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The 5 blade fan was introduced to the 2.7 911 in the mid 70's to make the engine run hotter in order to reduce exhaust emissions.
How do I know? I have a mid 70's 911 and every thing I have read about it tells me to upgrade to the 11 blade fan before the engine overheats and pulls the head studs out of the case.
So the 5 blade fan cools less.
To confuse things even more there were 2 sizes of pulley too.
Tim
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