[Henry Schmidt]

I quick note for those who want accurate information.
Most of the Porsche factory built 911 based engines (906, 907, 911R 956, RSR) ran a 225 fan and housing with a 1.3:1 ratio. the reason being that a fan like a propellor operates best at limited speeds. The closer the fan tips get to supersonic the less efficient the air flow becomes. Many IMSA teams actually ran a power pulley to reduce the fan speed and consequent horse power lost associated with driving the fan.
The reason Porsche went to higher ratios pulleys in later cars was to promote cooling at low engine RPMs. AC on while sitting in traffic and the like. As the engines got larger the red lines when down so max fan speeds were equally reduced. Also because the fan and alternators ran on the same pulley increased electrical demands created a need for higher speeds.
As soon as the 3.6 engine was developed with a two speed pulley system the alternator speed went up and the fan speed went down.

The best way to cool an oil cooled engine (yes 911 engines are primarily oil cooled) is with a proper oil cooler.The bigger the better. With a proper oil cooler a race engine can even run with a reduced speed 5 bladed fan. The big concern here is head temp which is why we run a head temp sensor whenever possible.

A fan does not have to go supersonic to be inefficient.
Generally a ducted fan will function it's best ( most flow/hp) at around 560 ft/s and still be relatively quiet.
Anything over 650 ft/s will tend to be very noisy.
Strangely enough when I calculate the tip speed of a 1.3:1 driven 245mm fan at 7000 RPM I get 541.33 ft/s.
Later model Porsches generally redline between 6400-6800.
Racing Porsches used small fan turning @ a reduced ratio 1.3:1 because they ran at higher RPMs. Sometimes as high as 8000 RPM.

diameter mm rpm pulley ratio fan tip speed ft/sec volume l/s

early 911.......245.......6100.......1.3.......334...... .1360

76-77 5 blade.......245.......6100.......1.8.......462.... ...1265

SC.......226.......6100.......1.8........426...... .1380

930.......245........6100........1.68.......431... ....1500

964/993........279.4.......6100.......1.6.......468... ...1010

993tt.......279.4........6100.......1.8........526 .......1210