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What is point of high flow cats?

This is a discussion on What is point of high flow cats? within the External Engine forums, part of the LSx Technical Help Section category; Looking at options for headers and in an effort to remain compliant, I want something with a catalytic converter. All ...

  1. #1
    Senior Member Naaman's Avatar
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    What is point of high flow cats?

    Looking at options for headers and in an effort to remain compliant, I want something with a catalytic converter. All the high flow cats I can find indicate that they are for "non-emissions controlled vehicles" or "off road use only" etc, etc...

    If I have an "off road" car, why would I even bother to drop cash on ANY kind of cat at all? Can high flow cats pass a sniffer test, or what?

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    Member TA98's Avatar
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    Not to sure if they can or cant. Probably depends on how strict your state is on that stuff. I know out here if you relocate the cats that it is no longer an emissions legal car, cats must remain in the factory location. They probably write that for legality purposes though. Call them and ask. Some high flow cats are also made like the factory ones where theyre a ceramic honeycomb core, or whatever theyre made out of. Others are rolled up aluminum that coated with catalyst metals. I believe the stock style would be better for you. Look at Dynatech headers. Great quality and will do what you want. Ive installed many sets of these.

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    ʢ ൧ ൨ ൩ ൪ ൫ ൬ ൭ ൮Ր Ց Ւ Փ Smittro's Avatar
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    Depends on your state laws..

    I ran a 3.5" high flow cat on my OBD1 LQ1 and passed the sniffer test every time well under state minimum requirements even with a change in cam timing (DOHC) and a stage-III chip.

    But here in my end of PA. the sniffer test is only performed on OBD1 cars.

    OBD2 cars get hooked to an emissions computer and are checked for emissions codes.

    If the service engine light is on they won't even look @ it here.

    I suspect if your state performs OBD2 dyno sniffer tests and your tune is within emissions limits with a high flow cat you'll have no codes/SES light..

    So therefore pass the emissions test.

    The point of any high flow cat is to have a less restrictive exhaust flow while still passing/complying with state emissions law and/or a visual inspection of your equipment.
    Last edited by Smittro; 01-19-2012 at 10:28 PM.

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    Veteran 0rion's Avatar
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    Aftermarket cats will reduce emissions but if you're running LT's you'll fail a visual if you get tested. Federal law says it's illegal to modify your exhaust from factory. There are only 2 standards for the states to follow. They either follow the EPA's guidlines or California's ( CARB's) guidelines.

    One thing you will notice not running cats is the car's exhaust smell will be much stronger than if you run cats and will have a gas smell. It's never bothered me much but I've read a bunch of people over the years that didn't like this part of not having cats on the car. I have been sitting at traffic lights and have people tell me my car "stinks".

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    ʢ ൧ ൨ ൩ ൪ ൫ ൬ ൭ ൮Ր Ց Ւ Փ Smittro's Avatar
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    When I lived on the west coast they did'nt raise the vehicle for the emissions test.

    The vehicle was put on a dyno, connected to a computer, and a sniffer probe was added the tail pipe.

    They would then have you run the vehicle to 55 mph or so for a sustained period of time while they monitored the cars emissions and looked for emissions codes to show up.

    If your vehicle passed you then recieved your plate tag.

  6. #6
    they keep the ses light off thats about it

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    Moderator 35th-ANV-SS's Avatar
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    Quote Originally Posted by 0rion View Post
    Federal law says it's illegal to modify your exhaust from factory. There are only 2 standards for the states to follow. They either follow the EPA's guidelines or California's (CARB's) guidelines.
    This is correct.

    Anyone who has LT's and passed state emissions means they did not perform a "true" visual on the car. All LT's installed on cars are in fact illegal because they relocate the catalytic converters.

    With that said OP, if your state only uses the OBDII plug-in, you'll be fine. If they do a generic visual like most states who do a visual, as long as you have cats you'll be fine.

    Most likely after the installation of LT's and cats, you'll need a tune. If you don't install cats and go with an ORY, you'll definitely need a tune to remove the SES light which will trigger due to you not having your rear O2 sensors.

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    Moderator 35th-ANV-SS's Avatar
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    On another note, you may want to ask FBJ since he lives in AZ if you are not familiar with what your state does for emissions on your year of vehicle.

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    She Moderator KahanaReef's Avatar
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    Quote Originally Posted by Smittro View Post
    When I lived on the west coast they did'nt raise the vehicle for the emissions test.

    The vehicle was put on a dyno, connected to a computer, and a sniffer probe was added the tail pipe.

    They would then have you run the vehicle to 55 mph or so for a sustained period of time while they monitored the cars emissions and looked for emissions codes to show up.

    If your vehicle passed you then recieved your plate tag.
    That's still how it's done out here. Only we're not allowed in the vehicle while it's on the dyno. And while they don't raise the vehicle, they do use a mirror to check under the car for the visual

    I have to get a smog next month.

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    ʢ ൧ ൨ ൩ ൪ ൫ ൬ ൭ ൮Ր Ց Ւ Փ Smittro's Avatar
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    Quote Originally Posted by KahanaReef View Post
    That's still how it's done out here. Only we're not allowed in the vehicle while it's on the dyno. And while they don't raise the vehicle, they do use a mirror to check under the car for the visual

    I have to get a smog next month.
    That's one of the nice things about my end of PA.

    If you're under 5k on the clock for the year you're emissions exempt even in the city.

    If you own a farm (which we do) any vehicles registered @ the farm address don't have to be tested for emissions.

    The Z34 used to pass emissions every year, but then I started messing with it's tune, timing on the cams, exhaust, and modifing the ECM (OBD1) when I still drove it.

    Around that time I started turning some things off because it would no longer pass, and it's been exempt ever since..

    The Camaro never failed a single test the whole time I had her about 5-6 years.
    Last edited by Smittro; 01-25-2012 at 11:27 PM.

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    Veteran 0rion's Avatar
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    if we tested here mine would fail as soon as I pulled on the parking lot.

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    Moderator 35th-ANV-SS's Avatar
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    I don't have cats and I pass here in IN. Of course I have the rear O2's tuned out. It will be interesting to see how emissions go with the Procharger on this year. I wonder if they will question anything. I'm going to guess no since it's just OBDII plug-in.

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    Senior Member Naaman's Avatar
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    I just smog'd my car at school. Passed with flying colors as is (Magnaflow cat-back). Will need to do some more research, but I might go with shorties instead of long tubes, depending on whether the shorties can improve power in the lower/mid ranged rpms by a noticeable amount. High flow cat in my area might be okay, since my car will only be subject to a OBDII test, no visual inspection... A tune should clear up any possible DTCs that result from high flow cats.

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    Senior Member 1MileCrash's Avatar
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    looking into headers again and seeing this thread brings up a question in my mind if the original poster doesn't mind me theadjacking for a minute.

    looking around i see several companies offering headers with and without the egr hookups. personally i'd like to leave as much of the factory stuff on the vehicle as possible when it comes to emissions stuff. dynatech supermaxx looks great...egr hookups and a great y-pipe with cats...but at $1,400 for the setup...well the fiance would slaughter me.

    texas-speed also makes a very nice looking product in a long tube header and a catted y-pipe. the only question being is that the long tube headers they make don't have egr hookups, but they have the O2 sensor bungs on the collectors. would this cause any issues...having the O2 system hooked up but not having the egr system hooked up. i'd imagine that the most apparent would be an ses light calling an egr fault due to the egr not being hooked up but that could be taken care of with a tune. would there be any other driveability issues?

  15. #15
    Veteran 0rion's Avatar
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    no.......the tune will take care of any issues you would have. The light will come on for the egr but there is also a timing adder table for the egr that would need to be zero'd out. If not it would cause some tip in knock from time to time and pull a couple degrees timing. Like I say though......a tune will take care of all that.


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