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Exhaust question?

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Old 12 February 2018, 02:28 PM   #1  
BlackBetty78
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Default Exhaust question?

Hi all


I currently have the original downpipe and centre pipe on my 06 Hawkeye WRX and a Pro-drive back box.


I have stock piled a sports cat downpipe and a second hand de-catted centre pipe from an RB320 ready to complete the exhaust system.


The sports cat has a port for the lambda sensor in it but the centre pipe does not.


If I was to have the centre pipe fabricated so it would allow for the second lambda sensor could I install both the downpipe and centre pipe and not need a remap at this stage and run the car with no issues?


I'm not sure on this and have found conflicting information on the internet so far.


My plan would be to have it remapped eventually so I can increase the power.


I hope that makes sense.


Thanks


Dave
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Old 12 February 2018, 03:21 PM   #2  
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any Hawk (apart from JDM) including the RB320 will have the main Lambda in the OS collector pipe of the exhaust manifold (basically pointing into the back of the drivers wheel)

it wont have a Lambda in the Sports Cat but should have a boss in the secondary pipe
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Old 12 February 2018, 05:10 PM   #3  
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So there is no lambda in my current OEM downpipe so that shouldn't stop me fitting the sports cat then if its in the exhaust manifold should it? So does that mean I need a bung for the sports cat as that's plumbed for a lambda and I will need fabrication on the centre pipe? I'm assuming that's what you mean by a "boss"?
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Old 13 February 2018, 04:12 PM   #4  
Don Clark
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The big question is what do you mean by centre pipe?? A lot of people/stores name the rear pipe as the centre pipe.

The OEM turbo back exhaust for a UK newage or later car has 4 parts, although with differing model years the flange positions have changed slightly.
Below is the layout for a newage (upto MY05) WRX and as it has a pre-cat in the up pipe it also has an EGT sensor. An STi doesn't have either of these items



Using the above diagram for naming conventions, All newage cars have a resonated or straight thru' rear exhaust pipe.
PPP'd cars usually have the 2nd cat pipe (midpipe above) replaced with a non catted version (2nd cat delete pipe) with boss to accept the rear O2 sensor.
A Hawk model one will have the front flange at an angle to suit the OEM downpipe

Is your sports cat down pipe one piece (down pipe and mid pipe all in one) - boss showing near rear flange.
If it doesn't have a boss for O2 sensor it maybe a classic version

or is it just the down pipe.


Pictures are so useful
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Old 13 February 2018, 04:24 PM   #5  
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Thanks for the reply Don I am a complete novice when it comes to car parts or working on cars. I was actually looking at that very diagram on my lunch. The sports cat I have is from Japspeed and appears to be the down and mid pipe all in one as the picture you posted up suggests and it does have a boss near the flange. That would make what I referred to above as the centre pipe a rear exhaust pipe using the diagram above however there is no boss on it.
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Old 13 February 2018, 04:42 PM   #6  
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If the Janspeed "all in one" down pipe is for a newage car then all should be fine and the rear O2 sensor will go in the boss near the rear flange.

This is what you currently have under your car...

Downpipe (Hawks had the joint flange moved to the base of the cat at an angle)


2nd cat pipe or mid pipe (angled flange on right hand side to match and boss for O2 sensor)


Rear or centre pipe ( with resonator)


The STi version has different part numbers for all the above items so will differ in some respect. The RB320 rear/centre pipe may have a more free flowing resonator.
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Old 13 February 2018, 05:07 PM   #7  
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This has been really helpful Don thanks very much. For some reason I was under the impression that there are 2 lambda sensors on the Hawkeye with one connecting to the down pipe and the other in the rear/centre pipe and given the rear/centre pipe I have has no boss for a lambda I was thinking I needed it fabricating to allow for a lambda to be fitted to it. From what you have described and shown today the lambda connects to the Japspeed downpipe near the flange the second cat comes out as it's replaced by the Japspeed downpipe and the centre/rear pipe goes in it's place and connects to the already installed Pro Drive back box? Does that sound correct?
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Old 13 February 2018, 06:17 PM   #8  
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Yep, about right, the lambda sensor (A/F sensor) is, as shown on the diagram and as mentioned by Alyn, situated in the manifold just before the up pipe, accessible through the access flap built into the right front wheel arch liner.


Last edited by Don Clark; 13 February 2018 at 06:19 PM.
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Old 13 February 2018, 06:54 PM   #9  
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[QUOTE=Don Clark;11995198]Yep, about right, the lambda sensor (A/F sensor) is, as shown on the diagram and as mentioned by Alyn, situated in the manifold just before the up pipe, accessible through the access flap built into the right front wheel arch liner.

So with it established I have all the required pieces 2 replace my factory down/mid and centre/rear if I did that would i need to remap at same time? I am looking to remap at some point but for the minute id just like to open up the Subaru rumble by switching out my exhaust if its safe to do so
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Old 13 February 2018, 07:08 PM   #10  
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Quote:
Originally Posted by BlackBetty78 View Post
Quote:
Originally Posted by Don Clark View Post
Yep, about right, the lambda sensor (A/F sensor) is, as shown on the diagram and as mentioned by Alyn, situated in the manifold just before the up pipe, accessible through the access flap built into the right front wheel arch liner.

So with it established I have all the required pieces 2 replace my factory down/mid and centre/rear if I did that would i need to remap at same time? I am looking to remap at some point but for the minute id just like to open up the Subaru rumble by switching out my exhaust if its safe to do so

As you are going "turbo back" changes then a remap would be on the cards.
Just changing to the RB320 rear pipe may generate some more noise.
Whilst changing the down pipe it would be a good idea to change the up pipe for an STi version without the pre-cat.

Last edited by Don Clark; 13 February 2018 at 07:16 PM.
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Old 13 February 2018, 07:58 PM   #11  
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Quote:
Originally Posted by Don Clark View Post
As you are going "turbo back" changes then a remap would be on the cards.
Just changing to the RB320 rear pipe may generate some more noise.
Whilst changing the down pipe it would be a good idea to change the up pipe for an STi version without the pre-cat.
Based on the fact it will need mapping then I might just wait and do it all at once then. I was hoping to be able to do it in 2 stages. I didnt realise I had an up pipe as well?
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Old 14 February 2018, 07:47 AM   #12  
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Quote:
Originally Posted by BlackBetty78 View Post
Based on the fact it will need mapping then I might just wait and do it all at once then. I was hoping to be able to do it in 2 stages. I didnt realise I had an up pipe as well?
It's all there in the diagram

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Old 14 February 2018, 08:56 AM   #13  
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shouldn't be a "pre-cat" on a 2.5 Hawk Don........

you could swap the up-pipe anyhows for a custom Harvey Smith 'trick' up-pipe to maximise spool & response however
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Old 14 February 2018, 10:59 AM   #14  
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Quote:
Originally Posted by AS Performance View Post
shouldn't be a "pre-cat" on a 2.5 Hawk Don........

you could swap the up-pipe anyhows for a custom Harvey Smith 'trick' up-pipe to maximise spool & response however
https://www.facebook.com/pg/ASPerformanceUK/photos/?tab=album&album_id=553517494659990
So true, my bad, it even gets confusing for us oldtimers

Last edited by Don Clark; 14 February 2018 at 11:10 AM.
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Old 14 February 2018, 02:42 PM   #15  
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No worries Don at all I appreciate all the other info you gave me :-) it's been really helpful.


Least I now have a better understanding of how it all works with the front 02 sensor located on the up pipe and the second 02 sensor located at the back end of the centre/mid pipe.


Just gotta decide whether to install my exhaust and not remap or wait until I have the funds to do both together
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Old 15 February 2018, 08:55 AM   #16  
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later cars run very close to lean as std so personally i would not alter the cat side of things until your ready for a map

centre & rear box will make little difference so you could have a little more sound in the mean time if you wished
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Old 15 February 2018, 02:27 PM   #17  
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Yeah I think I will have to bow to superior knowledge and just wait until I can afford a remap at the same time as much as I wanted to put the exhaust on asap!


Appreciate everyone's input :-)
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