So I have an s4 na and it’s really not loud enough for me. It’s a stock exhaust to an apexi catback single exit. Could I reasonably straight pipe from the header to the catback and keep enough back pressure to open the 5th and 6th ports? Maybe 2.5” piping for reference.

 

I don't think so. See this thread for the explanation why.
https://www.rx7club.com/2nd-gen-gene...ssure-1137769/
Just buy this header Rotary Exaust Header for 86-92 RX-7 Non-turbo - Racing Beat
and this pre-silencer Presilencer for 86-88 RX-7 Non-turbo - Man Trans - Racing Beat
Your not loud enough problem will be solved.

 

what are the consequences of just not letting the 5/6 ports open?

 

That depends. If you just disable them you will lose top end power if they were working before. If you're going to disable them then you want to wire them to their open position. You will lose some low end below 3500rpm which is where they open. How much I'm not sure. Most act like it's pretty noticeable. May be on a stock S4 with stock gearing.

 

It is very noticeable.

In most cases, it's too easy to get them properly working for one to just give up and wire them open. I wired mine open long ago when I was young and dumb and after having them working now for several years, I don't know why I didn't fix it properly to begin with.

 

Mine were wired open for about 17 years and now they are completely gone so I have no basis for comparision. I never missed them. I partially attribute it to the 4.30 gearing and I just adjusted my driving so I don't shift till about 4500rpm.They worked at the time but I removed the air pump( probably for no better reason than to save 10lbs, I was young). I don't know that I would recommend it on a stock S4 though.

 

So this header and presilencer will completely replace the cats? No other piping needed from motor to catback?

 

That's correct, other than the tube that connects to the pre-silencer for the 5th & 6th ports.

 

that tube is a part of the presilencer, no? Should bolt right to the stock location

 

I think so but someone with a S4 would have to confirm but the tube on the pre-silencer has a flange on it. I imagine it bolts to the tube that originally went to the cat.

 

Confirmed. You are 100% correct. Just 2 bolts and you're set.

 

Is it really as loud as everyone claims? Rb header -> presilencer-> apexi n1 single? I don’t wanna buy the header and presilencer if it’s gonna be quiet.

 

I can't tell you how loud it is as I've not heard one in person but it won't be quiet. Here's my reasoning. I have the old HKS Sport cat back that's was discontinued years ago. Between it and the RB cat back it was the quieter of the two and you can still hear my car until I'm out of sight. The Apex'i N1 is louder than the RB cat back they say. Also singles are usually always louder than a dual set up. Here is a video I took once while my mufflers were off. RB header and pre-silencer only. It doesn't convey how loud it is in person. I would imagine the Apex'i setup would sound similar but at a lower volume, yet still be loud.