I'm wondering if you can install a cat backwards. Most aftermarket ones I've seen do not list an inlet or outlet. I know there are also some differences with 2 way and 3 way cats. Anyone have a definitive answer?

EDIT: I'm actually curious about both sides of this. From a flow standpoint there should be little if any difference. From the catalyst standpoint though I'm unsure if changing the order of the stages will have a serious effect on emmisions or not. Hopefully someone know for sure.

[This message has been edited by Fieroseverywhere (edited 03-29-2010).]

i have a new in the box replacement supposedly HO (looks the same to me) cat and it's got an arrow stamped into it with "FLOW" inside the arrow. Also aren’t the inlet and outlet sizes different? I would think that the inlet would be 2" ID and the outlet 2" OD. I don’t have it in front of me but I would think that it wouldn’t fit on backwards.

Edit to say that I don't know what happens if you DO install it backwards, just saying that i don't think you can easily.

[This message has been edited by DLCLK87GT (edited 03-29-2010).]

I recently picked up a new cat for my 84SE. Unlike all the others I've used in the past this one DID have a flow arrow like yours does. This of course makes me question some of the others I've used in the past.

Of course being me I have to complicate things further...

I have been planing to re-design the exhaust for my 4.9 fiero. I'm currently using this cat for the swap...


So with this one the dual 2" inlets and a single 2.5" outlet are already different sizes. It also does not have a flow direction stamped on it anywhere. The only indication is the location of the O2 bung. This may not be a true indication though as many newer cars have second O2 mounted after the cat. Maybe being a "universal" cat has something to do with this?

Until the recent replacement on my 84SE I had never seen a direction stamped on one before. The design I had planed for my new 4.9 exhaust involves purposely reversing the cat and plugging the O2 hole. The O2 will then be mounted in the exhaust manifold instead, keeping it ahead of the cat. Now I'm not so sure if thats possible or not.

[This message has been edited by Fieroseverywhere (edited 03-29-2010).]

quote Originally posted by Fieroseverywhere: Of course being me I have to complicate things further...

yes, i agree! I don't believe the material inside a cat is directional but i am no expert.

Considering what you can see from the inside, and from what I know of how they work, my guess is that the flow arrow is so they don't get calls from people asking which way to mount it if it didn't have an arrow. Both inlet and outlet are the same size? Maybe the heatsheild is used on more than one converter and has an arrow for the ones with different sized inlet/outlets? Or if they have a bung for a second O2?

So my answer is I think it should work either way. I don't remember ever seeing a flow arrow on any others.

I've done a bit more checking. Its looking like the 3 stage cats ARE in fact directional. Because of the catalyst and how it works the 3 different stages need to go in order to function properly.

quote the converter is installed backwards. yes, the converters are 3 way dual bed catalytic converters and have a directional construction. it will reduce oxides of nitrogen first to free the oxygen to help with the hc and co, but it needs co to help reduce oxides of nitrogen. if it is backwards, then the co will be reduced first, and none will be available to reduce oxides of nitrogen. the converter should be marked with an arrow to indicate the flow direction.

I can't find a deffinate anwser yet but it appears the 2 stage ones do not matter. This is probably why some have a flow arrow and others do not. If this is indeed true then I should have no problem doing the exhaust the way I want to since this is a 2 stage cat. There are alot of "if's" there so I'll still be checking til I find a positive answer.

[This message has been edited by Fieroseverywhere (edited 03-29-2010).]

This might help

http://auto.howstuffworks.c...lytic-converter2.htm

------------------
Dave

www.ltlfrari.com

quote Originally posted by ltlfrari: This might help http://auto.howstuffworks.c...lytic-converter2.htm

Hehe. I've read that one twice today.

This is of some help. The differences between 2way and 3 way cats...

quote Two-way A two-way catalytic converter has two simultaneous tasks: 1.Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2 2.Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2] O2 → xCO2 + (x+1) H2O (a combustion reaction) This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on spark ignition (gasoline) engines in USA market automobiles through 1981, when the two-way converter's inability to control NOx led to its supersession by three-way converters. [edit] Three-way Since 1981, three-way catalytic converters have been used in vehicle emission control systems in North America and many other countries on roadgoing vehicles. A three-way catalytic converter has three simultaneous tasks: 1.Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2 2.Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2 3.Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O These three reactions occur most efficiently when the catalytic converter receives exhaust from an engine running slightly above the stoichiometric point. This is between 14.6 and 14.8 parts air to 1 part fuel, by weight, for gasoline. The ratio for LPG, natural gas and ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels. Generally, engines fitted with 3-way catalytic converters are equipped with a computerized closed-loop feedback fuel injection system employing one or more oxygen sensors, though early in the deployment of 3-way converters, carburetors equipped for feedback mixture control were used. While a 3-way catalyst can be used in an open-loop system, NOx reduction efficiency is low. Within a narrow fuel/air ratio band surrounding stoichiometry, conversion of all three pollutants is nearly complete. However, outside of that band, conversion efficiency falls off very rapidly. When there is more oxygen than required, then the system is said to be running lean, and the system is in oxidizing condition. In that case, the converter's two oxidizing reactions (oxidation of CO and hydrocarbons) are favoured, at the expense of the reducing reaction. When there is excessive fuel, then the engine is running rich. The reduction of NOx is favoured, at the expense of CO and HC oxidation.

Since 2 way cats no not catalyze NOX there is no reason for them to have a flow direction. Makes sense. Should have paid better attention in chemisty class!

So long answer cut short... Follow the flow arrow if the cat has one. If not no need to worry.

[This message has been edited by Fieroseverywhere (edited 03-29-2010).]

Oh, mine is a simple one, not a fancy 3 way dohickey

Or what you can do is take a really big pry bar and ram it through the cat smashing all the stuff inside and then dump it all out.....that way you'll know it works either way! .... and before anybody says anything about the environment and blab blab I'm just kidding....maybe.

[This message has been edited by DLCLK87GT (edited 03-29-2010).]

put in a straight pipe like i did.. they work either way... lol ..

If it has two bricks in it it should only be run way, if it has a single brick it most likely does not matter. There is no difference in flow running them either way.