Speeding up the fan -

[Trackrash]

Good info Henry. I am familiar with the "power pulley" concept, but was hesitant to use a '71 pulley and fan setup on my new 3,0 motor.

I will switch back to my '71 crank pulley once my motor is somewhat broken in. It will be interesting to see if there is any noticeable difference in temperature.

BTW, my '71 crank pulley measures 116mm overall diameter. Not sure why the chart lists it as 110mm.

[chris_seven]

The pulley diameters given on the table are the outer diameters.

As I tried to suggest using these values does not give the correct fan speeds.

As regards bearings most alternator bearings are good for around 20 000 rpm on a continuous basis and it the angular acceleration that generally causes them to wear out but they should tolerate 2000 rad/s^2 without too much trouble.

If it became an issue then hybrid ceramic bearings could be used and this would offer a significant improvement in speed and acceleration capacity.

Blade tip speeds are generally less than 50% of Mach 1 but is quite common for fans to run at speeds of up to about 1.8 x times Mach I depending on blade stiffness. The performance of the fan unit could also be improved by reducing blade tip clearances which would also reduce noise.

This thread now seems to have switched from increasing fan speed to reducing fan speed which will of course reduce the air flow and hence cooling.

Clearly parasitic losses will reduce but so will the airflow over the cylinder heads and without some basic measurements its all a bit of a guess.

By increasing speed by 10% losses will increase by 30% with a similar reduction if the fan is slowed by 10%.

[stownsen914]

Thank you for the input on this.

I had heard about some people decreasing fan speed (and the factory ran the smaller 226mm fan in its normally aspirated race engines - same goal). Is it possible that this is desirable for normally aspirated engines, but turbos require additional cooling?

I've heard at least one report of increasing fan speed on a small displacement, normally aspirated motor resulting in lower temps. I believe that was with the 130mm and 83mm pulley combination.

What are people doing on turbo race engines?

Scott

[chris_seven]

The cooling system with the small fan was generally used without engine mounted oil coolers.

This would have reduced the pressure drop and could well have improved air flow around the engine.

It would be interesting to use a pitot tube to measure the differences in pressure for various installations.

I will look to see if we can create a simple method to mount one and we could make some comparisons.

I have a small 906 fan and it would be interesting to see the differences.

[Trackrash]

What about blocking off the heater tubes? Isn't air constantly being blown into the heat exchangers? I know guys running headers eliminate these. Any benefit to cooling?

[Trackrash]

Originally Posted by chris_seven

The pulley diameters given on the table are the outer diameters.

Maybe mine is custom?

[Trackrash]

Sorry about the cross post, but maybe some of you saw this in the 911 Tech forum.
Interesting that the highest HP 935 used the vertical fan. Of course it had water cooled heads?

[chris_seven]

The early pulleys are a bit crummy and seem to vary between 115 and 117mm. The timing marks are not too accurate and can vary by 2-3 degrees.

They are supposed to be 116 but are said to rotate by 1mm for every degree which would mean they should be more like 115mm.

I would agree that the table has some errors but notionally they only provide the OD so the numbers are misleading.

The bottom of the V is 91mm.

I wonder if one of the issues is spark plug temperatures as this engine has twin cam, 4 valve per cylinder heads as well as water cooling.

Looking at the pulley sizes it may be running quite slow.

[stownsen914]

Originally Posted by Trackrash

Sorry about the cross post, but maybe some of you saw this in the 911 Tech forum.
Interesting that the highest HP 935 used the vertical fan. Of course it had water cooled heads?

The first water-cooled 911 racecars had only water cooled heads - the cylinders were still aircooled (for example the 935/78 engine shown in the pic above). So it still needed a fan to cool the cylinders. Since the heads were water cooled, Porsche must have figured a regular vertical fan was sufficient for cooling.

I think they went full water cooling (including cylinders) at some point on the 956/962 cars.

[Henry Schmidt]

Here is some research I posted about 10 years ago.
Of course, I am not an engineer so my conclusions may not be scientific.
What I have more than engineering to back up my claims are years of actual experience watching how 911 engines perform given differing perimeters.

Originally Posted by Henry Schmidt

Fan speeds for 911 cooling fans
Most of the Porsche factory built 911 based engines (906, 907, 911R, 956, RSR) ran a 226mm fan and housing with a 1.3:1 ratio. the reason being that a fan like a propeller 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

78-79911SC .......226..................6100.............1.8.. .........................426..................1380

930 Turbo ….......245..................6100.............1.68 ..........................431... …….....1500

964/993...............279.4................6100....... .......1.6..........................46 8................1010

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

__________________

[chris_seven]

All we need to do now is add some heads and oil temperatures to complete the picture.

[Henry Schmidt]

Originally Posted by chris_seven

All we need to do now is add some heads and oil temperatures to complete the picture.

I'm comfortable with how it will turn out (slight increase in head temp but well within parameters) but I look forward to your findings.
Cheers

[Walt Fricke]

One reason the 935 shown had the upright fan might be packaging - look at all the intake/intercooler stuff which had to be fitted into the engine compartment. Maybe it was later 935s which got the flat fan? After Porsche had time to figure out how to put all those parts somewhere else?

[Walt Fricke]

Starkey's book on the turbo Porsches says that when Metzger went to the four valve water cooled heads, the fan had less cooling to do, so they reverted to the upright fan. The fan was smaller and driven at 1.34 times crank speed, and absorbed 4php at 8,200 rpm. The result was even cooler cylinders. But the alternator was separate off to one side on a bracket, with its own belt and pulley (like the AC on street cars?).

[stownsen914]

Originally Posted by Walt Fricke

Starkey's book on the turbo Porsches says that when Metzger went to the four valve water cooled heads, the fan had less cooling to do, so they reverted to the upright fan. The fan was smaller and driven at 1.34 times crank speed, and absorbed 4php at 8,200 rpm. The result was even cooler cylinders. But the alternator was separate off to one side on a bracket, with its own belt and pulley (like the AC on street cars?).

Exactly my recollection

[Arnoud]

ORRATECH Motorenbau Shop für Typ1 / Typ4 Motor - Neuheiten---liegendes Gebläse----6Zylinder Porschemotoren 184.006.911

[stownsen914]

Originally Posted by Arnoud

Didn't realize it had been done for the 911 motor too ...

[manbridge 74]

Looks Corvair-ish. I would think at high RPM it might need a belt tensioner like the Yenko Stinger had to prevent belt from flying off. But maybe the ratio is different.

[boosted79]

Re. vert. fans. I would think the flow to 1 & 4 could be improved with better guide vanes attached to the shrouds to direct more air from the fan discharge directly to those two cyls. It would be interesting to have a cyl head temp sensor under each plug to read the variation. Then again I have to assume the factory tried all of that and came to the conclusion it didn't do much and just went with the flat fan.

[Trackrash]

Is this a bad idea? Something about trapping heat in the exchangers? Sorry, if it is off topic, but I was thinking it might channel more air where it is needed most....