I tried to figure it out and learned a bit from a thread on thesamba, a guy trying the same on his bug.
http://www.thesamba.com/vw/forum/viewtopic...asc&start=0The best bit
QUOTE
tzepesh wrote: I would say that correct results can be obtained only by placing the lambda probe after all pipes merge into one. I had some problems with mounting the probe in the exhaust pipe of only one cylinder, because it would have correct reading in only one of the 4 strokes (exhaust stroke). The other 3 strokes would lead to showing a lean condition, because the gas flows towards the muffler and it would cause a void. If the probe is placed after the merged pipes, it will get a continuous flow of exhaust gas, thus a correct reading.
-which means my 4 bung idea won't work, I think.
The consensus there is to use an extension fitting over the exhaust as that's the only place the 4 have come into 1 that can be accessed, but it needs to be extended to prevent false reading from air being drawn in.
The only such thing I could find is one on the bottom of this site
http://bugpatch.com/VWToolsCatalog/VWTools...79/Default.aspx but it would be small as it's for a bug. I'f you've got the center exit early tailpipe maybe that would work on its own?