Question on Pully Size - for increasing cooling

[Tidybuoy]

I've read somewhere on this site that you can aid cooling by changing the size of the pully.

My question are:

1) which pully gets changed - top or bottom

2) what is the stock size and what is the new size (I want to check to see if this has already been done).

Thanks............Vern

[Flat6pac]

Use the 77 to 79 crank pulley and the 80 to 89 fan and pulley half and a 9.5x725 belt of the 77 through 79 911.
Bruce

[Walter_Middie]

The small fan pulley sizes are 3.125" or 3.25" (80 vs. 83 mm). The pulleys consist of "pulley-halves". You can't simply change one fan “pulley-half” without also changing the other "pulley-half".

1965-74/75.. 110 mm crank pulley..95 mm fan pulley
1976-77... 130 crank...83 fan
1978-79....130 crank... 80 fan
1980-89.....120 crank....83 fan

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

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

BTW....aftermarket will sell fan belts at 9.5/10 x 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.

Early 911 245 mm fan with 1.3: 1 pulley ratio 1390 ltr/sec @ 6100 rpm

1976-77 five bladed fan with 1.8:1 pulley ratio 1265 ltr/sec @ 6000 rpm

1978-79 226 mm fan with 1.81:1 pulley ratio 1380 ltr/sec @ 6000 rpm


Fan Tip Speeds:
diameter (mm) rpm pulley ratio speed (ft/sec)

early 911 245 6100 1.3 334

76-77 5 blade 245 6100 1.8 462

SC 226 6100 1.8 426

930 245 6100 1.68 431

[Carrerax]

Does anyone know what length belt for an early crank pully (110mm) and a 78sc (80mm) fan pulley? Thanks, Mark

[Porchcar guy]

Perhaps the Porsche specialist at our host would have a quick answer via the 800 number :=)

[wwest]

Originally Posted by Tidybuoy

I've read somewhere on this site that you can aid cooling by changing the size of the pully.

My question are:

1) which pully gets changed - top or bottom

2) what is the stock size and what is the new size (I want to check to see if this has already been done).

Thanks............Vern

Engine..? Person (A/C)?

[Tidybuoy]

Originally Posted by wwest

Engine..? Person (A/C)?

Engine = 2.7 CIS

Person = ?

A/C = No

[mskala]

Originally Posted by Carrerax

Does anyone know what length belt for an early crank pully (110mm) and a 78sc (80mm) fan pulley? Thanks, Mark

I have the early pulley mated to an 80-89 fan (83mm) and am using 10x695 belt.

[Carrerax]

Thanks Mark!

[manbridge 74]

PO installed 130mm crank pulley on my 74. OE was 110mm.

Fan pulley is still 95mm.

Belt size: 10 X 740.

[Carrerax]

Originally Posted by manbridge 74

PO installed 130mm crank pulley on my 74. OE was 110mm.

Fan pulley is still 95mm.

Belt size: 10 X 740.

The 74 still had the larger fan pulley. I think I need the 695 or 700.

[wwest]

Originally Posted by Tidybuoy

I've read somewhere on this site that you can aid cooling by changing the size of the pully.

My question are:

1) which pully gets changed - top or bottom

2) what is the stock size and what is the new size (I want to check to see if this has already been done).

Thanks............Vern

How can you be certain that turning the engine fan/blower faster than the factory design would not result in a "compressor stall" at the higher ranges of engine RPM rather than additional cooling...?

[304065]

Originally Posted by wwest

How can you be certain that turning the engine fan/blower faster than the factory design would not result in a "compressor stall" at the higher ranges of engine RPM rather than additional cooling...?

Math

A few more ideas for the Fan debate.

[wwest]

Originally Posted by euro911sc

Ideally you would want to regulate fan speed by CHT. CHT goes up fan speed goes up and the opposite. Connecting it to the crank like it is approximates this behavior.

But, the blades are not dynamic and therefore are designed for a specific air velocity which will translate into an RPM band for optimal flow. Is this band narrow or big? *shrug*

So... does it matter if we're running outside the optimal band for the fan, but are still flowing enough air to keep the CHTs down? I do not think so. If we are not flowing enough air then yes... Will we know this if we are measuring individual CHTs? or should it be CTs? Or combustion chamber temps?

John's work is good and would be really really useful IF we knew what speed the fan was designed for... Then you could gear it for your average RPMs... higher for street and lower for track... or maybe its reversed.. hehehe

How about an electric fan slaved to CHT?

I have no idea where it would go though... Maybe a side mounted alternator and an electric flat fan setup

Best regards,

Michael

WOW...!

Yet another reason for using 2 12 volt radiator fans to help cool the rear lid condensor....and..

The engine.

[Tippy]

I have MegaSquirt and can read CHT's at anytime. On 90-100 degree days, once fully warm, the CHT usually stays around the 300 degree F mark give or take 5 degrees.

Fan speed seems to not matter much.

[wwest]

Originally Posted by Tippy

I have MegaSquirt and can read CHT's at anytime. On 90-100 degree days, once fully warm, the CHT usually stays around the 300 degree F mark give or take 5 degrees.

Fan speed seems to not matter much.

In heavy stop and go or slowing moving traffic...?

With A/C ?

[Tippy]

Moderate traffic.

I should add I'm 7.5:1 CR so my data may not be accurate for most.

[JohnJL]

Originally Posted by wwest

How can you be certain that turning the engine fan/blower faster than the factory design would not result in a "compressor stall" at the higher ranges of engine RPM rather than additional cooling...?

Originally Posted by 304065

Math

A few more ideas for the Fan debate.

And real world experience.

I spin my fan and alternator at 2x normal speed and have stable voltage, good CHT, no knocking and good oil temps. I have ~20 track days with that setup with no issues.