The race pipe, or test pipe, will throw a code.
The simplest way to get around this is with a drilled antifouler. Antifoulers are available from most auto parts stores for like $5. It's a small adaptor with the same threads and diameter as an O2 sensor. The trick is to drill one with a 1/2 inch drill just deep enough to allow the sensor to sit, but without opening the small diameter. You have to use only one though. They come in packs of two and some people have drilled one completely though and stacked it on top of the other one. This way sometimes doesn't allow the sesor to get hot enough and will still send a code.
I've used one for over 10,000 miles and it has never tripped a code.
They are also available through RRE for $20 (scroll down to Rear O2 sensor simulator);
http://www.roadraceengineering.com/evo.htm
