Thinking about keeping stock cat-back
 

It's been suggested to me that it may be possible to keep the stock cat back.

Quick background what I am chasing. AVO turbo kit, running 10psi or so, 250-280whp is all I am looking for. Trying to set the car up to be as stealth as possible.

I will replace the stock over pipe and front pipe. But wonder if I really need to replace the 'cat back' (mid and rear sections)

Sony's review of the Perrin exhaust is great http://www.ft86club.com/forums/showthread.php?t=16614 and I think if I have to go full exhaust, I will go Perrin. But, even Perrin's own videos show that just replacing the catback from stock to their unit increases volume. Add a over pipe and front pipe into that, and it will be louder.

As far as being 'stealth' the perrin cat back comes in a black finish, and the tips aren't OTT. So again, if I had to do the catback I would go Perrin. But, more 'stealth' that this, would be keeping the stock cat back.

In short, I'm replacing the over pipe and front pipe, but question whether I need to replace the cat-back for a FI 250whp setup.

If I have to do it, I will. But, I'd rather not. I'm not looking for any more volume at all. I don't think the stock exhaust looks bad, and would prefer to keep it.

At 10PSI which is alot compared to the stock 5-6PSI your going to want to larger diameter pipes for a more free flowing less restrictive exhaust. Not to say it won't work with the stock exhaust, it can't hurt to do testing. Not sure about the stock catback's longevity in a FI setting. Perrin does have a resonated version for there catback.

I say go for it. Just about every catback on the market makes little to no HP/TQ. I'd be interested to see what kind of gains you could see with the factory catback.

Agree. Most are purely for sound which can be just as important as a small bump in HP and TQ

Don't forget that adding a turbo will make the exhaust significantly quieter in itself.

I think you should be ok. I have personally seen cars (not BRZ's) with 300hp or more using 2.5 inch piping.

Our cars come with larger than 2 inch piping. So its not a stretch.

Ideal? probably not. I would say 2.5 inch or so for your power goals would be a bit more appropriate.

This comes straight from Perrin's blog:

"The stock exhaust system consists of a header, overpipe, front pipe, midpipe and muffler section. For now we will skip over the header as this isn’t something we plan on making right away. Starting from behind the header, every piece of the stock exhaust is made from tubing that measures 2.118″. That includes the overpipe, Midpipe and muffler section before the muffler. This great room for us as there is room to improve."

I'd replace it if I were you.

If you decide to keep it, please replace the tips. I could not stand those stock ones, so upsetting compared to the JDM, European and Australian tips

Full blown has made good numbers on a stock exhaust all the way back besides there turbo manifold. I say stick with it if that's what you want I do believe upgrading the exhaust will def. benefit as I've heard from guys already that with a 2.75 or 3" exhaust they have seen increased turbo spool and feels a little peppier. Up to you though man.

General rule of thumb is 2.5" up to 300whp, 3" for anything over. That's not to say you can make more than that suggests, just that you'll start to see gains at those levels. With any other turbo is say your goal is easy to achieve at that boost with stock exhaust (full blown has already done it). With the avo turbo being so small you may have trouble, but honestly I doubt it will be an issue.

Ill have my turbo car with full stock exhaust on the dyno tomorrow, so I can add more info after, but its a gtx2867r, so it won't be apples to apples.

In the end just build it the way you like it and don't worry about the numbers. You don't drive a dyno sheet. If your goal is 250 and you only make 235, would you even know without the dyno telling you? It's certainly not worth ruining your stealth style over.

^^^This, and you want as least amount of restriction as possible when you go turbo.

I personally think that a turbocharged setup needs as much "free flow" as possible, 2.5" for under 300whp, try going with a greddy or a blitz catback, I'm sure it'll be much better than building back pressure on stock diameter pipes. Plus you're gaining a bit of horsepower so why not? should be the question.

you can always add an exhaust later so run how you want now and if you feel the need upgrade later.