When replacing catalyser with midpipe, what should I do for the tube coming originally from intake plenum to catalyser? Just strip the tube and block the hole from plenum? What is that tube originally used for?

 

Yes. You can get a nice set of blockoff plates from the vendors who avertise on this forum.

It injects intake air into the catalyser to reduce emissions.

There is more that can/should be removed if you only have a midpipe. Such as the air pump and ACV.

 

https://www.rx7club.com/showthread.p...hlight=midpipe
https://www.rx7club.com/showthread.p...hlight=midpipe
There's also more that should be considered before adding a midpipe or removing the airpump. Your sig doesn't indicate, but if you have emissions in your state, you will NOT pass after doing either of the above. Adding a midpipe will likely cause boost creep which can cost you an engine. It also causes your exhaust to stink up the neighborhood and tends to make it much louder. Removing the airpump gets you some nice space under the hood, but no added performance. It can cause idle issues, stumble on acceleration and belt slippage. The idle and acceleration issues can be taken care of with a PFC and the belt slippage with a pulley kit or smaller belt and oversized waterpump pulley. Just decide for yourself if it's worth it.

 

Thanks for your quick answers.