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Take a look at the early Viper fans , they have small rubber flaps on all four corners that allow air to flow threw at road speeds and close at low speeds to force more air threw the fan . It is a two speed unit . I believe it is 2600 at low speed and 3400 CFM on high.I trimmed the factory mount and it slip right in . I have use it for five years and works great . You need two relays and an over ride switch and run it direct from the Alt.