Catalytic Converter Question

[FTL_Ian]

What's the scoop on these?  I've heard they negatively impact efficiency, and heard they help efficiency.   ???

[citizen_142002]

I'm not a mechanical genius or evn 100% sure how a cat works, but as I understood it, they help reduce emissions. Now a car may be more gas efficient, but not necessarily able to perform well. I thought that by removing the cat you got more power and less cleanliness/efficiency, but I'm just making inferences from what I'v heard of conversations among people who know what they're talking about.

[mvpel]

Random Technology:

For years, it has been a common belief that all catalytic converters place a choke-hold on performance. In 1993, Random Technology disproved that theory with the introduction of SuperStainless free flow catalytic converters.

Direct-fit, bolt-in SuperStainless converters are available for most vehicles. Click here for application charts that contain specific part numbers for direct fit converters (no welding reqired) Direct fit converters have all necessary flanges and oxygen sensors (if required) as the original equipment converters. Click here for dimensions of universal fit converters which are designed to be welded or clamped in place. Universal fit converters have the same flow characteristics as direct fit models with the same inlet and outlet diameter.
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On another page of this site:
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Owing largely to early designs, which were very restrictive, catalytic converters are widely viewed as horsepower killers. And while even the most free-flowing converter will increase exhaust back pressure, the effect on performance can be minimal. Independent dyno testing has repeatedly proven that most engines produce just as much power when equipped with high efficiency catalytic converters as they do when running through straight pipes. So it just doesn't make sense to drive a car that isn't emissions legal.
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[eques]

From what I've heard, the function of the catalytic converter in my Honda Civic 2002 is to reduce some forms of emissions by burning off unburnt gasoline... I don't know if it does some kind of carbon monoxide reclamation or reburning or whatever.  I think it's safe to say that catalytic converters probably reduce fuel efficiency in terms of miles per gallon, but they also reduce emissions.  It's sort of a trade-off.

I think that some catalytic converters might (MIGHT) send unburned fuel back to the engine or back to the fuel tank, but don't quote me on that.

Even without the burning off of fuel and the like, a catalytic converter restricts the flow of exhaust from the engine, and anything that does that (like a muffler, for example) would probably reduce overall fuel efficiency.

But we have mufflers on cars to reduce noise pollution, and the trade-off is that cars aren't as noisy.

Now, well, I'm no expert or nothin', so maybe all I've said about catalytic converters is just smoke that was blown up my ass by somebody else who got smoke blown up their ass in turn.  But, er, it kind of makes sense, if that's truly what the function of the catalytic converter is.

[mvpel]

Eques, Google means that there's never any need to repeat ass smoke.

The first hit, in fact, is "How Catalytic Converters Work."  To wit:

The Reduction Catalyst
The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the NOx emissions. When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:

2NO => N2 + O2 or 2NO2 => N2 + 2O2

The Oxidization Catalyst
The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidizing) them over a platinum and palladium catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:

2CO + O2 => 2CO2

But where did this oxygen come from?

The Control System
The third stage is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter is. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point [the ideal air-fuel ratio], and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.

[FTL_Ian]

If my catalytic converter is bad should it be replaced, or left bad?

[KBCraig]

If my catalytic converter is bad should it be replaced, or left bad?

Well... "bad" can mean two things. Once, that it's no longer doing its job, so your car is polluting more. If you have to have an exhaust emission test, it will fail, and you'd have to get it replaced to pass inspection.

The other "bad" is that you have increased backpressure, meaning it's restricting your exhaust. If your exhaust doesn't flow freely, it costs you power and mileage and can lead to other "bad stuff".

A good muffler shop can test the flow rate of your cat, but they also have a vested interest in selling you a new one. Risky, if you don't have a mechanic you can trust.

Modern cars are designed with the cat as part of the system. Removing it doesn't automatically give them more power or increase mileage; it may even decrease performance.

Kevin

[mvpel]

Normally a catalytic converter should last the lifetime of the car.  The precious metal catalysts are not consumed during the process.

Aside from the obvious issues of physical damage, like hitting a curb or the like, catalytic converters can become plugged up if oil or antifreeze has leaked into the engine's exhaust path, through a broken gasket.  The residue from burning off this leakage will remain in the cat and plug up the small channels.  This will obviously increase backpressure and decrease engine mileage and performance, and the cat should be replaced, but not before fixing the gasket problem that killed it in the first place.

The site I posted above has a lot of good information.

[citizen_142002]

I do know that when catalytic converters go bad, it can lead to an unplesant odor. Since the exhaust gets kind of back up. If the car starts to stink, then you might need a new cat.

[eques]

I must be having an intermittent problem with my catalytic converter, then, because every so often, when I'm driving, the car just smells BAD.

I'm sure that eating at Taco Hell has nothing to do with it. 

[David]

I'm sure that eating at Taco Hell has nothing to do with it. 

Are you sure about that?   

[Roycerson]

Catalytic converters can clog.  That's bad.  If your state has safety inspections that require a catalytic converter be present but no emmissions test than you can hollow out your catalytic converter and render it both useless and harmless and still pass inspection. 

I stopped for a guy at the side of the road once and his was literally glowing red from clog induced heat.  The car couldn't have been running well before that.  They are heat makers in the best of conditions and have been blamed for starting the occasional grass/forest fire.  Someone somewhere had numbers once that said if just one significant wildfire was caused by a catalytic converter than the CO2 and other pollutants emitted by that fire is greater than all the pollution prevented by all catalytic converters in the US since they became mandatory.  It was a long time ago; I didn't find the article with a quick google search.  If true it's another item the "the government is stupid" pile.

[coffeeseven]

I do know that when catalytic converters go bad, it can lead to an unplesant odor. Since the exhaust gets kind of back up. If the car starts to stink, then you might need a new cat.

I just put a brand new one on my Geo Metro. It already stinks. It's a trade off. Smell raw exhaust, or cat stink. I dunno why we're still buring fossil fuels anyhow.  Because we can?

[Recumbent ReCycler]

A while back, the catalytic converter in my Jeep Wrangler developed a hole in the outside, so I replaced it with a high flow performance catalytic converter.  You should be able to find a good one on ebay for a reasonable price.  That's where I got mine.

[KBCraig]

Just keep in mind that if you're looking to replace your current cat with a high-flow version, most muffler shops won't touch it. Believe it or not, it's illegal to remove a working catalytic converter, even if you're replacing it with another one. Even if the replacement would reduce pollution by increasing mileage. 

But if you've got the tools and skills to do it yourself, or if you know a shadetree mechanic who'll do the work, then go for it. Don't expect a quick payback, though.