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Discussion Starter · #1 ·
I’m building a turbo kit for my e46 m3 and would like to keep my cats as it’s a daily driver. But the cats are at the headers which will require heavy modifications as I have a bottom mount setup planned. N54 has a down pipe similar to e46 m3 but the cat is smaller and would required two or a y pipe either way. The biggest issue I see is the 3 sensors for the two cats. Can it be done and if so will it be worth it or just restrict too much and cause problems? Thank you in advance and if there is something similar posted sorry for that.
 

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2002 BMW E46 330i
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I’ll be honest, I’m kinda confused on what you’re wanting to do with the n54 downpipes. Just put a cat in the midpipe and call it a day. There’s more space and less headache.
 

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Discussion Starter · #3 ·
I’ll be honest, I’m kinda confused on what you’re wanting to do with the n54 downpipes. Just put a cat in the midpipe and call it a day. There’s more space and less headache.
So I’m trying to keep the car looking as stock as possible due to the smog tests in California. I’m not worried about it passing but would like to keep things close to stock. N20 down pipe looks like it could fit with the least modification. I know this is very hard in the space given but it’s worth a try so just looking for information.
 

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You’re not gonna fool anyone into thinking your car is stock if there’s a turbo on your m3. Additionally an stock n20 down pipe, or any other oem down pipe for that matter, will not be able to flow enough to make any marginal power. The stock downpipes will make a whole load of backpressure and make your egts go sky high. You might be able to get 60-70ish more hp out of your m3 with a turbo on an oem downpipe, but not much more. You can get close to the same gains with NA bolt ons. If you are determined to boost your m3 and keep it emissions agreeable and actually make notable power gains over stock, you should supercharge it instead. All of the cali turbo bmw guys I have talked to “know a guy” who will pass snog for them, and if you don’t know a guy, you’re not gonna pass.
 

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Discussion Starter · #5 ·
You’re not gonna fool anyone into thinking your car is stock if there’s a turbo on your m3. Additionally an stock n20 down pipe, or any other oem down pipe for that matter, will not be able to flow enough to make any marginal power. The stock downpipes will make a whole load of backpressure and make your egts go sky high. You might be able to get 60-70ish more hp out of your m3 with a turbo on an oem downpipe, but not much more. You can get close to the same gains with NA bolt ons. If you are determined to boost your m3 and keep it emissions agreeable and actually make notable power gains over stock, you should supercharge it instead. All of the cali turbo bmw guys I have talked to “know a guy” who will pass snog for them, and if you don’t know a guy, you’re not gonna pass.
cool thank you for the information I’m thinking about if I should now I’m only planning to boost up to 425 at most to start that’s why I was asking about the cats staying as you were saying it won’t be much boost power. Any shops they would recommend for the inspection?
 
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