 2nd, and less obvious job, is to pull heat from the cylinder and transfer it into the head where it can be cooled down by the cooling system.
The term colder plug and hotter plug refers to how fast or slow the plugs transfers the heat from the tip. A colder plug transfers heat faster while a hotter plug is slower. The goal is to keep the cylinder temperature between 550*c and 800*c which is about 1020*f to 1470*f. This temperature range is cool enough to prevent knock and not melt the plug tip, but still hot enough to burn off the carbon and prevent fouling.
For a stock car or even a car with full bolt-ons, running a colder plug isn’t going to benefit you much if at all. In fact, you’ll probably end up with more problems because the plugs will foul and cause mis-fires. You will see people reporting smoother revs or better idle after switching plugs, but this is most likely due to just being fresh plugs with correct plug gap and/or plug design differences and not so much from the heat range. These perceived benefits will soon fade.
The biggest problem is that running a colder plug when it isn’t needed, is that it will foul up quickly. This happens because the plug tip doesn’t stay hot enough to burn off the carbon deposits, and so a build up occurs. Another common problem is that colder plugs can make the car harder to start when it’s cold outside.
So in general, you should stick to your manufacturer’s recommended heat range until you actually need to run a colder plug. But when would that be? There seems to be a rule of thumb that states you should go 1 range colder for every 75-100hp additional you make over stock. However, I don’t follow that rule because it all depends on your setup. I’ve seen plenty of high power cars running stock heat range with no problems.
What I suggest is that anytime you make a change that will increase cylinder temperatures, you evaluate your need for a colder plug. Adding a turbo or super charger to a naturally aspirated setup, increasing compression ratio, making big changes to timing, changing fuel types and adding nitrous are a few examples of modifications that can increase cylinder temperatures. As always, if your tuner recommends you go with a colder plug, then you should listen. For a stock car, or even a full bolt-on, you’re not going to see any benefits, so don’t waste your time or money.
Now, if you do need to get a colder or even a hotter plug, keep in mind that there is no universal heat range system. Each plug manufacturer will have their own heat range system.
For example, NGK uses a system that ranks colder plugs with a higher number and hotter plugs with a lower number, while Bosch does the opposite. If you’re switching plug brands, you will need to do some research on the equivalent heat range for the new brand.