Yes, I used "sensor-safe" RTV silicone.  I should have mentioned it.  

To prevent confusion after the fact, the 2-1 manifold I refer to is the
cast iron elbow that joins the front and rear header pipes to the large
single pipe on the left side.  On other forums manifolds are commonly
referred to by their numbers of ports, like 4-2-1 or 4-1, I was simply
using the same shorthand here.  Sorry it was confusing.

Gregory Smith
'88 Vanagon GL

Smith Design Studio
Small Building Company
Olympia, WA
360-239-0429
gregoryfsmith.com

-------- Original Message --------
Subject: Re: '88 Exhaust help
From: Scott Daniel - Turbovans <scottdaniel@turbovans.com>
Date: Sun, September 02, 2012 9:06 pm
To: Gregory Smith <gfs@GREGORYFSMITH.COM>
Cc: vanagon@GERRY.VANAGON.COM

 You might want to check if your RTV silicone says 'sensor safe' on it.
 generally ..it's a no-no to use a sealant like that where there is an
oxygen sensor - sensor contamination can result. 
 Though sealants these days often say sensor safe on the tube.
 
 there's the 1-3 forward exhaust pipe, and the 2-4 rear one. But no 1-2
I can think of.
 
 you can run a straight pipe ...or even a straight pipe inside a hollow
cat.
 A piece of EMT conduit fits perfectly....just guessing.
 It might be that hardly anyone makes one as it would be illegal in many
places to do so. 
 There has been such an official part like that of course, once upon a
time.  Perhaps still. 
 
 it will help slightly with power.
 a good cat is best. A hollow cat is not that good, exhaust flow wise. 
 a straight pipe is very slightly better.