cast your braid upon the waters?

[Phil Rose <pjrose@servtech.com>]

Jonathan Fenton said,

>Date: Sat, 18 Jul 1998 22:10:04 EDT
>From: JEFDO@aol.com
>Subject: Re: Adding a boost guage to the S4/S6
>
>In a message dated 98-07-18 10:25:18 EDT, you write:
>
><< How can I add a boost gauge to my S4?
>  >>
>I put mine under the left air vent near the steering wheel. You can also mount
>it where the ash tray is, or at the bottom of the A-pillar if you can find the
>right mounting cup.
>
>I used a small (?1 1/8") VDO 0-30psi boost guage. Ned recomemded an absolute
>pressure guage (aircraft type), but the VDO was cheap, and the fancy one was
>~$125. With Ned's chip, and the right weather, I cat get up to 19PSI peak
>boost (that's above the 14.8PSI atmospheric(1.0Bar), or around 2.25Bar.
>
>Run good quality braid covered vacuum hose from the guage  behind the
>carbon(wood) trim, under the dash, to the vacuum line going to the computer in
>the floor of the front passenger side. I used a Y connector on the vacuum line
>coming from

Jonathan's suggestion raises again (in my simple mind, at least) the
significance of the "good quality braid covered vacuum hose" in manifold
hose applications--like bypass valve, aux. boost gauge, etc. Just what
"good quality" is added by the braiding itself? It can be hard to find this
braided stuff at sensible, non-Audi prices, so, just _when_ do we really
need it?

A function of the braid that most easily comes to mind is that it helps the
tubing resist expansion, right? After all, these hoses often have critical
functions under positive _pressure_, even though they're called "vacuum"
hoses. So, should we expect--for moderate environments--that there is
(without the braid) a good chance of failure by _bursting_? Is bursting of
bare*ss, ordinary vacuum hose really likely to occur at 2-2.5 bar? Well,
sure, when extreme heat is always nearby--such as in the case of the
infamous bypass valve hose in the '91 200q--even the braided tubing bursts.
But what about fairly mild situations, like the interior of the vehicle?
Or when a bypass hose is routed well-away from exhaust-manifold heat? Is
braided hose then of any benefit?

Alternatively, the braiding's function might be a more subtle one--such as
to improve the response time of the ECU and other components to rapid boost
increases. An interesting notion, but, considering the relatively huge
volume of the rigid intake manifold, it's hard for me to rationalize that
these itty bitty tubes might expand enough to have a significant effect on
the development rate of boost pressure.

So I'm back to wondering....I think I'll take out my bicycle pump and see
what these hoses actually do when pumped up to 19 or 20 psi.

Then, I can start tackling the really important issues...like snow tahrs,
Quattro driving school, erratic A/C, selling my 45K mile '89 Probe GT,
Monica Lewinski...

Phil

Phil Rose		Rochester, NY
'89 100
'91 200q		pjrose@servtech.com