Originally posted by soulcontroller

Originally posted by turbo8u please, explain.

you had to be there :)

Now that's just not nice, Matt. :P I'm sure Bill wouldn't mind divulging the information for the good of all 2gNTers. :)

Anyway, Bill and Adam were tuning the RS in preparation for the shootout. It's its full form with a full exhaust. I believe they had it to some number of horsepower. They wanted to remove the exhaust and run open downpipe to get a greater flow and less exhaust resrticiton (at the same time they put in race gas). Obviously when you do this the rear o2 is just hanging out. When they put the car back on dyno it dyno'd 50hp less. Knowing that was a problem they back tracked what had changed from the previous run which included removing all the C16 they had just put in. Still the same problem until the put the exhaust back on and plugged the rear o2 back into the exhaust stream.

The hypothesis goes like this. The rear o2 needs to see exhaust gas PERIOD. If the ECU failes to see the cycling voltage it may or may not throw the cat efficiency code and o2 failure code but it will change the timing somewhat so that the car will not run optimally but instead in some form of limp mode.

Well that is what I took away from lesson (and I say that in the loosest of terms) Bill gave the 2gnters on Friday. If I've remembered anything incorrectly please correct me. I was on my 7th beer by the time we started talking about that. :)