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I live in Oklahoma, so we don't have to get emissions checked. I have a 2000 v6 3.8l Mustang. I bought the car for $3500 at 70k miles (it was a really good) so I don't want to spend more money than I have already done with maintenance (around $1000/ brakes, other stuff, etc) to make the car "better".

Would removing the catalytic converters cause any negative effects on the engine? Would it affect how it runs in a negative way? I'm not to worried about the ECU popping up a code, because I could always take my ECU to get flashed (tuned) to run without the catalytic converters.

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    Why are thinking of gutting the cats? Are you thinking it will add power? Or are you just looking for a louder car? – Bob Cross Sep 3 '15 at 12:00
  • well, "cheap horsepower" is always nice! Mainly, I have had people I know that have done this or talked about doing it, so I figured I would jump on the bandwagon and give it a try if there are more pros than cons. – Chris Manning Sep 3 '15 at 12:29
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    Okay, my answer is below. To be clear: I support the quest for cheap fun. I just don't think this is a good idea. – Bob Cross Sep 3 '15 at 13:26
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    rm -f /bin/cat ;-) – Digital Trauma Sep 3 '15 at 19:22
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The catalytic converter is there to make your exhaust cleaner. It does nothing critical to the proper running of the engine itself. If you remove it, all that will happen is that you'll get dirtier exhaust and an error code P0420 (Catalyst System Efficiency Below Threshold (Bank 1)), which is to be expected because the "catalyst system" has been removed.

If you reflash the ECU to ignore this error, you won't have to worry about it ever again. I got the same error in my Subaru Forester after I had the downpipe decatted. When I reflashed the ECU, the error no longer popped up and it didn't affect my performance in any way. In fact, turbo spool is quicker because the exhaust is less restrictive now ;)

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    Thanks for the info, I feel like having a less restricted exhaust flow should help the engine run better. – Chris Manning Sep 3 '15 at 20:58
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tl;dr: I can't find any evidence that removing the catalytic converters on this model will improve the car. It will definitely confuse the ECU (and obviously pollute more).

I've looked hard for any dyno sheets that show any concrete evidence that cat delete alone will add power.

Removing the cats is something that I would almost never recommend outside of certain specific circumstances:

  1. Removing a pre-turbo cat on an early 2000s WRX. Those fail after years of use and will suck all kinds of turbo breaking yuck into the turbine.
  2. Building a race exhaust from the ground up where cats aren't required anyway.

Downsides:

  1. Louder: this might be something that appeals to you. However, I find the tinny cat delete sounds to be like a drill in my brain.
  2. Confused ECU: the ECU will almost certainly throw codes due to the change in airflow and composition. On my car, a confused ECU means lower performance.
  3. The exhaust fumes will be illegal in many states: unless you're planning to live in one place forever, you should consider whether it's going to be worth the hassle when you decide to move or even drive out of state. Local ordinances will win if they don't like your new volume and non-catted exhaust smell.
  4. Also, as Fred Wilson reminds me, it is against federal law to remove or replace a functional OEM catalytic converter.
  5. Hidden costs: getting any benefit from a no-cat exhaust will require tuning. This, this isn't really a zero cost solution.

Practical recommendations for other ways to get cheap fun:

  1. Get a cheap intake. Better airflow up front will certainly help.
  2. Remove the muffler / replace with minimal required: a lower weight muffler will likely sound much more fun than no cats. If I could get away without mine, I would. :-)
  3. Remove weight in the car: can you replace the spare tire and jack with a can of fix a flat?
  4. Better tires: are you driving on the original tires? Dump them for something new but affordable and be impressed.
  • Nice answer. I'm curious as to how higher exhaust flow will affect the ECU though, since the only sensors that live around the cats are the lambdas, which output AFR independent of flow rate. – Zaid Sep 3 '15 at 13:56
  • Should include the environmental impact as one of the downsides. I know many people in U.S don't really care about the environment, but unfortunately we share the same planet. – rana Sep 3 '15 at 14:06
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    It is illegal in all states to tamper with a vehicle emission system. It is Federal law as part of the clean air act. – Fred Wilson Sep 3 '15 at 15:21
  • Ill have to research what it can and can't do. From what Ive learned about engines though, a less restricted airflow, opens the door to more power. – Chris Manning Sep 3 '15 at 20:57
  • @ChrisManning Opens the door but doesn't bring any other benefits without further modification. To get any real performance benefit out of any change to the entire engine system would usually require a tune by a professional, the ECU just wont know that it has more it can make use of without being told. – Trotski94 Jul 20 '16 at 14:54

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