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I assume you're talking about the primary cats?

They can be opened in situ, gutted and welded. Evolve have proven this achieves minimal gains.

Ideally you want to replace the headers for decent power gains
 

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IMO I would do it right and remove the headers to remove the primary cats. If you want to preserve the stock sound as much as possible cut off the cats and weld in straight pipes, Ryan @ RMS can do it for $1k with a core exchange or if you want max hp go with Evolve Tubular headers or Supersprint but that will change the exhaust note drastically.
 

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I assume you're talking about the primary cats?

They can be opened in situ, gutted and welded. Evolve have proven this achieves minimal gains.

Ideally you want to replace the headers for decent power gains


it can be done but you need a lift and about 4-6 hours standing up with a see-saw and ALOT of elbow grease, and it does not achieve minimal gains

but expect to get about 10whp and thats dyno proven, obviously you can get up to 30whp with headers but it will cost you
 
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To answer your question directly, yes you can! I just gutted my cats a few weeks ago, while they were still on the car! It's a tough job, but after this and evolve tune it's a whole new car! Stinkier too ;)

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IMO I would do it right and remove the headers to remove the primary cats. If you want to preserve the stock sound as much as possible cut off the cats and weld in straight pipes, Ryan @ RMS can do it for $1k with a core exchange or if you want max hp go with Evolve Tubular headers or Supersprint but that will change the exhaust note drastically.
Hey, do we know for a fact that the chamber from the hollowed out cats is what leads to the deeper note?
 

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Discussion Starter #7
IMO I would do it right and remove the headers to remove the primary cats. If you want to preserve the stock sound as much as possible cut off the cats and weld in straight pipes, Ryan @ RMS can do it for $1k with a core exchange or if you want max hp go with Evolve Tubular headers or Supersprint but that will change the exhaust note drastically.
I wonder. Are the gains from the RMS process about the same as hollowing out the cats?
 

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Hey, do we know for a fact that the chamber from the hollowed out cats is what leads to the deeper note?
I am not sure on this I haven't heard a hollowed out primary cat setup in person but I don't experience this note you are talking about on my car. Mine is the same as RPi's setup with straight pipes welded in and the cats cut off. Do you have a video before you reinstalled your secondary cats so I can check it out?
I wonder. Are the gains from the RMS process about the same as hollowing out the cats?
Regarding the hollowed out cats from what I've been told by by someone who is more knowledgeable in the area than me, he said, that big empty chamber will actually slow down the exhaust flow making the engine work harder to push out the exhaust gases. In theory you want your exhaust system setup with the proper diameter piping so it can create a vacuum effect that way the gases get "pushed out" naturally. Again, I am not an expert in this exhaust area tuning, but I do know how to make an exhaust sound good though. hiha
 

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Discussion Starter #9
I am not sure on this I haven't heard a hollowed out primary cat setup in person but I don't experience this note you are talking about on my car. Mine is the same as RPi's setup with straight pipes welded in and the cats cut off. Do you have a video before you reinstalled your secondary cats so I can check it out?


Regarding the hollowed out cats from what I've been told by by someone who is more knowledgeable in the area than me, he said, that big empty chamber will actually slow down the exhaust flow making the engine work harder to push out the exhaust gases. In theory you want your exhaust system setup with the proper diameter piping so it can create a vacuum effect that way the gases get "pushed out" naturally. Again, I am not an expert in this exhaust area tuning, but I do know how to make an exhaust sound good though. hiha
Yeah, that's makes sense. A hollowed out chamber that is larger than the pipe it's dumping in to does seem like it might cause turbulence and not be optimal.
 
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For what it's worth, I recommend taking the headers off and having the decat done at any competent engine shop with a tig welder.

Have then cut the car off and punch out the material and then weld it back up. Should cost around $125 to $150 for the set.

If you want to ceramic coat them ( I recommend ), then that adds about $265 to it.

So you are looking at $150 to $400 max.

We charge $600 for decat headers and $265 for the ceramic coating, this is without a core charge. I have these on all my personal M5 and M6 (7-cars in total).

Best,


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