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Re: [T3] fuel damper??

On 7 Feb 2005 at 23:21, C.Rochambeau wrote:

> Think of the pulses/vibration in acoustic terms. The case walls need not flex to have a
> marked impact on the actual "sound" being carried along with the flow of fuel. I presume
> we're discussing one of these?
>  in v
>   _| |________
>  |________   _|
>           | |
>            v out

Yep, that's the one. Nice drawing.

>  The chamber could set up enough acoustic reflections to phase out a good deal of the
> carried signal, sort of like a super simplified muffler without concern over
> scavenging/back-pressure. I would guess that this would "look like" a really big choke to
> some resonant frequency hitching a ride on the liquid.

I was going to say that this would only make sense in air, which is 
compressible, but then I remembered that sound, which works on compression, 
also travels nicely in liquids and solids.

Since the pump runs at a constant speed, it should be possible to build a 
resonant cavity that would null out the dominant frequency of the pump, but I 
would expect such a device to be rather physically large, since the speed of 
sound in a liquid is much higher than in air.

I really don't know. It's possible that this chamber is made so that it always  
holds a bubble of air, and that would give it extra resilience, or maybe they 
injected some foam in there after brazing and that foam has all gone away in 
the last 30 years.

I'm sure we have some people out there who have simply left these out. Does 
this make the pumps noisier?

It may also be interesting to note that most of the D-Jetronic cars that 
followed us did NOT have such a device.

> That said, I like to be able to hear my pump loud and clear.

Well, I like to be able to hear mine when I listen for it, and yet be able to 
ignore it most of the time.  ;-)

Jim Adney
Madison, WI 53711-3054

List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org

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