Cutting cat out of perrin downpipe, thinking of replacing with flex joint

p951951951t · 02-10-2013, 01:34 PM

I'm going to be porting my wastegate. After I finish, I think I'm going to cut the cat out of my perrin downpipe. I have some 3" SS exhaust tubing laying around, but I was thinking of adding a flex joint to take some strain off the turbo. I already put one into the catback right where it meets the downpipe, but I figure another can't hurt.

Anyone done this?

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h00z0r · 02-10-2013, 02:15 PM

Honestly I would sell your Perrin dp, and buy a no name down pipe, mod the new one and pocket the extra, those Perrin pieces are worth good money

nhat · 02-10-2013, 02:58 PM

A flex joint in the downpipe?

How exactly is this going to take strain off the turbo?

If you're set on doing this, catless downpipes are a dime a dozen.

p951951951t · 02-10-2013, 05:17 PM

Because right now, the whole exhaust is rigidly connected to the turbo and mounted to the trans as well if I recall (don't really feel like goigng to peek under there at the moment)... Theoretically, shock generated from the exhaust's own inertia as well as from movements in the trans can put physical strain on the turbo and any parts attached to it, such as up-pipes.

I have heard a lot of people mention that aftermarket up-pipes have a lot of leaks, I wouldn't be surprised if that had something to do with it.

Then again, I'm still pretty new to the STi, I could be completely wrong. That's why I added the first flex joint anyhow.

Ihoozer: like the perrin piece, it's well made. I'd just as soon modify what I've got.

I'm cutting up the perrin either way, just trying to decide whether to weld in pipe or to add the flex joint.

bmw325ix · 02-10-2013, 05:54 PM

I am very in favor of a flex section in the DP, the only probably is a good quality 3" flex section can get pricey.

p951951951t · 02-10-2013, 06:53 PM

I think last time I paid like 40-50 for a SS flex pipe at summit racing.

The thing is I still have a 5' section of 3" SS tubing from summit I've never used, so if I decide on the flex joint I can just exchange that and only pay a few bucks out of pocket.

DeleriousZ · 02-11-2013, 12:47 PM

Definitely agree with putting a flex pipe on the downpipe. Will save your up pipe, manifolds and everything else related to the exhaust.

CalvinT · 02-11-2013, 08:35 PM

The cat in a Perrin down pipe is sized for a 6 liter engine. Not sure what you'd gain by removing it. It's made by FlowMaster. Take the number that's laser engraved on the cat and do a search on the FlowMaster site for more info.

Also aftermarket flex sections aren't noted for durability. If a flex was needed, why didn't the factory put one in? The stock downpipe is also rigidly mounted to the trans.

p951951951t · 02-12-2013, 03:26 PM

I hear what you are saying, but the difference is enough of a restriction to prevent boost creep with all else being the same, which means it's having a tangible effect on the flow characteristics.

As for the factory... well, you have a point. Then again, how many people have experienced leaks when moving to an aftermarket up-pipe? Personally I think that's in part due to the rigid mount. I could be wrong, and I guess there is no way for me to know for sure. I've also seen several up-pipes with built in flex joints over the last few days - that may be the way to go.

Factory engineers make decisions for all kids of reasons... cost/simplicity/ease of manufacturing/etc. I do tend to be the type that thinks generally the engineers at the factory know what's best, but obviously sometimes they have to make compromises that an individual like myself might not.

What gives you concerns about the durability of a quality aftermarket flex joint? I've never had any trouble before but I'm interested in what you know.

DeleriousZ · 02-12-2013, 05:33 PM

For anyone that's going to do a flex joint should make sure they use the right style as well.

This is not what you want:

This is what you DO want.

Notice how the 2nd one has the internal flex pieces? That style holds up to abuse much better than the 'regular' style of flex pipes.

RockNRace03 · 02-12-2013, 08:14 PM

im not so sure i agree with the flex joint. i can imagine it causing you more harm than good. they break all the time. even the "good" ones break and leak. i work for a custom exhaust shop and i would say 80% of our warranty repairs are from flex joints. we haven't used them in any products for the past year or so now. if you're looking for some extra flex, used a slip joint with accu-seal clamp

CalvinT · 02-13-2013, 12:32 AM

Grimmspeed warns not to wrap a flex section, even theirs. Jeff Perrin is the one that warned me off after market flex sections. If you'll notice, they don't use them. The factory flex sections are more robust, have double walls.

p951951951t · 02-13-2013, 08:04 AM

Yeah, the ones I use are the ones like the second pic, high quality stainless pieces.

Since I already have one just behind the downpipe, I'll probably just gut the cat or weld in some 3" SS pipe, especially now that I've seen the up-pipes with the flex joint are available and seem to work well.

02-10-2013, 01:34 PM	#1
p951951951t Scooby Newbie Member#: 276541 Join Date: Mar 2011 Chapter/Region: South East Location: North Georgia Vehicle: 2008 STi Snow	Cutting cat out of perrin downpipe, thinking of replacing with flex joint I'm going to be porting my wastegate. After I finish, I think I'm going to cut the cat out of my perrin downpipe. I have some 3" SS exhaust tubing laying around, but I was thinking of adding a flex joint to take some strain off the turbo. I already put one into the catback right where it meets the downpipe, but I figure another can't hurt. Anyone done this? * Registered users of the site do not see these ads.

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02-10-2013, 02:15 PM	#2
h00z0r Scooby Newbie Member#: 212540 Join Date: May 2009 Chapter/Region: Tri-State Location: the 315 (normally) Vehicle: 2002 WRX Battlewagon PSM	Honestly I would sell your Perrin dp, and buy a no name down pipe, mod the new one and pocket the extra, those Perrin pieces are worth good money

02-10-2013, 02:58 PM	#3
nhat #Belieber Moderator Member#: 140163 Join Date: Feb 2007 Chapter/Region: MAIC Location: 3MI Racing Vehicle: 03 STi hellaflunctional	A flex joint in the downpipe? How exactly is this going to take strain off the turbo? If you're set on doing this, catless downpipes are a dime a dozen.

02-10-2013, 05:17 PM	#4
p951951951t Scooby Newbie Member#: 276541 Join Date: Mar 2011 Chapter/Region: South East Location: North Georgia Vehicle: 2008 STi Snow	Because right now, the whole exhaust is rigidly connected to the turbo and mounted to the trans as well if I recall (don't really feel like goigng to peek under there at the moment)... Theoretically, shock generated from the exhaust's own inertia as well as from movements in the trans can put physical strain on the turbo and any parts attached to it, such as up-pipes. I have heard a lot of people mention that aftermarket up-pipes have a lot of leaks, I wouldn't be surprised if that had something to do with it. Then again, I'm still pretty new to the STi, I could be completely wrong. That's why I added the first flex joint anyhow. Ihoozer: like the perrin piece, it's well made. I'd just as soon modify what I've got. I'm cutting up the perrin either way, just trying to decide whether to weld in pipe or to add the flex joint.

02-10-2013, 05:54 PM	#5
bmw325ix Scooby Newbie Member#: 204652 Join Date: Mar 2009 Location: newburyport, ma	I am very in favor of a flex section in the DP, the only probably is a good quality 3" flex section can get pricey.

02-10-2013, 06:53 PM	#6
p951951951t Scooby Newbie Member#: 276541 Join Date: Mar 2011 Chapter/Region: South East Location: North Georgia Vehicle: 2008 STi Snow	I think last time I paid like 40-50 for a SS flex pipe at summit racing. The thing is I still have a 5' section of 3" SS tubing from summit I've never used, so if I decide on the flex joint I can just exchange that and only pay a few bucks out of pocket.

02-11-2013, 12:47 PM	#7
DeleriousZ Scooby Newbie Member#: 323869 Join Date: Jun 2012 Chapter/Region: W. Canada Location: Calgary, AB Vehicle: 2000 ST-outback White/Grey	Definitely agree with putting a flex pipe on the downpipe. Will save your up pipe, manifolds and everything else related to the exhaust.

02-11-2013, 08:35 PM	#8
CalvinT Scooby Newbie Member#: 329025 Join Date: Aug 2012 Chapter/Region: SWIC Location: AZ Vehicle: 2002 WRX Wagon Aspen White	The cat in a Perrin down pipe is sized for a 6 liter engine. Not sure what you'd gain by removing it. It's made by FlowMaster. Take the number that's laser engraved on the cat and do a search on the FlowMaster site for more info. Also aftermarket flex sections aren't noted for durability. If a flex was needed, why didn't the factory put one in? The stock downpipe is also rigidly mounted to the trans.

02-12-2013, 03:26 PM	#9
p951951951t Scooby Newbie Member#: 276541 Join Date: Mar 2011 Chapter/Region: South East Location: North Georgia Vehicle: 2008 STi Snow	I hear what you are saying, but the difference is enough of a restriction to prevent boost creep with all else being the same, which means it's having a tangible effect on the flow characteristics. As for the factory... well, you have a point. Then again, how many people have experienced leaks when moving to an aftermarket up-pipe? Personally I think that's in part due to the rigid mount. I could be wrong, and I guess there is no way for me to know for sure. I've also seen several up-pipes with built in flex joints over the last few days - that may be the way to go. Factory engineers make decisions for all kids of reasons... cost/simplicity/ease of manufacturing/etc. I do tend to be the type that thinks generally the engineers at the factory know what's best, but obviously sometimes they have to make compromises that an individual like myself might not. What gives you concerns about the durability of a quality aftermarket flex joint? I've never had any trouble before but I'm interested in what you know.

02-12-2013, 05:33 PM	#10
DeleriousZ Scooby Newbie Member#: 323869 Join Date: Jun 2012 Chapter/Region: W. Canada Location: Calgary, AB Vehicle: 2000 ST-outback White/Grey	For anyone that's going to do a flex joint should make sure they use the right style as well. This is not what you want: This is what you DO want. Notice how the 2nd one has the internal flex pieces? That style holds up to abuse much better than the 'regular' style of flex pipes.

02-12-2013, 08:14 PM	#11
RockNRace03 Scooby Newbie Member#: 322678 Join Date: Jun 2012	im not so sure i agree with the flex joint. i can imagine it causing you more harm than good. they break all the time. even the "good" ones break and leak. i work for a custom exhaust shop and i would say 80% of our warranty repairs are from flex joints. we haven't used them in any products for the past year or so now. if you're looking for some extra flex, used a slip joint with accu-seal clamp

02-13-2013, 12:32 AM	#12
CalvinT Scooby Newbie Member#: 329025 Join Date: Aug 2012 Chapter/Region: SWIC Location: AZ Vehicle: 2002 WRX Wagon Aspen White	Grimmspeed warns not to wrap a flex section, even theirs. Jeff Perrin is the one that warned me off after market flex sections. If you'll notice, they don't use them. The factory flex sections are more robust, have double walls.

02-13-2013, 08:04 AM	#13
p951951951t Scooby Newbie Member#: 276541 Join Date: Mar 2011 Chapter/Region: South East Location: North Georgia Vehicle: 2008 STi Snow	Yeah, the ones I use are the ones like the second pic, high quality stainless pieces. Since I already have one just behind the downpipe, I'll probably just gut the cat or weld in some 3" SS pipe, especially now that I've seen the up-pipes with the flex joint are available and seem to work well.