A few more ideas for the Fan debate.

[304065]

I think it is reasonable to say that the question of whether to change fan ratios is one that remains unresolved in the Pelican community. Should a street engine be overdriven? Should a race engine be underdriven? Or vice versa?

In order to better understand the issues involved, I did a few calculations in Excel. For many years here, there's been concern about whether the tips of the fan blades go past the speed of sound. I think the reason people are concerned about this is because in the aviation world, fast-turning 88" seaplane props experience the tips going sonic, resulting in a tremendous noise and potentially harmful vibration.

But it turns out that even the most overdriven, largest diameter fans don't go anywhere near the sonic limit. As the below graph shows, the popular combinations of ratio and fan diameter are well below sonic, and even combinations that never originally came from the factory, e.g. a 250mm fan with a 1.8 drive ratio, are still comfortably below the limit.



(Interestingly enough, an 88" seaplane prop at 3000 RPM runs at 102% Mach. No WONDER they are so noisy)

[911 tweaks]

great topic John. I am sure that Yoda (Henry) can enlighten us all with what is available, has been done, and possibly what not to do.

I think a hypothetical premis needs to be stated here, John, so the info generated is relevant. i.e. if the cylinder heads are reading xyz degrees F, a fan speed and/or set up as such is required to not allow the heads to reach what ever the max safe op temp is.

Bob

[304065]

I'm hopeful that Henry will help us with some empirical data about head temps. Another Pelican asked what the CHT limit should be, but I believe RoninLB is currently on the road and may not have his log of CHT data handy.

There are a few different dimensions to the cooling issue. First, our engines are air and oil cooled, with oil cooling the pistons (for those cases which have squirters installed) and providing cooling along with lubrication to the valvetrain and other moving parts. Air plays a role in cooling the cylinders and heads. And importantly, air plays an important role for cars with an engine-mounted cooler, insofar as airflow through the cooler is critical to maintaining low oil temperature.

The fact that the 911's cylinder head is a much better design than the 356/912 or VW head does not obviate the need for air cooling, which is why Henry said, and I hope I am quoting him correctly, and I agree, that it's critical to monitor head temperatures if you reduce the drive ratio to the fan.

It's like the three little bears: you want it JUST RIGHT, just enough airflow to maintain CHTs in an acceptable range, while maintaining oil pressure between 180 and 210F or thereabouts. More may help from a longevity standpoint but it costs horsepower to drive the fan.

[Jeff Alton]

John, is there concern about cavitaion if the fan turns to quickly? If so, I wonder how to measure that effectively?

I am not surpirsed the tips do not reach mach 1 as the fan would be very noisey I would think.

Gotta love the sound of a C185 on floats climbing off the water with a load on.........

Cheers

[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

[Henry Schmidt]

I quick note for those who want accurate information.
Most of the Porsche factory built 911 based engines (906, 907, 911R 956, RSR) ran a 225 fan and housing with a 1.3:1 ratio. the reason being that a fan like a propellor 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

SC.......226.......6100.......1.8........426...... .1380

930.......245........6100........1.68.......431... ....1500

964/993........279.4.......6100.......1.6.......468... ...1010

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

[304065]

Here's a scan of the factory spec book. The engines listed are the 911T, 911 Normal, and 911S, only up to December, 1967 (1968 Model year).

[Henry Schmidt]

Although the spec book states 1.4:1 ratio on the fan, the actual measurement can vary based on the stretch on the belt. The higher the belt rides in the v grove the lower the ratio. The early belts were most often 9.5 x 710 and the new belts are generally 10x710 changing the ratio yet again.
During our research we have found that the early pulleys measure 117 mm crank and 90 mm fan on average.

[304065]

On the left is a used pulley, part number 930 102 128 01. It measures 134mm diameter.

On the right, the darker color, is a brand new, $200, pulley of the same part number. It measures 132mm.

I am really looking forward to performing back-to-back tests of these pulleys against the stock pulleys on my '66 911 and '71 911E. I plan to take CHT and oil temperature measurements with a data logger, which will also record vehicle performance information, e.g. flexibility numbers, 20-50, 60-100 numbers.

[Jeff Alton]

John,

It will be nice to see some actual data from a back to back comparisons....

Cheers

[Henry Schmidt]

Originally Posted by 304065

On we go. One invisible mod is the 130mm pulley instead of the small '66 one, this turns the fan faster for better cooling (yes, I know we debate this). The question: will it fit inside the '66 engine mount? Answer?

130mm pulley: no way to change the belt without a partial drop.

The reason to abandon the early motor mount (cradle) is the fact the mount offers less structural rigidity than later versions. All of the later mounts offer a reinforcing plate that mounts directly to the case.

As for the fan cooling better with a larger pulley, the only way there is a debate is if one side totally ignores the aeroelastic characteristics of a ducted-fan configuration. The rpm of the engine/ crank dictates the pulley diameters. Low rpm engine can use a large pulley. 6100rpm defined as low rpm. John's engine will most likely spend a great deal of time well above that number.
All factory Porsche race engines that used a vertical fan addressed this issue. They used small diameter fan to reduce tip speed and smaller pulleys.
I have posted speed charts and ducted fan theory and the opposition has used anecdotal evidence. There's no debate, science and math vs. "we did it and it worked !" It's the big lie. No matter how many times you tell a lie, it's still untrue.

Even the later street engine addressed this issue with a two speed pulley. This pulley allows the fan to spin slower than the alternator. Why is that? Porsche factory doesn't understand dynamic harmonics as they apply to ducted fans? No, the factory gets it. It's the goofy "bigger is better" folks that don't understand.

Rant over, you can put your head back in the sand. Thanks for coming up long enough to listen.

[304065]

Henry,

As far as I know nobody has ever posted back-to-back dyno readings from the same engine on the same dyno on the same day using a faster-turning fan vs. a slower one.

That is why I have two pulleys, one 130mm and the smaller version. When this engine is run on the engine dyno, the debate will hopefully be resolved. Empirical testing is the essence of the scientific method.

The reason for the double pulley arrangement for later cars was to allow the alternator to be run at a higher RPM than the fan for more electrical output. The fan could not be run at the higher speed for noise purposes. The cup pulleys available for the 964 and 993 run the entire assembly at the lower fan speed at the expense of reduced alternator output, which makes sense considering that race cars generally operate at higher RPM and have fewer current consumers (HVAC, etc.)

LOL comparing me to an Ostrich, this is why it's great having you on the board, have a great day!

[Henry Schmidt]

Oh John
This board is read by hundreds if not thousands of people.
Please don't take my "head in the sand " comment personally. It was my intention to shine the light of day on everyone who buys into performance legend without reasoning through them.

As for back to back dyno testing. All that testing was done years ago (by the factory and others) when air cooled Porsche engines were still a viable racing option. You don't really believe that the Porsche factory didn't (double negative, sorry) dyno test fan output at varying speeds do you? If you believe they did extensive testing on their racing engines you have to believe the small fan turning at slow speeds is the best option. I say this because [I repeat] all vertical fan 911 based factory race engines ran this way.

It's the same thing with boat tailing. This nonsense has permeated the Porsche performance world for years. Where's the back to back testing? We did it back in the 80s and boating makes no measurable difference. Using the fact that the factory did back to back testing and never did the boat tail modification perhaps we should lay this to rest also. But no, the "head in the sand" people prevail and boat tailing runs rampant.

I can shine the light of day but everyone must open their eyes to see.

[1982911SCTarga]

I never noticed that the spec book listed a maximum oil temperature. That number across the model range -- 130 degrees Celsius -- is 266 degrees Fahrenheit. Seriously hot.

Brian

[304065]

Yep, tell me about it.



Hence my interest in determining the optimum fan speed for a street engine that spends a lot of time idling and driving around town below 7000 RPM.

[1982911SCTarga]

Heck, John, you had six degrees of separation before you hit the max temp.

Brian

[Henry Schmidt]

The temperature refers to oil temp. The fan speed changes have little effect on oil temp.
The real effect is to the head temperature.
Remember that Porsche engines are primarily oil cooled not air cooled as "air cooled flat 6" might imply.
If you are truly after a cool running 911 engine the best modification is increased oil cooler capacity.

[TdiRacing]

Henry,

Side note question: Where do you put the CHT sensor? Which head does it go in? And position? Want to know so I can get one installed when my heads are out. Well machined for one anyway. Will use it for data loging on my EFI conversion.

Anyone have a picture on one in a head?

[cgarr]

Get a pickup under the spark plug, not sure if #4 or #3 would be the hottest, depends on how the air flows I guess. Or it could be one of the center cylinders?

[dfink]

Was wondering how much HP the fan actually draws. Would it be feasible to remove the alternator, make a mount just for the fan, install an external alternator perhaps where the A/C was then drive the fan with an electric motor at constant volume. I would think at some point the fan is producing way more air volume than is needed.