Cat removal: Worth it for MPG? How will the computer handle it?

Discussion in 'Performance & Fuel' started by SurrealOne, Apr 23, 2013.

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  1. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    So I've been considering removal, bypass, or emptying of my cats for performance and fuel economy reasons. Does anyone have an idea of what kind of MPG improvement this might yield due to the reduced exhaust restriction in the system? Also, how will the computer handle it given that the upstream and downstream O2 sensors will read about the same? Will it throw idiot lights? Would I need to disable my downstream O2 sensors with my handheld programmer?

    Advice wanted. I'm weighing the benefits versus the known drawbacks (among which I include the various fines). I'm just doing my homework for the time being so that I can make an informed decision.


    P.S. Yes, I know I won't pass emissions tests if I do anything other than a bypass (a la electric cutouts). I'm not worried about that.
    Last edited: Apr 23, 2013
  2. dobey

    dobey Epic Member 5+ Years 500 Posts

    You will basically see no difference in fuel economy, unless your cats are old and worn out, and need to be replaced. Then you might see a little bit of an improvement, but probably only 1 MPG at most, maybe 2 if they are really bad. In fact, you're more likely to lose MPG, because the ECM will have less data about how much fuel to burn.
  3. stchman

    stchman Active Member 2 Years 1000 Posts

    You will have to check if what you want to do is legal in your area.

    Remember, the ECU is programmed to have a catalytic converter(s) present. Bypassing them without re-programming your ECU will more than likely have unintended bad effects.
  4. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    [MENTION=57614]stchman[/MENTION], the legality is a separate issue ... one I'm well-versed with at both the local and federal level ... and something not to be confused with what I'm asking. I do, however, appreciate you looking out for me by mentioning it.

    I would, indeed, expect to have to turn the downstream O2 sensors off in my custom tune if I did something like this -- so I'm prepared for that.
    [MENTION=40835]dobey[/MENTION]'s input matches what I surmised -- that there'd be performance gains but likely MPG losses due to lack of data (a la the disabled downstream O2 sensors). Some people claim there's nothing but gains by removing these restrictive devices. I've read pros/cons from both sides.

    I'm really after hard data from someone who has DONE it... rather than mere speculation.
  5. tbplus10

    tbplus10 Epic Member Staff Member 5+ Years 5000 Posts Platinum Contributor

    From my experience on modern vehicles (1990's and up) removal of a good working cat rarely achieves anything and normally results in slight loss of torque at low rpms, and a small loss of MPG.
    Early model cats were real restrictive, but by the late 80's manufacturers learned how to make them work much better with an engine and designed them to be integrated instead of just a restrictive add on.
  6. TimTom64b

    TimTom64b Epic Member 5+ Years 500 Posts ROTM Winner

    I 2nd this.... I think you will loose mpg. It will definately have a different sound.... maybe even more drone. If it were me I replace them with high flow cats
  7. SurrealOne

    SurrealOne Former Member ROTM Winner 1000 Posts

    Thanks, all. That's enough people whose opinions I trust for me to have a clear picture that it's a bad idea no matter what I've read, elsewhere. The advice given, here, is also directly in line with what a mechanic (who I trust, completely, and is one of two I will let touch my truck) mentioned, today, while chatting over lunch.

    My mind's made up; I'm not going to consider the matter, further. @tbplus10, would you be so kind as to close/lock this thread so that no more time is wasted on it ... and also to keep it from being resurrected and posted in two years from now when someone does a search, sees it, and posts a response without looking at dates? :neutral:
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