| It seems like they are recomending the Iridiums for anything other than stock cars. Text from Website... "For years we searched for the "right" plug for high HP, high boost applications on our own Zs. After testing numerous plugs we found the absolute best plug money can buy. Trying to find an adequate plug colder than the PFR6B-11B proved to be tough, but these plugs deliver the best spark around. We offer these plugs in the heat range of 7,8, and 9. Our recommendation for choosing the correct heat range: Stock Boost Levels up to 15 psi: Stock Plug PFR6B-11B "7" Heat Range: Stock Turbos (any boost level), Sport 500s. 530s, & 600s. Great for street use. "8" Heat Range: For those who race at high boost levels and any upgraded turbochargers. Good for nitrous as well. Great for street use. "9" Heat Range: Race use only! Great for high boost and nitrous use. Not recommended for daily driving."
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