Found this on the Stewart website... Very interesting.
When using a mechanical fan, a properly designed shroud must be used. Most mechanical fans are not designed for high RPM use: they can have serious vibrations problems, due to air turbulence, when run over 6,500 RPM. This is a turbulence problem, not a balance problem, and will destroy the water pump and components in front of it. The large fans preferred by dirt oval track racers can consume up to 18 horsepower at 6,500 RPM. Do NOT run a mechanical fan that is any larger than required for the application.
Flex fans are a poor design for performance applications. They move less air at higher RPM, and only consume a fraction less power than standard fixed pitch fans.
Clutch-style fans are inconsistent and we do not recommend their use for any application, if possible.
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