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3 inch vs 2.5inch exhaust system


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Hey Guys 

 

Just wanting to know if anyone has had experience switching between 3inch and 2.5inch exhaust systems when tuning a vehicle.

 

Basically I am running a 3inch RCM downpipe and 3inch HKS catback with a VF22 and made 226kw which I am happy with the power etc. 

 

However I am considering changing my catback to something different to give the car more of a stock look as the HKS system is quite big but it does sound very nice. I was thinking of possibly pulling my 2.5inch catback system out of the shed that is running a remus rear muffler and making this fit my front pipe. Will changing to the 2.5inch system from the catback cause any restriction and possibly limit my power?

 

I have made a couple of other changes to the car sobit will be going on the dyno again but i am still running the VF22 so ideally would be nice to still make the same sort of power.

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24 minutes ago, Loren said:

My system is 3 inch downpipe into whatever the stock STi size is... 63mm? It works @ 300kw.

Sweet. Thats what I wanted to hear. I'm not going to be pushing any more than that 230kw mark.

 

Out of interest does your downpipe itself reduce to 63mm? Or do you have the mid pipe just bolting directly to the downpipe. 

You running stock STI rear muffler too?

 

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3 minutes ago, ginganinja said:

Sweet. Thats what I wanted to hear. I'm not going to be pushing any more than that 230kw mark.

 

Out of interest does your downpipe itself reduce to 63mm? Or do you have the mid pipe just bolting directly to the downpipe. 

You running stock STI rear muffler too?

 

 

The downpipe is all 3 inch.I don't know if the mid section is stock or not, but it's 63mm with a small 3 inch section where the first resonator once was... second resonator is still present.

The rear section is also 63mm... not the stock muffler.

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1 hour ago, ginganinja said:

Cool as. So it likely has a smooth transition from 3 inch to 2.5in in the mid pipe. Think I might look at doing the same and get something made up.

 

It must be smooth because I only just found out it wasn't 3 inch the hole way a few weeks ago when I got a different muffler put on.

I've had the car for 5 years and spent a great deal of time under it.

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  • 4 weeks later...

@Loren

 

Is your cat back a custom made one or off the shelf item? 

 

Still leaning towards something either more stock looking or Group A style. The HKS sits a little low and is a little big for my liking. Although it sounds great and flows great.

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1 hour ago, ginganinja said:

@Loren

 

Is your cat back a custom made one or off the shelf item? 

 

Still leaning towards something either more stock looking or Group A style. The HKS sits a little low and is a little big for my liking. Although it sounds great and flows great.

 

Looks like stock mid section and off the shelf rear section and muffler... both have been hacked since I've had it. It sits nice and high like a stock exhaust.

Edited by Loren
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4 minutes ago, ginganinja said:

Yer ok. I am trying to look at what 3 Inch options there are that sit nice and straight like a stock but offer better flow. Would like to stay 3 inch rather than 2.5 inch.

Seems like there isn't much option in NZ unless you got custom. Where as in the UK there are a few options but shipping kills it.

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I probably wouldn't go from a 3 inch downpipe into a 2.5 inch Catback. There is conflicting evidence as with anything but if you research I think there is something about increased backpressure compared to just running 2.5 inch straight through.

 

Also something about the gases expanding and contracting etc with some people doing dyno runs with this setup and getting worse results compared to either 2.5 inch through or 3 inch through.

 

I just would leave it with 3 inch personally or redo the whole system 2.5.

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For turbo cars, big is better, period. No such thing as back pressure for turbo cars. The exhaust with least restriction will yield the most power, spool etc. Obviously other factors come into play like you've mentioned going back to stock look. If that is of more importance, a 2.5 cat back system won't make tooo much of a difference, but you will definitely lose some power. 

 

The downpipe is the critical part. Don't change that if you want to keep most of your tuned gains. 

Edited by Niran
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On 14/08/2021 at 7:50 PM, ginganinja said:

Seems like there isn't much option in NZ unless you got custom. Where as in the UK there are a few options but shipping kills it.

I picked up a new 3" inch rage system recently in NZ via my tuner so definitely still possible to find something local? It was in the same realms of an HKS exhaust in terms of dollars as well but i also just prefer the mild steel set up vs stainless etc.

 

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  • Admin

I dunno. If I were in your shoes, unless its hitting the ground or sounds bad etc I probably wouldn't change? 

I do not rate the rage exhaust systems tbh. For what you pay and what you get and need to do it's a bit 'meh'. 

At this point it's a case of 'Do as you want' but there very likely will be some power loss. 

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1 hour ago, Dairusire said:

I dunno. If I were in your shoes, unless its hitting the ground or sounds bad etc I probably wouldn't change? 

I do not rate the rage exhaust systems tbh. For what you pay and what you get and need to do it's a bit 'meh'. 

At this point it's a case of 'Do as you want' but there very likely will be some power loss. 

 

What don't you like about them? 

 

I had one of their 3 inch downpipes once... what annoyed the hell out of me was that it wasn't designed to fit with the existing oem mid pipe... they made it about an inch too short and put the flange on a different angle.

Infuriating.

 

It was also was made from mild steel which was covered in surface rust in a short period of time.

Edited by Loren
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1 hour ago, Loren said:

 

What don't you like about them? 

 

I had one of their 3 inch downpipes once... what annoyed the hell out of me was that it wasn't designed to fit with the existing oem mid pipe... they made it about an inch too short and put the flange on a different angle.

Infuriating.

 

It was also was made from mild steel which was covered in surface rust in a short period of time.

 

What you stated but also their divorced downpipe works about as well as two people who actually had a REALLY bad divorce. 
It's a poorly cut basically gas axed hole in the pipe... 

The rust is a big one but also the fact the exhaust doesn't actually come finished, and the welds overall are trash in comparison to the near HKS level of pricing you pay for it. 

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3 hours ago, Dairusire said:

I dunno. If I were in your shoes, unless its hitting the ground or sounds bad etc I probably wouldn't change? 

I do not rate the rage exhaust systems tbh. For what you pay and what you get and need to do it's a bit 'meh'. 

At this point it's a case of 'Do as you want' but there very likely will be some power loss. 

 

I have noticed one scrape on the centre resonator. However I am getting some group N exhaust hangers to see if that helps lift it a bit.

 

I am pretty fixed on staying 3inch the whole way through now.  And needs to be stainless. The problem i have, is the HKS sounds very nice, and works well, and I already have it, however its just the the more look at it the more im not sure about how it looks. You can see the whole rear muffler hanging out the bottom of the car and it just looks a bit wrong. If the exhaust had 2 x  90 degree bends at the muffler so that it sat straight and high like a stock muffler it would look fine. The 4.5inch tip is just HUGE! 😀

 

20210820_143002.thumb.jpg.06f48db07d7dc3c0b4973d6c0cb78b75.jpg

 

I am leaning towards getting a custom Cat back made with an adrenalinR muffler hidden up behind the rear bumper and then a 3inch stainless pipe bent in an S shape and coming out nicely out from just below the bumper.

 

Kind of like this  https://www.haywardandscott.com/subaru/impreza-classic/subaru-impreza-classic-group-a-replica-3-rear-silencer.html

 

Or this factory look would be nice too. https://www.haywardandscott.com/subaru/impreza-classic/subaru-classic-3-backbox.html

 

It gets expensive getting stuff like this from the UK. Pity we don't have many people doing this type of thing for these older cars over here.

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Bro that exhaust looks mad! Mine comes out on an angle now and I love it! I can see literally most of the muffler and it just looks aggressive and wicked.

 

Honestly bro I would not change your exhaust it looks bloody awesome!

 

Edited by Bugeye01WRX
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2 hours ago, Bugeye01WRX said:

Bro that exhaust looks mad! Mine comes out on an angle now and I love it! I can see literally most of the muffler and it just looks aggressive and wicked.

 

Honestly bro I would not change your exhaust it looks bloody awesome!

 

It is very JDM however I've never been a huge fan of big bore exhausts. The only reason I bought it was because it was the only bolt on readily available 3 inch system available at the time for a reasonable price. I didn't even consider custom at that time.

 

As I have got older and the car gets older I think either OEM look or rally car style look suits the GC8 best.

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I suppose that bumper cutout doesn't really support dual tips all that well. Something like Niran has done on his but only on one side with a big Adrenalin resonator/muffler and twin tips straight off it suits older cars quite well and is a bit less in your face big.

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Posted (edited)
8 hours ago, Andy_Mac said:

I suppose that bumper cutout doesn't really support dual tips all that well. Something like Niran has done on his but only on one side with a big Adrenalin resonator/muffler and twin tips straight off it suits older cars quite well and is a bit less in your face big.

I did have a 2.5inch Remus exhaust on it with twin tips before I fitted the HKS system.

 

I still have the Remus exhaust so could always go back to that. I am only making 230kw so surely 2.5inch is big enough to support that power??

 

Here is a pic with the Remus from a few years ago before I did the restoration. Still quite like that look.

 

20210821_183859.jpg.1b4c5b85cf6301c5f89baec5ebba3ca8.jpg

Edited by ginganinja
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3 minutes ago, ginganinja said:

I did have a 2.5inch Remus exhaust on it with twin tips before I fitted the HKS system.

 

I still have the Remus exhaust so could always go back to that. I am only making 230kw so surely 2.5inch is big enough to support that power??

 

Here is a pic with the Remus from a few years ago before I did the restoration. Still quite like that look.

 

20210821_183859.jpg.1b4c5b85cf6301c5f89baec5ebba3ca8.jpg

 

I wonder if I just modify the Remus muffler and cut the 2.5inch pipe off it right at the muffler and then run 3 inch pipe work right the way up to it.

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7 minutes ago, ginganinja said:

I did have a 2.5inch Remus exhaust on it with twin tips before I fitted the HKS system.

 

I still have the Remus exhaust so could always go back to that. I am only making 230kw so surely 2.5inch is big enough to support that power??

 

Here is a pic with the Remus from a few years ago before I did the restoration. Still quite like that look.

 

20210821_183859.jpg.1b4c5b85cf6301c5f89baec5ebba3ca8.jpg

 

Yea I like that look way more. Keeps everything way up tighter to the car than a massive cannon. Surely you can make it work again somehow.

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  • Admin

I mean, if you want to consider selling that HKS exhaust, I'm fairly certain my RA could do with a nicer exhaust than this s****y looking 3" I've got on my car currently lol. F*cking mushroom tip muffler that subirex put on it is heinous 

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