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Replacing OEM Central Catalytic Converter with High Flow


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Recent thread by Metal_Head titled  "Rear Muffler Delete" included posts about an ideal exhaust system size application for the Aurion. Preliminary research confirmed his suggestion of replacing the last catalytic converter with a 3" version as an ideal starting point for an improved flowing exhaust system.

On the 40 series Aurion, there is a very large resonator after the catalytic converter. Approx, exhaust pipe measurements [after the flex joint for the front Y pipe] increase from 60mm to 72mm so this should readily accommodate fitting a 3" high flow stainless steel catalytic converter. In theory, there should be improved exhaust flow which hopefully will result in better fuel economy and acceleration particularly on the highway.

Nice theoretical musings then Karma came to the girlfriend's birthday celebration on Sunday along with an unexpected windfall. Phoned a Brisbane Southside Exhaust shop today and accepted his phone quote of $240. Car is now booked in for Thursday morning. I will give a follow up about whether this change was worthwhile or save your $$$  

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Nice to see u invest in the exhaust. Just remember it will only flow as good as the bottleneck in it. You may have to follow through and do the whole lot to see gains.... Which wouldn't be a bad thing 🙂

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5 hours ago, Metal_Head said:

Nice to see u invest in the exhaust. Just remember it will only flow as good as the bottleneck in it. You may have to follow through and do the whole lot to see gains.... Which wouldn't be a bad thing 🙂

OEM catalytic converters are designed to last the life of the vehicle. However, over time they can become less efficient or even clogged or more restrictive of the exhaust flow and even damaged for various reasons. Any gain from replacing the catalytic converter will be more about restoring lost engine performance and fuel economy. Anyway the build date on my vehicle is Oct. 2006 with 221K on the odometer so maybe more potential efficiency gains than a 50 series Aurion with much lower mileage. Soon find out by a visual inspection of the element in the catalytic converter whether it really needed to be replaced or not. This may be a good indicator of whether to consider replacing the catalytic converters in the headers.

5 hours ago, Metal_Head said:

Or even just delete the last cat........ 

Not an option for a street legal vehicle. It is something like a $10K fine. The Aurion has a 3 cat designed system to meet vehicle emission standards. While it may be possible to still pass an EPA test with just the catalytic converters in the headers, you are still legally required to keep the last catalytic converter. I will confirm this point with the exhaust shop owner. 

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And replacing the factory cat with a lower cpsi version is illegal too. I guess at least it "looks" like something is there so that option is preferable. 

Exhaust place will fit or mod anything for you, but just a friendly tip.... If u walk in their all holy & start going on about legalities, the bloke will be hesitant to do anything. 

Keen to see how u go with it, and crossing my fingers you end up doing the whole exhaust like I want to do, but the coin will never allow me to 😞

(sorry about the edit, the post sounded nothing like how I wanted to come across) 

 

 

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13 minutes ago, Metal_Head said:

And replacing the factory cat with a lower cpsi version is illegal too. I guess at least it "looks" like something is there

Since there's already 2 cats there, I personally would do away with the 3rd cat and I doubt anyone who would have to check your car in the future for any reason would know off the top of their head an aurion has 3 cats standard. Getting caught & fined won't happen, because the "last owner must've did that" , or if it's a one owner then an "exhaust place years ago must've did that" and you were unknowingly driving around like that. 

Exhaust place will fit or mod anything for you, but just a friendly tip.... If u walk in their all holy & start going on about legalities, the bloke will be hesitant to do anything. 

Curious if you've ever modified a car before, because it sorta looks like it's your first rodeo

 

I have just done some preliminary research. It is a bit of a maze with no clear cut one size fits all answer. Main point is that the catalytic converter are the means by which vehicle manufacturers meet vehicle emission standards and these standards have increased over the years but are not retrospective.    Emission_Standards_for_Petrol_Cars.pdf

https://www.infrastructure.gov.au/vehicles/environment/emission/index.aspx

Let's just say that I prefer to not be in a situation where I am having a roadside or courtroom discussion about a missing catalytic converter.

I am dealing directly with the owner of a long established exhaust shop. We will be having a confirmatory discussion that the replacement will be legally compliant for my specific vehicle. My thinking at the moment is that a 3" version with 200 cpsi should readily pass an EPA test.

See that you asked I have never modified a vehicle which explains how I happened to survive my younger driving years.

These days, I prefer to have a stock standard looking vehicle. I am more into preventative maintenance and using upgraded replacement parts for efficiency gains. When it comes to exhausts, I want to have the most quiet, definitely without any drone.

 

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If u go 200cpsi, then get one with a larger body, that way u get the filtration along with larger area for flow. 100cpsi are good for tight spaces whilst still allowing flow

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I did proceed and got a 2 1/2" high flow 200cpsi stainless steel catalytic converter fitted.

Check of the OEM catalytic converter showed that it was not blocked or damaged and still in good condition after 14 years. Therefore, it was not critically essential to replace it so could have saved the $240. 

Initial observations are as follows: 

1. the exhaust is still relatively quiet, only very marginally louder.

2. as expected, there is no exhaust drone. 

3. engine idle rpm is occasionally lower

4. as expected, no apparent performance gains

5. as expected, some possible increased torque in lower rpm range.

6. Noticed speed increased from 60kph to 70kph with little change in accelerator or engine rpm. Mental note to be gentle on the accelerator and pay more attention to the speedo. 

I can also see how any expected fuel economy gains will be quickly offset by being too playful with the accelerator. So planning to be responsibly playful and enjoy the change then look for fuel economy later. Good excuse to go for a drive on the highway.

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Yeah when I dropped the rears, I drove like an idiot, purely the sound encouraged me to do it. 

When resonators went on it, I calmed down alot. Strange phenomenon haha

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6 hours ago, Metal_Head said:

purely the sound encouraged me to do it.

I am thinking that the engine at idle has a deeper throatier sound to it.

What I am am noticing is that the engine seems to be more free revving. Appeared to be effortless to get up to the 80kph speed from a standing start. Before I was more watching the tachometer trying to keep the revs in the 2000 range for fuel economy. Now I am more watching the speedometer to ensure that I keep within the speed limit. 

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I didn't notice that. Considering 3" is ideal for this engine, the 2.5" cat will be a bottleneck for any future exhaust mods. I'd say exhaust bloke didn't want the awkward job of merging 3" with the factory pipe

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2 hours ago, LordBug said:

Just curious, why'd you opt for the 2.5" over the 3" in the end?

More the recommendation of the exhaust shop owner after measuring up the exhaust pipe on either side of the existing catalytic converter.  2.5" ended up being a neater fit.

26 minutes ago, Metal_Head said:

I didn't notice that. Considering 3" is ideal for this engine, the 2.5" cat will be a bottleneck for any future exhaust mods. I'd say exhaust bloke didn't want the awkward job of merging 3" with the factory pipe

 Both the 2.5" and 3" catalytic converter had the same size main body diameter i.e. 4". If there had been a notable difference then I would have been more insistent upon having the 3" version fitted. 

For insurance purposes, my rationale is that I have not modified the exhaust, I have upgraded an existing component. It is very much the same as replacing the existing mufflers with high flow versions. 

The main bottlenecks are the header catalytic converters and then the merge pipe of the front Y pipe. My thinking is that by increasing the exhaust flow of the last catalytic converter, this should be a viable alternative to modifying the front Y pipe and not impact the driveability of the vehicle. 

Certainly enjoying the differences and I have particularly noticed that the Aurion now coasts downhill faster.

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  • 1 month later...

I thought that I would do a follow up after an experience this afternoon. I got a bit impatient waiting for a break in traffic while stopped at a T intersection. I had previously let an opportunity pass thinking that I would get a better opportunity soon. When an opportunity did finally come, I accelerated a bit harder than usual. Accelerator was probably at mid point and most certainly not to the floor. Anyway I got a bit of axle tramp.

Last time that happened was well before this upgrade and I did have the accelerator flat to the floor and had to back off to get traction. 

Certainly thinking that there is more torque in the lower rev band which I do prefer. Possibly related or at least contributing, I have noticed that the transmission moves up into higher gears a bit quicker and stays in high gear longer.

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Interesting topic Ash. I'm not too fussed about exhaust systems these days as I was when I was younger. Remember when we ran around in our sporty versions of road cars and the exhaust sound was practically everything ? I had a few old Fords before I bought an XD 351 Fairmont. It had a 4 into one header system that merged into a single 3" system. It sounded great. Then as time rolled on I got more into the bikes and started driving regular cars till the M3 came along. That car is totally stock and I much prefer it that way to be honest.

I did, however come across a good deal on Ebay recently for an Aurion 50 series catalytic converter and thought why not buy and keep it as a spare for my long term ownership plan. It came off a Sportivo model with only 60K and so it's still newish. It even looks like new compared to some you see advertised. It came complete with O2 sensors too. What a bonus.

My idea was that as the car ages, the catalytic converter will degrade with time and start to become restrictive as you guys mentioned and with this I thought by having a spare low kay one I could just swap it out when the time comes.

 

s-l1600.jpg

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22 minutes ago, Tony Prodigy said:

My idea was that as the car ages, the catalytic converter will degrade with time and start to become restrictive as you guys mentioned and with this I thought by having a spare low kay one I could just swap it out when the time comes.

I am thinking that it could be a long time before you will need to swap the pipe out.

Very handy to have those O2 sensors which will eventually need replacing.

Hope you have sealed off the openings to keep out any dust, insects etc.

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1 hour ago, campbeam said:

Very handy to have those O2 sensors which will eventually need replacing.

They were a bonus I thought. Not a cheap part either and there are two. There is also a ding on one of my Y pipes on the bottom. (not by me either) It was there when I bought the car and it has always bothered me, so win win for this replacement piece as not only will it do away with the cosmetic damage but refresh the potential flow restriction when that occurs.

1 hour ago, campbeam said:

Hope you have sealed off the openings to keep out any dust, insects etc.

It's bubble wrapped from tip to toe don't worry and stored safely.

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48 minutes ago, Tony Prodigy said:

It's bubble wrapped from tip to toe don't worry and stored safely.

🤣 Knowing that you readily admit to being OCD, I would have been more surprised if you had NOT stored it safely. 

I have got a ding on the bottom of the Y pipes and I was the one that did it. I should have been a bit more careful when parking up against raised concrete edging at the local Tavern. 

When you get a CEL for the O2 sensors then that will be the time to fit your standby assembly.

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59 minutes ago, campbeam said:

When you get a CEL for the O2 sensors then that will be the time to fit your standby assembly.

Yep. Good advice. I've heard )2 sensors are difficult to remove and you can risk damaging them. Is that generally the case Ash ?

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For background information purposes, the Aurion has 3 catalytic convertors of which 2 are on the exhaust headers. Last one is on the exhaust pipe after the Y pipe join and before the resonator. Generic term is O2 sensors. However pre-cat sensors are actually air/fuel ratio sensors and post-cat sensors are O2 sensors. There are no O2 sensors for the 3rd catalytic convertor. I understand that it was there to meet stricter European emission standards. 

Sensors on the exhaust headers can be difficult to remove. Internet tip posted by a Toyota Technician is to use brake fluid as a rust and thread penetrator. Apparently, it works better than WD-40 and other similar products. Definitely worked for me. 

I purchased a 3 piece O2 socket kit from eBay and ended up using different length sockets. 

https://www.ebay.com.au/itm/3-Pc-Automotive-Oxygen-Sensor-Socket-Wrench-O2-Remover-Tool-Set-Kit/162972133984?_trkparms=aid%3D1110009%26algo%3DSPLICE.COMPLISTINGS%26ao%3D1%26asc%3D228209%26meid%3De588bd643ee4471a8d9c77da086c5e6d%26pid%3D100008%26rk%3D2%26rkt%3D12%26mehot%3Dpp%26sd%3D203043681465%26itm%3D162972133984%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3Ddefault%26brand%3DUnbranded&_trksid=p2047675.c100008.m2219 

As for the O2 sensors on the Y pipe, access is a bit tighter. I ended up using a 22mm ring spanner on one of these sensors after disconnecting the electrical connector. When refitting, you use some anti-seize being careful to apply to the threads only. 

In my one-time experience, difficult part is more the disconnecting of the electrical connectors. Sometimes they unclip nice and easy and others are just stubborn. A bit of silicone or WD-40 spray can help. Not exactly a good idea to be pulling [or too hard]on the electrical wires. 

 

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14 hours ago, campbeam said:

For background information purposes, the Aurion has 3 catalytic convertors of which 2 are on the exhaust headers. Last one is on the exhaust pipe after the Y pipe join and before the resonator. Generic term is O2 sensors. However pre-cat sensors are actually air/fuel ratio sensors and post-cat sensors are O2 sensors. There are no O2 sensors for the 3rd catalytic convertor. I understand that it was there to meet stricter European emission standards. 

Sensors on the exhaust headers can be difficult to remove. Internet tip posted by a Toyota Technician is to use brake fluid as a rust and thread penetrator. Apparently, it works better than WD-40 and other similar products. Definitely worked for me. 

I purchased a 3 piece O2 socket kit from eBay and ended up using different length sockets. 

https://www.ebay.com.au/itm/3-Pc-Automotive-Oxygen-Sensor-Socket-Wrench-O2-Remover-Tool-Set-Kit/162972133984?_trkparms=aid%3D1110009%26algo%3DSPLICE.COMPLISTINGS%26ao%3D1%26asc%3D228209%26meid%3De588bd643ee4471a8d9c77da086c5e6d%26pid%3D100008%26rk%3D2%26rkt%3D12%26mehot%3Dpp%26sd%3D203043681465%26itm%3D162972133984%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3Ddefault%26brand%3DUnbranded&_trksid=p2047675.c100008.m2219 

As for the O2 sensors on the Y pipe, access is a bit tighter. I ended up using a 22mm ring spanner on one of these sensors after disconnecting the electrical connector. When refitting, you use some anti-seize being careful to apply to the threads only. 

In my one-time experience, difficult part is more the disconnecting of the electrical connectors. Sometimes they unclip nice and easy and others are just stubborn. A bit of silicone or WD-40 spray can help. Not exactly a good idea to be pulling [or too hard]on the electrical wires. 

 

Great info Ash. I'm thinking that in the event one needs to remove an O2 sensor, perhaps soak everything well and leave overnight to soak in to allow for best possible penetration.

Just a thought.

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On 9/17/2020 at 12:44 PM, Metal_Head said:

Yeah when I dropped the rears, I drove like an idiot, purely the sound encouraged me to do it. 

When resonators went on it, I calmed down alot. Strange phenomenon haha

That same kinda of phenomena has me driving my Aurion like an 'ol fart/Hybrid driver & the 86 like a pace car on a racetrack 😐  It's the way the car makes you feel. Interesting also is that I feel more 'awake' in the 86 as opposed to the Aurion.

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47 minutes ago, ZZT86 said:

That same kinda of phenomena has me driving my Aurion like an 'ol fart/Hybrid driver & the 86 like a pace car on a racetrack 😐  It's the way the car makes you feel. Interesting also is that I feel more 'awake' in the 86 as opposed to the Aurion.

Hahaha, so true. The Aurion has nothing sporty about it so it doesn't really inspire you to drive it like you stole it. I can understand the 86 though. That car would have more of a roadster feel. If I had spare cash I'd really love a BMW Z4. Superb car !! Roadsters are fun to drive like the Mazda MX5 too.

My M3 is on a whole different level again. It feels solid, planted and very neutral in the corners thanks to the 50/50 weight distribution and the ///M suspension. Handling is sublime, with linear power delivery from the engineering masterpiece S50B32 engine. I love that car.

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My Hachi has the loud boxer rumble coupled with a short throw manual gearbox, rwd & go cart handling - it stirs the emotions, I love it :wub:

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86 being a manual will do that. 

Anything auto kills the vibe. Manual can give it random throttle in any gear & stills feels kind to the vehicle. Flooring it in an auto means long pause followed by huge stall into lower gear & harsh kickback & wheelspin, then transmission thump when you've backed off & confused it. 

Autos suck. 

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