A lot of time and effort went into designing these valve springs specifically for the 4g63. The unique single spring beehive design requires less horsepower to operate, and handles high rpm harmonics better than any dual spring on the market. These springs have been tested to over 10,000 rpm, and are used in our record setting EVO 2. These are a must have for any 4g63 spinning over 8,500 rpm. Available for EVO 4-9.