Duct Noise Attenuation?

[Paulporsche]

My car is missing the under dash fittings that direct air from the duct and fan. The noise of the air is a loud annoying plasticy sound. There is no adverse fan motor sound, it's just the sound of the rushing air, which changes with the fan speed, of course.

If I buy the factory parts (911571347/4801) or the Kool'N parts I realize I can get better airflow direction, but will it cut down on the noise?

If not, has anyone come up with any other way to make it quieter?

Thanks.

[Paulporsche]

OK. Well I guess not. Looks like it's just something that is part of this car's DNA!

[patkeefe]

Paul, I think the ductwork should make the annoying sound go away. However, it will be replaced by a different annoying sound, of air wooshing through two smaller ducts to the vents.
I am just putting my A/C back together, and I will face the same issue you have now. I will advise as to any resolution.

[piscator]

Paul,

I agree with Pat. I suspect that any hose you use for ducting will reduce the noise by containing and directing the air flow. The URO brand hoses I've purchased have been very good quality.

If the noise still troubles you, try stuffing fiberglass or foam insulation up and around hoses, etc. -- just to see if that helps. If it does, you might craft a way to permanently insulate those areas. Or go with the ultimate upgrade -- orange shop-vac hose! :-)

[patkeefe]

I am flattered

[RSBob]

My ducting is quite quiet even with the factory air removed. If you are going to replace the missing components, you want to place a WTB n the classifieds. If not, you will need to custom cut plastic pieces and use epoxy to block-off all openings.

[Paulporsche]

I'm about to replace the vent pieces at the ends of the footwell ducts. I've noticed that high velocity HVAC systems for buildings have foam wrapped around the inner surface of the nozzle portion to attenuate the noise. I'm going to try something like that before I reinstall the vents.

The hoses are, I assume, the original factory ones, so I may go with replacements from another source. Maybe URO, maybe Shopvac. Who knows? Time for some trial and error.

[Jgordon]

I've thought a bit about this as well. The fan has three speeds:
1. Oh, it's on?
2. Meh.
3. Hurricane.

One upgrade would be the griffiths variable speed fan, but that's a band-aid, rather than a solution. There are a lot of bad angles in the ducting, and not much thought given to aerodynamics of the internal pieces in the 911 AC system. I'm sure someone with some plastic molding expertise could design and fabricate some smoother pieces. One other idea is to carve and glue some foam to ease the air flow around corners. It does get annoying and loud; that's for damn sure.

[GH85Carrera]

Originally Posted by Jgordon

I've thought a bit about this as well. The fan has three speeds:
1. Oh, it's on?
2. Meh.
3. Hurricane.

One upgrade would be the griffiths variable speed fan, but that's a band-aid, rather than a solution. There are a lot of bad angles in the ducting, and not much thought given to aerodynamics of the internal pieces in the 911 AC system. I'm sure someone with some plastic molding expertise could design and fabricate some smoother pieces. One other idea is to carve and glue some foam to ease the air flow around corners. It does get annoying and loud; that's for damn sure.

I installed Griffith's variable fan speed knob, and the bigger blower motor. It is nice to just dial in the speed that is perfect for the situation. For sure at wide open with even more air than stock, it blows a lot of air, and is noisy. But if I driving down the highway, it is not that loud. It does not take long before the car in cool enough I can turn the speed down to "just right" and not have a lot of noise, and plenty of air in my 85 911.

Oh, I do also have his knee pad vents. Those tiny little dash vents are pretty useless, so the knee vents help. And I have his center vent that replaced the bowtie. They help move air away from the stock vents and let me aim the air flow.

[piscator]

The variable blower modification is great!

There's a very good 'how-to' in the Pelican Parts tech articles. I followed that and the result was wonderful. I did modify the 'how-to' slightly by using a potentiometer with an internal switch, they are called... are you ready for this... a 'switching potentiometer' !! You can find them at Amazon.

The switched potentiometer eliminates the additional switch outlined in the 'how-to'. I found one with a shaft that matches the Porsche fan speed knob. Looks almost factory!



!

[UROParts]

Originally Posted by piscator

The URO brand hoses I've purchased have been very good quality.

Thank you for the feedback Piscator! We make quite a few Hot Air Hoses for Porsche and VW in the original aluminum / paper construction style, perhaps the below list will be helpful.

901 110 295 00: 28 x 560mm (Shiny Black)

901 110 296 00: 35 x 550mm (Shiny Black)
901 108 291 00: 35 x 780mm (Black)

644 211 921 05: 50 x 270mm (Black)
911 211 274 00: 50 x 720mm (Tan)
911 211 274 00 B: 50 x 720mm (Dark Brown)
028 129 087A B: 50 x 1080mm (Dark Brown)

930 211 196 00: 60 x 130mm (Tan)
930 211 622 01: 60 x 330mm (Tan)
930 211 622 01 B: 60 x 330mm (Dark Brown)
901 211 194 01: 60 x 380mm (Tan)
901 211 194 01 B: 60 x 380mm (Dark Brown)
901 211 195 01: 60 x 458mm (Tan)
901 211 195 01 B: 60 x 458mm (Dark Brown)
911 211 522 00 B: 60 x 1110mm (Dark Brown)
911 211 522 00: 60 x 1110mm (Tan)
644 572 971 00: 60 x 1500mm (Tan)

(We linked to several of these hoses on Pelican's site, but were unable to find all of them.)