Cooling fan questions

[David Baugh]

As I understand it, the factory punched the eight holes in the rear of the cooling fan so that it would draw cooling air thru the generator. This leaves me with a couple of questions.

1. Why did they decide to cover the holes with a removable plate?

2. Would removing the plate aid in cooling of the generator?

3. What would be the downside to removing the cover plate and replacing it with a shim?

Dave

[Ron LaDow]

David,
I think Al Z removes the aluminum pan, and claims to find some improvement. If I remember correctly, his point is that removing it draws some cooling air across the generator, not that it aids engine cooling.
You might contact him, assuming anyone 120 years old is still posting. Hey, Al when's the 121st birthday party?

[Vic Skirmants]

Ron; minor correction. The plate/pan is steel, not aluminum.
Since the fan is a VW item, I guess VW wanted the holes for cooling (or lightness?).
Porsche added the plate, I assume for better engine cooling. I'd rather cook the generator than the engine.

[David Baugh]

The reason I'm asking is that when I rebuilt my engine 3 yrs ago, I decided to leave the cover plate off, in order to try to prolong the life of my newly converted 12V generator. Everything seems to be working flawlessly. I haven't noticed any excess engine heating issues, but I changed so many components during the rebuild that I have no base line for comparison. Now, I'm fixin' to pull the engine to install an aluminum oil cooler and a large-pump 3rd piece with a Pre-Mat full flow filter adapter, among other things. I am wondering if I should re-install the cover plate at the same time. If the holes are causing turbulence that adversely affects air flow, then they should be covered.

I appreciate the responses so far, but was hoping to find someone with experience in this arena.

Al .... you out there?

Dave

[David Baugh]

In hopes of attracting information from some of the history buffs on this board, I resisted the urge to plant a suggestion which may have poisoned the well of independent research. I don't seem to be getting anywhere with that plan, so here it is.

Long ago and far away, I read somewhere that the factory covered the holes in an effort to reduce noise. Apparently, the holes caused the impeller to make a whining or whistling sound thru certain RPM ranges. I can't vouch for this explanation, but if it's true, I've heard some worse reasons for defeating a useful function, but none come to mind at this time. This one's up there with the plastic sprocket teeth on the 5 liter Ford V8 cam gears that gave up at 100K miles, leaving the operator stranded with significant towing and repair expense .... all in the interest of noise reduction.

When I decided to leave the cover off, I reasoned that every other generator application I can think of (automotive or otherwise) includes its own cooling fan. There must be something to it. So, unless I hear convincing evidence to the contrary, I'm going to leave the cover off, and reap the rewards of a cooler running generator. Besides, with my 75% hearing loss, a little more noise isn't going to bother me.

Dave

[Dan Epperly]

David Baugh wrote:
> In hopes of attracting information from some of the history buffs on this
> board, I resisted the urge to plant a suggestion which may have poisoned
> the well of independent research. I don't seem to be getting anywhere with
> that plan, so here it is.
>
> Long ago and far away, I read somewhere that the factory covered the holes
> in an effort to reduce noise. Apparently, the holes caused the impeller to
> make a whining or whistling sound thru certain RPM ranges. I can't vouch
> for this explanation, but if it's true, I've heard some worse reasons for
> defeating a useful function, but none come to mind at this time. This
> one's up there with the plastic sprocket teeth on the 5 liter Ford V8 cam
> gears that gave up at 100K miles, leaving the operator stranded with
> significant towing and repair expense .... all in the interest of noise
> reduction.
>
> When I decided to leave the cover off, I reasoned that every other
> generator application I can think of (automotive or otherwise) includes its
> own cooling fan. There must be something to it. So, unless I hear
> convincing evidence to the contrary, I'm going to leave the cover off, and
> reap the rewards of a cooler running generator. Besides, with my 75%
> hearing loss, a little more noise isn't going to bother me.
>
> Dave

If the cooling holes were not needed I wonder why Porsche kept using the 40hp VW style fan beyond the point VW used it? In other words, VW stopped using that style of fan with the 1300/1500/1600 engines starting in '66 but my '67 912 still has the 40hp style fan.

[David Baugh]

Another good question, Dan. Is it possible that the Porsche factory wanted to leave it up to the individual owner to decide whether or not he or she wanted better generator cooling at the expense of higher noise levels? Perhaps a racing modification, where sound doesn't matter?

Again, I'm thinking that someone out there has the answers.

Dave

[Sterling Vaden]

Here's another data point. The '69 912 fan, which uses the larger diameter VW generator, has a cooling slot built into the fan housing generator mounting plate.

[Dan Epperly]

Sterling Vaden wrote:
> Here's another data point. The '69 912 fan, which uses the larger diameter
> VW generator, has a cooling slot built into the fan housing generator
> mounting plate.

The same is true with post 40-horse VWs. I bet Porsche used the same deflector plate. Since Porsche continued to use the early style stale air 40hp VW shroud through '69 they probably stuck with the 40hp fan as long as they were using the small bodied generators similar in size to the six volt VW units. When they went to the big VW style 12 volt generator they probably adapted the same deflector that VW used starting with the 1300 engines, which in Europe were 12 volt but in the US were still six until 1967. At that point they went with a fan that was the same size as the 40hp fan but had no holes in it. In 1971 they went with the wider shroud and the "doghouse" and used a wider fan than the earlier versions.

[David Baugh]

An update on this discussion .... and an unsolicited opinion.

I've put over 5,000 miles on my car with the fan hole cover deleted. As near as I can tell, there has been no effect whatsoever on engine cooling. Nor did I expect any. Since these holes are located in the low-pressure section of the fan, they should not affect air output volume or pressure.

When I replaced my generator with one of Al's alternator conversion kits a couple of months ago, I noticed that the outside-rear surfaces of the wires inside the generator were coated with dust. I take this as mute testimony to the fact that cooling air is being drawn thru the generator.

I did notice a slight increase in fan noise .... that is, until the "Polar Vortex" encouraged me to hook up my heater, which had been disabled for over 25 years. As a sidenote, my car underwent a complete body restoration during that period of time. I'm running "naked" j-pipes (w/o heater boxes), and had removed the heater valve/mufflers from the rear of the longitudes. So, I made adapters which I attached to the front of the flapper boxes, and ran 2" VW aluminized heater hoses directly to the tubes that exit the rear of the longs.

The first couple of times that I revved it in 2nd gear with the heater knob cranked open, it was "Holy body dust and rust scale storm"! I couldn't see where I was going. That stuff was 1/4" deep on the dash pad, and I looked like a furnace had exploded in front of me. Once I'd recovered from that, I noticed a whole new array of sounds inside the car. Gone was the transaxle melody that I'd become accustomed to. It had been replaced by a symphony of engine sounds, piped into the cockpit in stereo. I was pleased that, other than a bit of valve clatter, the engine was very quiet. The fan, however, was another story.

At 3200 - 3400 RPM, it emits a piercing whistle. Above that, it quiets back down, until I exceed 4000 RPM or so. From there on up, it shrieks like a jet engine compressor. Now I understand why the gearheads in my neighborhood have asked if this thing is turbocharged. Fortunately, I'm a half-deaf noise-junkie. I'm adicted to the whine of straight-cut cam gears and the whistle of the old Chevy Rochester fuel injection with the silencer removed.

My conclusion .... and unsolicited opinion .... is that what I'd read all those years ago was correct. The factory engineers did punch the holes to aid in generator cooling. Other than this rig, I can't recall ever seeing an electrical generator without a cooling fan on its armature. It must be a good idea. But, as often occurs, the marketing division overruled the wisdom of the engineers, and the benefit was lost.

My dog isn't too crazy about it, but I can live with the extra noise. Cruising along at 3300 RPM, it commands a lot of attention.

Dave

[Craig Richter]

Porsche didn't just make parts for the heck of it. Generator plates and those silencer cans between the chassis and the heater boxes serve a purpose. I'd rather have a fast, quiet car, than scare my dog. The Bursch exhaust is as loud as I can take these days...

[David Baugh]

Porsche didn't just make parts for the heck of it. Generator plates and those silencer cans between the chassis and the heater boxes serve a purpose. I'd rather have a fast, quiet car, than scare my dog. The Bursch exhaust is as loud as I can take these days...

I don't suppose they punched those holes for no reason, either. And I never did like the idea of blowing air across the exhaust pipes into the cabin. A pinhole leak can kill you and anyone else around you, including the dog. That's why I disconnected it in the first place. Don't need the heater much here, anyway.

To each, his own. But, this is my "fun" car .... and, I like having "fun" with it.

Dave

[Matt Kreeve]

An update on this discussion .... and an unsolicited opinion.
...
At 3200 - 3400 RPM, it emits a piercing whistle. Above that, it quiets back down, until I exceed 4000 RPM or so. From there on up, it shrieks like a jet engine compressor. Now I understand why the gearheads in my neighborhood have asked if this thing is turbocharged. Fortunately, I'm a half-deaf noise-junkie. I'm adicted to the whine of straight-cut cam gears and the whistle of the old Chevy Rochester fuel injection with the silencer removed.

Dave

Dave I am running the 28 blade fan as I live in a hot climate (on my 56A Coupe) ... Is your fan the 16 blade? I am also getting the high pitched whistle at 3400 .. haven't gone much north of there as car is just back on the road and being run in .. but is there a mod to remove this noise or is it a correct characteristic of the fan housing? Or could it mean that the blades are somehow unaligned or weak at the ends and reverberate at high revs?

The only other mention of a whistle around 3.4k is this article talking about catastrophic failure of the fan! Is that whistle maybe a signal?
https://porsche356registry.org/article/35

"You are enjoying a drive on a lovely fall day. Cruising along at 3400 RPM’s, suddenly there is a loud bang! You quickly shut off the engine and coast off the highway. Getting out of the car for a closer examination, you see a big puddle of oil, a smoking engine, and realize that shutting off the engine was a wise decision."

[Leo Dreisilker]

Gents, don't forget that carbon dust from the brushes wearing will enter the air stream. It won't be a lot but there will be some floating in the air. It is not just carbon but has metal(copper) in it too.

[Al Zim]

Porsche did not design a cooling fan, the used the VW ones. Why they put a cover over the holes in the fan rather than designing a solid piece of metal will remain a mystery. Perhaps it was my VW training or thinking that cooling the generator was more important that the noise the fan made, The result was that I probably never installed the cover. I have a stack of them in the warehouse. In our alternator kits our generator/starter rebuilder has told us not to install the shield over the holes in the fan. Concerns about the generator brushes throwing carbon and/metal into the oil cooler are unfounded. The brushes wear forever and the dust is very fine which will pass through the fins of the oil cooler. The metal from the armature or copper in the generator will be short lived since the generator will stop working and have to be rebuilt. Regarding the whistling noise an air leak past the cardboard ring is probably the cause. al zim 2019/02/17