In general, you want to run the coldest plug you can that won't foul.
For non-turbo cars and turbos running factory boost levels, stick with the factory-specified plug.
For cars running higher boost (16 psi or more), you might want to move to a one-step-colder plug. The higher boost pressure generates intense combustion heat, and heat removal is critical. An easy rule of thumb is for every 75-100 horsepower you add, use one range colder plug. (We can supply you with two- or even three-range colder plugs; give us a call.) Note that if you drive around without using the boost, your two-range-colder plugs may foul because they won't heat up enough to burn off the carbon deposits.
See the plug diagnostic section for more help on figuring out if you need a colder or hotter plug.
Sign up to get the latest on sales, new releases and more …