So.. As I have my motor tore apart, I am going to take this time to replace the downpipe/cat. Mine is blasted so time to replace it. I've got so many questions and the more I look, I just have more questions than answers.

When shopping for an OEM replacement on Minimania, I saw that the Catalytic Converter for the N14 in the Cooper S and the Catalytic Converter for the N14 in the JCW are not the same. I emailed Mini Mania about this and they told me that the OD for the Cooper S exhaust is 2.25" and the OD for the JCW exhaust is 2.5"... What is confusing is how so many aftermarket brands say their downpipe fits both the Cooper S and JCW... So i am very confused.

Does anyone have any insight here?

My next question is about sport-cat. I want a high flow sport cat. Thing is, I have to be able to make that work without throwing a Check Engine Light and I cannot fix this by tuning it out. We have Emissions to pass here in Arizona. When you tune the CEL off for the downstream O2 sensor, it always fails the emissions test. Has anyone ever used a defouler or diverter bung to get the CEL to stay off?

Any insight here?

I was looking at Magnaflow's Catalytic Converter and they insist that it fits both the S and JCW and based on Mini Mania's info to me, I don't see how this is possible.

 

Switching out your downpipe to any aftermarket one, it will throw a check engine light for p0420, so stick with stock one or you will have to run tune.

 

As for the question about size, all the aftermarket downpipes are all about the same size. Both models have the same engine bay, so it makes sense that the aftermarket would make just the one size for both models.

 

The engine bay fitment isn't the concern so much. The JCW having an additional 1/4" in diameter would be by design for increased airflow from the larger turbo. What I don't want to do is end up with a downpipe/Catalytic converter that is sized down and creates undue backpressure. I asked Magnaflow specifically about their measurements and got no response.

The reason I've been leaning towards the Magnaflow is because they state that their Catalytic Converter will pass emissions and for almost half the price of the the OEM converter, it is a very attractive option. However, if it comes in at 1/4" under OEM, than it will certainly create undue back pressure.

 

What part you arent getting here? Why would these manufactures these downpipes in different sizes, when they can make one for both models, that is the same. Im pretty sure any downpipe you get is going to be 2.5 due to size, its cramped in there. I think you can get a 3 inch in there, but you have to loose something due to the size. Search button is up there in the right hand corner, you can search this forum and google, but from the 5 years i have been on here im pretty damn sure any downpipe that isnt stock is going to throw an CEL, so you might want to just hunt for a stock one and not waste your money on the magnaflow. There is a forum member, MikeHKS, might have a stock downpipe you can purchase, he buys them off here all the time. I had an N14 for 3.5 years sunk 10k into it, so what do i know?

 

The N14 JCW turbo isn't much bigger than the S version, and the size difference is only on the intake side. Turbine housing is the exact same between the two turbo versions. I wouldn't bother with a catted downpipe. Get the catless and a tune, and enjoy. @Lou@Prototype-R can give you an "Emissions+" tune option, and you'll have no problem passing inspection. A 2.5" catless down pipe will flow more than enough for these engines. There are plenty of engines making ~300hp on a 2.5" down pipe.

 

I suppose I will address your comment here in order...

"What part you arent getting here? Why would these manufactures these downpipes in different sizes, when they can make one for both models, that is the same." - The reason they would only make one that "fits" all models is to streamline manufacturing to turn a larger profit. But what isn't stated is which standard was used for their baseline prodcut? Since Cooper S models out number JCW's to a fairly large order of magnitude, do you think it is likely a manufacturer would make it to the JCW standard, or the Cooper S standard and then just sell them to fit everything? I actually don't have an answer here, which is the very heart of my question... you also don't have the answer, or you would have provided it.

"Im pretty sure any downpipe you get is going to be 2.5 due to size, its cramped in there. I think you can get a 3 inch in there, but you have to loose something due to the size." - Honestly, this makes sense and I am leaning towards you being right here. I am just trying to get more than just an educated guess. Seems a 15 year old platform would have more info on the matter is all...

" Search button is up there in the right hand corner, you can search this forum and google" - I post here as a last resort after I have searched extensively. I've searched the internet, I've emailed manufacturers directly, and I've searched this very forum. Nothing. I post here as a last resort because this is the typical reception you get on forums these days. It's crap like this that has killed forum culture BTW. Forums used to be amazing communities of knowledge....

" but from the 5 years i have been on here im pretty damn sure any downpipe that isnt stock is going to throw an CEL" - If using a standard cell-count catalytic converter, there won't be a CEL. There are several aftermarket downpipes with Code-Compliant converters in them. The Magnaflow I mentioned happens to be one of them at a significantly reduced price.

"so you might want to just hunt for a stock one and not waste your money on the magnaflow" - Also been hunting for a stock converted, not easly found.

"There is a forum member, MikeHKS, might have a stock downpipe you can purchase, he buys them off here all the time." - Thank you, I will reach out to him.

"I had an N14 for 3.5 years sunk 10k into it, so what do i know?" - I've been wrenching on cars since the 90's doing all manner of modification, tear down, rebuilt, etc. All I can say is, the longer you wrench on cars bud... the more you will realize that you can fill a library with "what you DON'T know". And the more you know, the more you will realize, you DON'T know.

Overall, more posting on the forums is good for forums. The more questions that get asked, the more information gets shared. A healthy forum needs more posts, not less.

 

The CEL is hit or miss. I know those who have a 200 cell sport cat fitted and they don't get a CEL light. It's a different story if it's straight downpipe, CEL real quick.

 

I only know of a few manufacturers that offer a 3" downpipe for the R56 but they are not catted at all and are straight thru. Unless you pair this with a 3" exhaust, it defeats the purpose because you are restricting air flow with the reduced exit gas velocity if you go with a 3" DP to a 2.5 exhaust. The other issue is if you go with a 3" downpipe, then the flange will be different and you'll need longer studs to the turbo.

As for the CEL, it's hit or miss with a sport cat. I have a Scorpion 200 cell sport cat and I don't have a CEL. I know others who have the more expensive Milltek 200 cell sport cat DP and they also do not have the CEL. It seems the CEL is more relevant with the cheaper brands. From the outside the welds look good but the filter material that is supposed to reduce the pollutants isn't good enough which triggers the O2 sensor fault and that's why we have the CEL issue. The other option is I know there are those who have installed a space for the O2 sensor which removes the sensor itself from the direct air stream within the DP and have had luck to remove the CEL. The size of the OEM downpipe is the same for the S and the JCW models. The difference between them is the cell material within. I could be wrong since its been so long since I researched this but I believe the S model is 400 cell and the JCW model is 300 cell.

 

While not ideal having a larger 3" DP going to a stock Cat-Back will still net gains, not as much as a full exhaust but more flow always helps.

Also you can use stock studs with a 3" DP. Maybe some builders do goofy stuff but not needed for most.

Also a note for AZ emissions if you tune out the rear 02 you will still pass emissions just fine. Arizona only checks if there are codes, if the code has been turned off that you pass.

I cannot recommend which cat might work, I know that high flow magnaflow cats even with a few different style of defoulers will still throw one or two error codes, either cat efficiency or to low of temp on sensors if you move it to far out of the exhaust stream. One option might be take a stock DP and cut out the stock Cat and have someone weld that unit into larger 3" DP and that will give you the increase in flow and reduced restriction of the stock pipe while maintaining the OEM cats ability to do it's job.

 

I do not think you are correct about AZ emissions. My previous car, a 2016 Veloster Turbo, had a catless DP. My tuner fixed the CEL thru the tune. When I went to emissions the vehicle, they told me I did not pass because the downstream O2 sensor was in a constant state of "pending" and not actually reading the values.

 

I have been in AZ for going on 19 years with all levels of modified cars, And I do love how AZ on older cars doesn't care what motor or mods are done as long as the tailpipe is clean per chassis spec. I have gotten more than a few funny looks and laughs at my older cars when I bring them in for the rolling test only for it pass the sniffer with flying colors.

For 1996+ cars I can state that in my experience with others at least up until 2021 doing emissions that I have seen more than a few vehicles that are tuned and without rear 02s even installed have passed just fine. I might be tempted to state that maybe the tuner didn't set up that flag correctly, was it a local tune or online tune?

Maybe since 2021 they have changed standards for what they look for but from what I had seen it was a non issue. Newer federal requirements have been kicking tuners In the *** lately which might of required some tuners to be unable to legally set the flags needed.

I will see if I can test out a car or two locally later today to see if they show that test as passed.

I just checked a 2005 and 2012 and both show all emissions systems ready. So again your experiance with the previous tuner might not of been ideal compared to maybe the Mini unless something new has happened with AZ emissions that I am not aware of.

​​​

 

To go back and answer some questions. After clearing out some other issues when I got the head off, I'm finally ready to put the car back together. I bought the Magnaflow Cat because their customer service said it was 2 1/2"...

well... it wasn't. The exact thing that I was concerned about to begin with is exactly the issue here. The parts are made to Cooper S specs and sold as "fitting" all N14 models. This just isn't the case and if I actually mounted this thing, I'd have backpressure and turbulance and probably premature turbo failure.

So here I am tracking down a part that fits.

Here is the OEM downpipe/Cat


Here is the Magnaflow (notice the insanely shitty build quality to boot)

 

I do not think it is made to the S specs as I think my stock S DP is the same ID as yours (I will measure when I get home).

I think it is just aftermarket is garbage unless it is more performance based and custom made.

 

Who's best to tune out the check engine lights....associated with these down pipes?

 

I went with the JMTC tune which being local was nice.

https://jmtcperformance.com/jmtc-per...ce-ecu-tuning/

You might want to check with Dyno Comp as they are local if you are in AZ as that is another option.

 

My issue with DynoComp is that they often outsource their tune to local "private contractors" and then take a cut. That cut is substantial which makes their tunes cost double or more the national average rate. Experience does tell me that they do fantastic work, even when subbing the work out and they stand behind it as well from what I've seen.

 

That is almost how most of the tuning world is it seems. I do not know about mini tuners but I have heard about a lot of other drama from other local tuners and how they trash talk other tuners since that person used to work at their shop.

Lots of talk around here about the prototypeR turner. I am tempted to give them a shot for my r58.

I would guess that most even mail order tunes for the mini take info account the most common mod.

 

You can’t go wrong with @Lou@Prototype-R His tunes are not “off the shelf” - everything he does is specific to your car and modifications. And he will work with you to get everything right until you’re happy the way the car is performing.