The Flapper opens directly over the hole, there is no added back pressure. Independent tests have been done that showed a significant decrease in back pressure with the EPM O2 Dump. You would have to expect some suety buildup from the exhaust gas staining the flange and gasket. And not to be overly dramatic but if you have a turbo you will have boost creep, it is unavoidable to some extent, but again in tests which can be seen in one of the largest Threads on EvoM that the EPM O2 Dump will also lessen those effects too. More Horse Power, Increase in Boost, Quickening of Spoolup, etc. are just a few benefits of the EPM O2 Dump.

If one can think of more boost because you are running the EPM O2 Dump then a consequence would have to include a new tune of anyone's Boost controller to lessen the effects of running more boost, hence boost creep. By putting the oem, or ebay O2 Housing back on you will then be running less boost, less creep. The EPM O2 Dump will give you more boost much quicker in the rev zone, which you will have to tune for it. An added benefit ofcourse. But also you will have to adjust your boost controller or risk a little more boost creep.

I hope this helps answer this issue..any more questions feel free to let me know and I will try my best to explain any issue or concern.

thnxx