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Join Date: Jun 2004
Location: Friendswood, TX
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Question 11 Blade Fan upgrade fan belt question

Hey Guys.

Looks like the PO of my C3 engine upgraded the fan to the 11 bladed type. The engine pully is still the original 130 mm, but the fan is the pre-76' 95 mm. Not too worried about the ratio, but when I put the 710x10 belt on it was too tight with all 6 shims installed. Probably explains this...

Should I be worried about this fan housing weld?

My question is what is the best belt size for this half upgrade?

Thanks
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John F. Lewis

74' 911S Targa + 930/02
76' 914 - Eternal Type IV project FOR SALE
Old 03-20-2005, 07:24 PM
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Maybe you can work your way through this info that I posted in another thread:
---------------------------------------------------------------------------

Guys:
There's still some misinformation floating around here ....
the small fan pulleys are 3.125", or 3.25" ( 80 vs 83 mm). Why such small change was ever made we'll never know. Notice that the pulleys are "pulley-halves". You can't simply change the upper fan pulley without also changing the mating fan ( which has the other "pulley-half" surface on it).

From my notes...and this is inconsistently shown in the various parts books, so don't hold me to it, it may be one model year "off":
1965-74/75.. 110 mm crank pulley..95 mm fan pulley
1976-77... 130 crank...83 fan ( may be 80?)
1978-79....130 crank... 80 fan ( or as that 83?)
1980-89.....120 crank....83 fan ( or was that 80?)

for fan sizes / blades / air flow:
1965-74/75....245 mm / 11 blade... 1.3:1 ratio..1390 liters air/sec
1975/76- 77.....245 / 5 blade.......1.8:1 ratio....1265 l/sec
1978-79...........226 / 11 blade ...1.8:1 ratio...1380 l /sec
1980-89.........245 mm / 11 blade...1.67:1 ratio...1500 l/ sec

Turbos used the 245/11/ 1500 set up.

Interestingly... the later curved fans for the 964 were quieter ( by being curved)...but resulted in lower flow:
-- 1.6:1 ratio..1010 l /sec (!).

fan belt sizes:
1965-75....9.5 x 710
1976-77.....9.5 x 725
1978-79....9.5 x 725
1980-on.... 9.5 x 710

There are "borderline" years where these specs don't quite match...and the goofy 80 vs 83 negligable fan size difference doesn't help to sort this out either. It won't make a difference in performance but watch out of you mismatch a 80 and 83 pulley half. BTW....aftermarket will sell 9.5/10x 710 as being "equal" to 9.5. Don't buy it. There have been reported problems with this. If possible, buy from dealer or Pelican and ask for the right size.

--Wil
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Wil Ferch
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Old 03-21-2005, 08:14 AM
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Thanks Wil, great info

Did the math using your data and I show that I need a 9.5 x 740.

The way I calculated it was adding half of the circumference of the fan and crank pullies and figuring the run between the two pullies by knowing the distance between the pullies and the difference in the radius. Here is the formula I came up with:

Belt lenth = Cf*((r1*pi) + (r2*pi) + (2*sqrt((L^2)+((r2-r1)^2))))

Where:
r1 = fan pully outer radius
r2 = crank pully outer radius
L = distance between center of fan and crank pullies
Cf = correction factor

I calculated the correction factor by comparing actual belt sizes to my calculated using the data you pulled together. The correction factor was ~0.97. I figure this is due to the fan belt riding inside of the outer diameter of the pullies.

Now I have got to go find the right belt
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John F. Lewis

74' 911S Targa + 930/02
76' 914 - Eternal Type IV project FOR SALE
Old 03-21-2005, 08:34 AM
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I didn't check ( point-for-point) your math...but your reasoning is sound and I did the same math once before to come up with the data I presented.

Yes...you need to know center-to-center distance of the crank and fan pulley centers, and the circumference of each pulley. A "correction" factor of sorts needs to be applied because the quoted speed-increasing ratios don't fully match the actual sheave sizes. Apparently, where the belt rides within the pulley is a factor.

Good show... !

Wil
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Wil Ferch
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Old 03-21-2005, 10:03 AM
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For what it is worth...

I put a 10x735 on my engine and it fit with 6 shims.

So, if you have an engine that was modified from a 5 blade fan to an 11 blade fan without changing the crank pully, do the math and go with the larger belt to save your fan housing.
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John F. Lewis

74' 911S Targa + 930/02
76' 914 - Eternal Type IV project FOR SALE
Old 04-11-2005, 02:22 PM
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Just being anal..... asking to verify 735...... not 725 ...... right ??

- Wil
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Wil Ferch
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Old 04-12-2005, 04:44 AM
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Yep, 735. I calculated 740. I found a Continental 10x735 and gave it a try.

The extra shim (6 inside rather than 5) just verifies the math

Or my luck
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John F. Lewis

74' 911S Targa + 930/02
76' 914 - Eternal Type IV project FOR SALE
Old 04-12-2005, 05:58 AM
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730 and 735 are the most common sizes used when you swap pulleys around and the 710/725 don't fit anymore.
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Old 04-12-2005, 07:11 AM
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