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Engine Fan Drive Ratio

Hey all. New to the BBS.

I have trouble keeping my oil cool in stop and go traffic in my 89 930. I'm already planning on installing a lower temperature fan switch on my oil cooler fan.

I think my engine fan has the 3.25" pulley. Has anyone on the list increased the engine fan drive ratio? I'm not worried about the high RPM power loss on this daily driver and it seems air would be the easiest way to keep this thing cool. Plus, it would help the A/C condenser efficiency and keep the intercooler cooled down.

Anyone else overdriving their engine cooling fan?
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Old 06-29-2004, 09:16 AM
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I think you have the best ( fastest turning) ratio available among the various years' Porsche pieces already.....

There were something like 110, 120 and 130mm crank pulleys...and something like 83, 85 and 95 mm fan pulleys available.If the numbers are correct ( memory at work...caution !...don't have my notes in front of me)...then you have the largest crank pulley and either the 83 or 85 fan pulley. Why Porsche ever made a 83 and 85 is beyond me. The number of tightening shims you use has more ratio effect than the 83 vs 85 mm size. Why bother ? Oh, yeah, right...."German" engineering...with all their complexities... silly me.
---Wil
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Old 06-29-2004, 09:29 AM
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Re: Engine Fan Drive Ratio

Quote:
Originally posted by philipguziec

Anyone else overdriving their engine cooling fan?
I do.
I'm not sure if if does any good. Efficency from added CFMs from the fan can become a complicated subject
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Old 06-29-2004, 09:44 AM
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Phil,
You are on the right track. More air - the better. True for engine cooling and A/C.

As Wil said you may have the maximum ratio already. The max Factory stuff is 1.81:1 ratio with an 84 mm fan pulley and a 134 mm crank pulley (9.5x725 belt.)

You possibly can have made an even larger crank pulley. The limiting factor is the clearance to the engine mount assembly and the case mounting studs. A custom pulley might be worth the effort in your situation. Remember the A/C pulley and belt clearance also.


Ronin,
I donít think efficiency is the issue here. Phil is willing to expend energy to keep the engine cooler (to save the machinery) and have the A/C be more effective in hot weather (to keep him cool.)


Best,
Grady
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Old 06-29-2004, 11:24 AM
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I think the proper fan belt is 710 circumference..not 725.

I also think the point that RoninLB is making is related to going too far. At some point you draw more hp than you get in additional cooling...or worse...the fan goes into "stall" when severely overdriven, and the amount of air flow decreases substantially.

---Wil
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Old 06-29-2004, 11:58 AM
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Could changing the ratio (up to 1.8:1) adversely impact the alternator?
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Old 06-29-2004, 11:59 AM
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Wil,
A í79SC is spec with 1.81:1 ratio, 84 mm fan pulley, 134 mm crank pulley, and 9.5x725 belt. Unfortunately it used a 226 mm fan.
Yes, the í80 911SC and others used the 710 belt.

I have this configuration with a 245 mm 11-blade fan on several 2.8S engines and turn them above 7200 RPM regularly. I find no indication of ďstallĒ or the blades going supersonic. It pumps a lot of air.


Jim,
I have never had a problem with the alternator turning this fast.


I think every 911, 2.4 and later can benefit from this configuration. It addresses the cooling issue at the core of the problem. You must keep the heads and cylinders cool. Keep the heads and cylinders cool and the ďhearts and mindsĒ (oil temp) of the engine will follow.
There are other threads on this issue.


Best,
Grady
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Old 06-29-2004, 12:23 PM
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Grady,

You're thinking along the same lines I am-get the heat out at the source. Any idea who makes an overdrive crank pulley? How about thoughts on how much to overdrive? I think fan power is the square of speed but I'm not sure the form of the equation of airflow or heat rejection relative to RPM for an axial flow fan. Any suggestions?

Phil
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Old 06-29-2004, 05:01 PM
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Quote:
Originally posted by philipguziec
Any suggestions?

fwiw.
my stock 5 blade system was described by Grady except I have the large fan housing. "A í79SC is spec with 1.81:1 ratio, 84 mm fan pulley, 134 mm crank pulley, and 9.5x725 belt. Unfortunately it used a 226 mm fan." , i believe because I didn't dig up the hard info. So when I did a large 11 blade in the stock housing I was told it may generate more noise, I didn't notice any noise problem. I kept the high ratio because I wouldn't be pushing the envelope.
So imho my large 11 blade and high ratio is the best sensible street and DE system.
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Old 06-29-2004, 05:27 PM
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So, you have the same drive ratio as my car and the same fan, but with a smaller shroud? Know of anyone with a higher drive ratio?
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Old 06-29-2004, 06:43 PM
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Quote:
Originally posted by philipguziec
So, you have the same drive ratio as my car and the same fan, but with a smaller shroud? Know of anyone with a higher drive ratio?
I have the stock large fan housing.. it came with the 5 blade and high ratio. If you have the small fan get a large 11 blade fan housing and fan imho. and some years were slightly different for bolting up the ignition wires.
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Old 06-29-2004, 06:54 PM
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An '89 930 stock has fan is 245mm, w/a ratio of 1:1.67 and flows 1500l/s @ 6000rpm stock drivebelt is 9.5x710, these specs are the same for n/a Carrera.

Like Ron, My C3 had a 5 blade 245mm fan w/ a ratio of 1:1.8 which I updated to a Carrera fan w/ retaining the stock pulleyi.e.245mm fan, 1:1.8 ratio. this gives a little more air flow, but not enough to noticably lower oil temps.

The best thing to improve cooling in traffic is the lower fan t-stat trip point
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Old 06-30-2004, 05:27 AM
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Bill,
You are right on. I think that fan combination was consistent for 911s from í80 thru í87?
I thought the 930 raised the ratio to 1.81:1 at some point.
Can someone shed some light there?

Phil,
I have not seen an aftermarket pulley larger than the Factory 134 mm pulley.
All 911 fans are overdriven, the question is how much.
Yes, I think most fans are exponential power vs. speed.
I would assume the air flow would be close to the same. It certainly isnít a liner relation.
All that said; there are fan speeds where all the rules go out the window.
I donít think we can get to that point with regular 911 parts.

The limiting factor for maximizing the ratio is:
First, it is impractical to manufacture a fan with a pulley smaller than the Factory 84 mm.
Certainly from a cost standpoint.
It is possible to have a machine shop make a crank pulley larger than 134 mm,
possibly for less $ than a new Factory part.
The limiting factor there is clearance to the engine mount assembly and ultimately the case studs.
The mount can be cut away some (necessary with a 134 mm pulley
on a 914-6 mount) or a new one fabricated.
I donít think there is any (reasonable) way to get around the case stud limit.
You should consider being able to install the fan belt with the pulley in place.
You could make it so tight that the belt goes around the pulley and then the pulley is bolted to the crank.
Lets see; to change the fan belt you remove the muffler, rear tin, support the engine,
remove the engine mount, change the fan belt, and reassemble Ė I donít think so.


At 7200 RPM and 1.81:1 ratio, my fan is turning 13,032 RPM. Works great.
Here is the info for í79 and í80 911SC.
"

"
(C) 1980 Porsche AG, "Porsche 911SC Service-Information Model 80." 4591.21

Given the above numbers, an 11-blade 245 mm fan driven at 1.81:1 and 6000 RPM
should deliver 1616 liters/second assuming a liniar relation.
It should deliver 1741 liters/second assuming an exponental relation.

It is mass air flow that cools a radiator (heads and cylinders.)


Does anyone have the Porsche numbers for the early ('66-'74) fan and ratio?
What did Porsche list as pulley diameters, ratio, and fan air delivery?


All this is like putting an early 36 HP crank pulley on a 40 Horse. It works.

Best,
Grady
Old 06-30-2004, 07:45 AM
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Wait, I'm confused. What is my crank pulley size and is there a larger factory crank pulley than mine available or not?

I follow on the limits to the crank pulley size and I don't want to try to downsize the fan pulley so, if there isn't a factory part bigger than mine, I'll figure out the biggest one that will still allow the fan belt to be changed and get one made, assuming it's meaningfully bigger and not ridiculously big.

Thanks for the help.

Phil
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Old 06-30-2004, 09:04 AM
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Anyone happen to know the size of fan on a 2.4L? Can it accept the small fan pulley? I'd love to forgo the cost of a turbo fan and housing if I can just buy pulleys and a belt, attach them to my 2.4E's fan/housing and enjoy most (or all) of the improved airflow. Or am I dreaming? Thanks!
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Old 06-30-2004, 09:48 AM
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Phil,
I donít know the answer to your question but Iím sure someone here knows.

Jim,
Your fan housing is OK. To achieve the 1.81:1 ratio you need a new 245 mm fan and attendant 84 mm pulley parts, a new 134 mm crank pulley, and a new 9.5x725 belt. Not inexpensive but worth it.

No, you canít put 84 mm pulley parts on an early fan like your í73.

Yes, you can replace just the crank pulley with the 134 mm pulley and keep your existing fan. It wonít turn as fast or be as effective. You will need to find a belt slightly longer than 725 mm. It wonít be a Porsche standard part.

Best,
Grady
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Old 06-30-2004, 10:12 AM
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Thanks Grady! I was trying to get by on the cheap, but it looks like I'll have to open the wallet a bit wider to implement this mod. Once again, thanks!
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Old 06-30-2004, 10:24 AM
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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 was 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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Last edited by Wil Ferch; 07-30-2006 at 10:24 AM..
Old 06-30-2004, 10:49 AM
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Wil, I thought the pulley halves can be completely removed from the pulley. Otherwise, how can the assembly be diasassembled to allow replacement of alternators and, more importantly, power-coating of fans?
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Jim:
I don't understand your question. The fan center itself is "one-half" of the fan pulley. The outside half is removable to allow belt changes. One can then ( conceivably) have the wrong "outer" half attached to the fan's inner half. Different fans not only have different number of blades and diameters...they embody different "pulley-half" diameters ! Beware.
---Wil

EDIT-- the half attached to the fan is ( I believe) riveted in-place, so yes..it's possible to have this removed / replaced...but as a practical matter it's semi-permanently attached, as I recall.
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Old 07-01-2004, 05:19 AM
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