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Why 2 CATS?  (Not the purring kind)

Discussion in '80-Series Tech' started by XXXR, May 2, 2003.

  1. XXXR

    XXXR

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    Why in the heck does my rig have two cats anyway? No other vehicle I own has morethan one. What if I leave #2 off? I'm assuming that there is one O2 sensor... if there are two why??? Please help the ignorant stepchild on this!
     
  2. archie

    archie

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    Re: Why 2 CATS?  (Not the purring kind)

    my 88 fj62 has two cats and two O2 sensors. never figured it out also. Wondering in Chicago.
     
  3. Jacknasty

    Jacknasty

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    Re: Why 2 CATS?  (Not the purring kind)

    Good question.

    My little brother works at a muffler shop and I am going to get him to do some custom exhaust work done to my rig. My 4runner had a high flow cat and a Super Dynomax muffler. It sounded great. But, this LC has a lot better tone to it out of the box. I can't wait to hear the rumble I get from the new exhaust.

    I haven't talked to him about the two cats, but I would rather not have one of them to allow my LC to breathe a little better. I hope I don't "need" both of them
     
  4. Scamper

    Scamper

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    Re: Why 2 CATS?  (Not the purring kind)

    Seeing as nobody else took a stab, I toss in a WAG, daredevil of flame that I am:

    If you assume that any single cat can scrub exhaust clean with 50% efficiency (removes 50% of the pollutants), then you could get to 75% clean by adding a second cat (0.50 x 0.50 = 0.25 or 25% pollutants remaining). Adding a third cat would get you to 87.5% clean in this example, etc.

    This sort of process is very common in scrubbing in chem labs where you do consecutive scrubs/washes rather than one large scrub for efficiency purposes.

    I don't know what the actual efficiency of a cat is, nor do I know if this explanation is anywhere's near true. But it would make sense in the real world. It's also possible that the two cats are constructed of different materials where one works best at very high temps (upstream, near engine), and the other works better at slightly cooler temps (downstream)--but again, I don't know :G

    Tom
     
  5. Gumby

    Gumby Supamod Staff Member s-Moderator

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    Re: Why 2 CATS?  (Not the purring kind)

    A couple more WAG's.
    One cat works in country X, US needs two.
    One cat is not efficient enough and a bigger one wouldn't fit without heat/ fitment problems.

    If you live in an emmisions state, I would be hesitant to put on only one replacement cat. Of course, you could retrofit a second one if you fail. My state, IL, runs a dyno test that I don't have to take 'cause they only equip the stations with one axle dynos. :D
     
  6. yomama

    yomama

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    Re: Why 2 CATS?  (Not the purring kind)

    Had two cats 2 1/2"....now I have one cat 3" (High Flow) and a Highflo muffler 3", still need both o2 sensors though. One o2 sensor reads before cat and the other is after the cat, the reason for this is to determine your air/fuel ratio, or I should say fuel ratio as I don't believe the air is adjustable (no butterfly).
    Also, I have a Y pipe at the rear so I have dual chrome tips coming out the back. Looks like a dual exhaust system and looks cool however I am lacking part of the cool sound I probably would have due to the turbo, but at idle it sounds throaty.

    Yomama
     
  7. moralien

    moralien

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    Re: Why 2 CATS?  (Not the purring kind)

    XXXR,
    I don't think it is so much 2 cats to clean twice as much as it is fitment issues. It would be a huge cat if you had to put all that honycomb material into one large unit. Also remember that the #1 sensor has a heater installed in it. Upon startup before the cats have a chance to reach nominal operating temps, this heating compensates for it. So it could be the first cat is for lower temps and the second cat is for normal temps? Dunno... All I know is they are buggers and I don't think any one else has to run them. Maybe somebody from across the pond can chime in...

    Yomama,
    What's the point of the dual tips? It makes no difference if you Y it since there is only one exhaust manifold... Seems to me that's like putting socks on a rooster :dunno: Also it's just more stuff to "trim" on the trails.
     
  8. PHAEDRUS

    PHAEDRUS

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    Re: Why 2 CATS?  (Not the purring kind)

    I am guessing your rig is a 93-97 as I have a different reason if it is a 91-92. on the early models there are twO cats, first three cylinders and second set of cylinders. two seperate exhaust manifolds wich feed into two seperate cats wich y down to the exhaust line back.
    If yours is the later model then I believe scamper is on the ball on this one although there might be a temperature range issue as well.
    The long and short of it is you might be able to go to one cat that is high flow but I would be wary of it on the later series (1fze motor) as they are obdII compliant with the second 02 sensor. make sure the cat is designed to be able to filter enough so that it will not impede the performance of the secondary 02 sensor.
    just my .02
    Dave :beer:
    P.S.
    just a little dig here... 3FE KICKS ASS!!
     
  9. moralien

    moralien

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    I have a 95Re: Why 2 CATS?  (Not the purring kind)

    I have a 95 with the two inline cats... Now just the one high flow cat. So far so good with no codes as of yet. The O2 sensors should not pull a code unless something is wrong with them, there is a blockage of the cat material or there is no cat present at all. Again just guessing. FWIW there are O2 simulators available to simulate the presence of the sensor as to not foul up the OBD. So anything is possible if you’re creative enough. :)
     
  10. yomama

    yomama

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    Re: Why 2 CATS?  (Not the purring kind)

    "Yomama,
    What's the point of the dual tips? It makes no difference if you Y it since there is only one exhaust manifold... Seems to me that's like putting socks on a rooster Also it's just more stuff to "trim" on the trails. "

    Huh? moralien,
    Are you kidding? &nbsp:Did you know that 80% of the Dodges, Fords and Chevy's have both manifolds connected and then Y them at the rear for sound, you didn't really think they were REAL dual exhaust systems. Yes it does a make a sound difference. They are also as high if not higher then stock.
    Hope that made sense, too many :beer: :beer: :beer: last night...

    Yomama
     
  11. moralien

    moralien

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    Re: Why 2 CATS?  (Not the purring kind)

    Not to get in a s$@t kicking contest with you but, if the big 3 choose to do that it's their prerogative. You’re taking your flow and equalizing it from TWO different sources. Not a single source and splitting the difference (what your doing). Also if you have a turbo it's seems to me it would sound better coming from one pipe. Just my .02 and to each his own... :beer:
     
  12. XXXR

    XXXR

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    Re: Why 2 CATS?  (Not the purring kind)

    Well, I will assume the WORST since I live in the mighty state of evironmental socialism. (California) Boy, two cats look like they are worth their weight in gold. Hummm... thanks guys for the overwhelming responses.

    Des
     
  13. moralien

    moralien

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    Re: Why 2 CATS?  (Not the purring kind)

    It should read they are worth their weight in platinum, palladium and rhodium. These jokers are so expensive because they rely on trace amounts of said materials to be coated all across the cat media. The kicker is (total laymen working from memory) that the vast amount of palladium is marketed thru the Former Soviet Union. Unfortunately the Mob controls everything lucrative now, thereby making cats even more expensive. Imagine them driving up costs to the big players of the auto industry. Ford, Honda, Gm... All getting collectively screwed by the mob. Talk about poetic justice :eek: