fidelity101
fidelity101 Dork
3/28/14 1:16 p.m.

So I wanted to see the hive's mind/input on this.

  1. Application: streetported 4pt 13b rallycross
  2. Requirements: stainless exhaust
  3. Muffler: open to suggestion

Current setup is a racing beat header to 2.5" stainless pipe that connects to a racing beat dual catback. here is info on current exhaust: http://www.racingbeat.com/RX7-1986-1992/Exhaust-Headers/16130.html http://www.racingbeat.com/RX7-1986-1992/Cat-Back-Exhausts/16420.html

the problem(s):

  1. The mid pipe is broken because its thin stainless and was done to replace the presilencer that failed.

  2. 2 mufflers are the heavy

  3. 2 mufflers requires a y pipe and it is harder to service (by yourself) and gets in the way

The goal is 1 muffler from the header back, header outlet is 2.5" and just take that all the way to 1 muffler. BUT what diameter will I need? 2.5" or 4" right now I'm going from total 4" outlet to 2.5" or 3" will that hurt flow or am I just moving my torque peak around. I know rotaries have a direct correlation between power and noise. I'm okay with sacrificing some top end power for mid range/throttle exhaust velocity.

I'm no exhaust engineer but I think I'm heading in the right direction.

Thoughts/suggestions/recomendations?

Right now this muffler has my eye. 2.5" or 3" http://www.racingbeat.com/RX7-1986-1992/Exhaust-Universal-Parts/16031.html

GameboyRMH
GameboyRMH MegaDork
3/28/14 2:23 p.m.

Just to be clear, you have 2x 2.5" flanges coming from the exhaust headers right?

You're going to have to choose between "super restrictive" and "still bloody loud" if you can only use 1 muffler in total though.

fidelity101
fidelity101 Dork
3/28/14 3:12 p.m.

In reply to GameboyRMH:

no it is one single 2.5" flange from the header to the rest of the pipe.

GameboyRMH
GameboyRMH MegaDork
3/28/14 3:19 p.m.

Well in that case you should stick with 2.5", always avoid any step-up in exhaust diameter or it will decrease exhaust velocity. Factory exhausts often even have a step-down to keep velocity up.

fidelity101
fidelity101 Dork
3/29/14 1:42 p.m.

Yeah I called racing beat on some info and they were saying that by just using one 2.5" muffler would be more restrictive than the stock exhaust. But in this application isnt the bottleneck already at header outlet? My understanding of airflow science is that you are limited to your maximum power at your smallest orifice and that the exhaust tubing size will just shift your torque peak. By having a 3 or 4" id exhaust tube the torque curve shifts to the right on your dyno graph so it results in a loss of low end for a trade off on high end and viceversa.

DaveEstey
DaveEstey UberDork
3/29/14 2:21 p.m.

You're basically running a short primary system that mixes the exhaust pulses pretty quickly. A long primary system would allow two presilencers before collecting to exit through a single muffler. Quieter without getting restrictive - but you'll need a new true dual header.

I have a used RB true dual I could make you a handsome deal on.

Don49
Don49 HalfDork
3/29/14 9:05 p.m.

I run a RB header on my 13b streetport. At the y it goes to 3" and into an expansion chamber. 3" all the back to a Magnaflow 3" muffler. 96to 97db as measured this past weekend at Road Atlanta and putting just north of 200HP to the rear wheels.

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