What is my fan ratio???

[Robs72T]

I sent Gandy a PM asking how to measure fan ratio and this is what I got back (thought I would share)

Gandy, how do I determine fan ratio? I have a "new" 2.7 that needs to
be installed and I want to make sure I have the optimal ratio. At this point all
I know is that I have a 11 fan blade.

The easiest and best way is to paste a 360 degree protractor (a paper photocopy
works well) oto the fan. You will need a large center hole for the hub and one
split to get it around the fan belt. A simple piece of wire from a fan shroud bolt
will work as a pointer.

Turn the engine CW an intergral number of complete revolutions - say ten complete
turnd OT-to-OT.
It is much easier to turn the engine if you remove the sparkplugs.

As tou turn the engine, count the integral number of complete turnd of the fan.

As you get to the last partial turn of the fan, note the number of degrees when
the engine gets to the tenth complete turns. You then add this fractional turn
of the fan to the total intiger number of turns. For example, you measure a 267
degree fractional turn. That is 267/365 = 0.73 in decimals. If the fan had
turned ,say, 18 turns plus the fraction for ten turns of the crankshaft, the ratio
would be 18 + 0.73 = 18.73:10 or 1.87:1

The Factory claims that 1.82:1 is the highest ratio. The actual ratio is dependent
on which fan hub you have (80 mm or 83 mm, Porsche doesn't distinguish between
the two) and the number of shims between the pulley halves. There is also a slight
difference between a 9.5 belt (correct) and a 10.0 belt.

Please feel free to post this on Pelican.

If you want more, please email me so I can send images.

[Robs72T]

Can't you just measure the pulleys?

[klaucke]

The issue with just measuring the pulleys is that the effective pulley size is different than the overall diameter. The effective diameter varies depending on the size the pulley, the amount of wear (likely minimal in this case), and the belt in use.

[Eagledriver]

While this is certainly the most accurate way, I would say it's much easier to just look at the pictures in a book and compare to your car. In addition each year and model will determine it usually unless there is a mod.

Early cars (like yours) are usually 1.3 to 1. 78-79 SC's are 1.8 to 1 with a smaller fan. 1980 and newer are 1.67 to 1 with the larger fan like the early cars. The early cars with the 1.3 fan ratio have 8 holes in the face of the fan pulley.

Hope this helps.

-Andy

[Wil Ferch]

Look what I say here...especially post #18 --->

Engine Fan Drive Ratio