That is dependant on the engine modifications that were performed. Always start at the stock heat range and refer to the specifications of the performance part suppliers. Generally, go one heat range colder for every 75-100 Horsepower increase. This is only good for internal engine modifications, add-on nitrous, supercharger, or turbocharger. The heat range does not have to be changed on bolt-on items such as: air intakes, exhaust (mufflers), or throttle bodies.