Does anybody know what for is that small pipe which goes to second cat?

 

That's your split air pipe. It should go to a rubber hose coming out of the manifold that your secondary sleeves are on. If my foot wasn't broken i'd go outside and take a pic for you

 

I`d like to replace my old second cat and put there full flow “cat” to reduce exhaust fumes resistance. I am wondering if I have to weld this small pipe (see pic above) to “cat” because I do not know if it is some kind of safety - **** or maybe I can forget about it.
Also I remove my turbo to clean it and I`d like to clean my fuel injectors too. I will be grateful for all smart suggestions, diagrams, schema how to do it.

satanicmechanic I wish you fast return to health !!

 

If your main cat is that rusty, it may be useless by now. I bought the Bonez high flow cat and had a pipe made from exhaust manifold to new main cat. Big improvement as my two pre-cats were quite plugged.

 

Thanks for best wishes!

Also, if you remove the split air pipe the secondary sleeves will not open on an N/A which will result in a loss of power over 3,800rpm. Your best option is to get a replacement high flow cat with the pipe from an aftermarket supplier to RX-7's.

 

the preceding post only refers to S4's. S5's will still have their ports working.