When your Rx-7 was shiny and new, you may have thought the original catalytic converters should last the life of the car. Some people may still feel that same way. Unfortunately, that theory has a few caveats. Before going into why your cat is probably dead, let me take a minute or two to explain what a cat is, and how it works.

You probably already know that a catalytic converter is a piece of smog equipment. A converter's interior design determines what type it is. If the converter is filled with hundreds of metal beads, it's called a pellet type converter. If the inside of the converter looks like a honeycomb, then it's called a monolithic type converter. Monolithic type converters are a newer design, much less restrictive, and should be used in place of pellet type cats whenever possible. Cats are required because aside from carbon dioxide (CO²) and water (H²O), your engine produces hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). The theory behind a catalytic converter is to get hot exhaust gases to pass over a surface that has been coated with rhodium, platinum and/or palladium. These metals are known as the catalyst material. When the exhaust enters a catalytic converter, it cannot leave without coming into contact with the catalyst material. The chemical reaction between the precious metals and the hot exhaust results in a slightly less poisonous gas than what initially left the exhaust port. Some people argue that what leaves the cat isn't actually poisonous. If you've spent any time behind my car, you know the fumes are toxic.

Now that you have a basic understanding of how a catalytic converter works, how does it wear out? In a perfect world, the cat's internal temperature will be about 1200 degrees, the air fuel ratios are correct, and the cat will work forever. There are no moving parts, and the catalyst material doesn't get burned up in the chemical reaction, so what could possibly go wrong? Plenty. I've explained what happens in a best-case scenario. Over the life of a car, a number of things can cause a converter to fail. Usually, catalytic converters fail because either the engine has been running too rich, physical damage, or exhaust contamination.

These are pictures of a high mileage converter. Notice how the converter is clogged. This converter is clogged from antifreeze due to a blown coolant seal.

As far as physical damage, bottoming a car out on a speed bump, or hitting something (like a tire chunk from a big rig) at 80+ on the freeway will usually do the trick. That same friend who can't see the warning lamps typically performs these stunts. If the converter's outer shell crushes the inner monolithic brick, the brick will fracture. At that point, it's only a matter of time until the core rattles itself into oblivion. In the case of a pellet type cat, they have an inner seam that holds the pellets in place. If the cat gets hammered, and the seam rips, the pellets will escape the cat and possibly clog your muffler. At that point, you'll have to buy a new cat AND a pair of mufflers. Unless of course, your muffler happens to be an N1. In that case, anyone fortunate enough find themselves driving behind you will get bead blasted.

Exhaust contamination happens when anything, other than regular exhaust fumes, finds its way into the cat. An example is when a rotary blows a coolant seal. Coolant can escape past the seal, enter the rotor housing and exit through the exhaust port. The coolant's final destination will be the converter. Antifreeze will cause a converter to heat up as the converter tries to burn it. Unfortunately, the converter never does completely burn the antifreeze, and a residue is left over. This residue clogs the monolithic bricks. The same thing can happen with oil, transmission fluid, and to a lesser extent, poor quality fuel with too much sulfur. Transmission fluid typically finds it's way into the exhaust tract when owners repeatedly use it to build compression on a flooded engine, or when the vacuum modulator from an automatic transmission equipped car fails.

Suppose you don't fall under any of these categories. You haven't modified the car, it's always been in perfect running condition, and you don't have any friends. Do you still need a cat? If not now, eventually you will. Sulfur is present in crude oil. It's also present in gasoline. As sulfur passes through the exhaust, it naturally passes through the catalytic converter. Sulfur poisons the catalyst material in converters. Sulfur poisoning is a very slow and steady process that takes hundreds of thousands of miles to kill a California cat. Why are California cats any different from others? They're not. The fuel in California is though. In gasoline, sulfur content is rated by the number of parts per million (PPM). In California, the legal limit is 30 to 40 ppm, depending on the county. In 2002, the nationwide average was 330 ppm. In 25% of the United States, the sulfur content is 500 or more ppm. In recent years, the federal limit for gasoline has been as high as 1000 ppm. Based on that, I think you can see that where you live has a great bearing on how long your cat will survive. On the bright side, it appears the days of catalytic converter sulfur poisoning are coming to a close. The federal regulations are changing to more closely match those of the State of California.

Now that you have decided you need a new cat, what should you buy? If you want a high performance cat, pellet types are out of the question. That leaves monolithic cats. As you've probably guessed, all monolithic cats are not the same. What actually constitutes a high performance cat? Two words; high flow.

The first question should be, "How much flow should a new cat be capable of?" The obvious answer is, "If it flows as much as the engine, you're set." Figuring out how much air the engine flows, is another article. Really. Stay tuned, and we'll have an upcoming article on rotary flow numbers. Explaining the how and why of both the NA and turbo flow numbers would pull us in another direction. Although related, this article is about cats. Just use these numbers. An NA 13b, with 95% volumetric efficiency, at 8000 RPM, flows 352 CFM. Got boost? Using the same 13b, with 95% VE, at 8000 RPM, and 5 pounds of boost flows 472 CFM. 7.5 pounds of boost flows 531 CFM, 10 pounds of boost flows 591 CFM, 12.5 pounds of boost flows 651, CFM and 15 pounds of boost flows 710 CFM. These numbers are not meant to indicate that a 13b runs at 95% VE, or that a particular turbo produces a certain amount of boost at 8000 RPM. They are simply ballpark numbers. A word of caution about estimating flow and cat sizes; It's better to err on the high side. This is because if a converter is undersized, aside from being an exhaust impediment, it will swell up, turn purple, and should fail the visual portion of any smog test.

Now you know what you want; a high flow monolithic beast. OEM parts won't do for two reasons; (1) OEM cats cost a fortune (the retail price for my OEM main cat is over $1400), and (2) being a stock part, I'm not sure they qualify as a "high flowing monolithic beast". Lets look at two approaches some people take when choosing an aftermarket high flow converter. Some take the approach that if a converter is certified for one type of vehicle, it should work wonders for their application. For example, a person may look up the part number for a universal high flow cat that's federally certified for a 82-85 Mustang GT, and use that. It seems logical. If a single cat meets the needs of a 302 cubic inch V8, it should be able to handle the needs of an 80 cubic inch rotary. Another way of choosing a cat is to look for the physically largest unit you can find. If the inlet and outlet pipes are three inches or larger, and the converter body is larger than the original equipment part, it seems logical that the converter should flow more than a smaller unit. Unfortunately, using either one of these methods to choose a high flow converter may, or may not, result in you purchasing a true high flow converter. On a federal level, aftermarket catalytic converters are certified based on engine size and vehicle weight. All we really know about the universal high flow unit that is "certified" for the Mustang GT is the Feds said it was a good match for a 302 cubic inch engine in a 3400-pound vehicle. The federal certification for that exact same universal cat may also make it a match for an F150 pickup with a 300 cubic inch inline six. That high flow cat doesn't sound so impressive when "F150 pickup" is used in the description, does it? As for the super sized cat, all we really know is that it's big. The problem with using either method is the buyer hasn't been told anything about the airflow capacity of either cat. Airflow is what we're looking for. The monolithic bricks within a cat are the sole impediment to exhaust flow. The most common way of making the cat flow more is to have short bricks. This is the exact opposite of the "bigger is better" train of thought. Think about this - which is harder, to blow through a long straw or a short one? The problem with using short bricks is cost. The catalyst material must be more heavily applied with short bricks to make up for the lack of surface area. Since the catalyst material is the most expensive part of building a catalytic converter, a heavy application drives the price up. If a manufacturer were to use short bricks and a light application of the catalyst materials, the car would fail smog.

So what's the best way to find a good high flow catalytic converter? Two choices come to mind. You can let someone else do the legwork, or you can do the research yourself. How can you let someone else do the legwork? Contact the largest rotary engine based car club you can find, and ask them. Ask the members who have a car like yours what they use, and if they are satisfied. Car clubs are where the real knowledge base exists. The members of a large car club will have real experience from a number of different perspectives. I believe this perspective will prove superior to that of the clerk at your local auto parts chain store. If you go the other route and do the research yourself, contact catalytic converter manufacturers and ask for specs. Ask them about the warrantee, how long they have been in business, whether the cat comes with a CARB exemption, and don't forget to find out about the flow numbers. If a manufacturer produces a cat that can truly flow four to five hundred CFM, they probably mention it in one of their brochures. If the company doesn't have any air flow information in writing, ask them why. A company without product specs really isn't doing much to earn your business. Companies that do put their specs in writing can either back it up, or they find themselves being exposed. Once exposed, it's only a matter of time before the Feds step in and make the company change their advertising practices.

While we're on the subject of the Feds, now would be a good time to wrap this up by talking about what we are legally allowed to do. I live in California, where it is illegal to use any aftermarket cat that does not have a valid CARB (California Air Resources Board) certificate. Basically, the CARB certificate is proof that a particular part is certified by CARB to be used on a particular vehicle. As an example, it isn't legal to bolt a Hyper-Flow cat on your Rx-7. Although Hyper-Flow cats have CARB certificates, the certificate would not be valid, because it does not apply to the Rx-7 application. You can also run into problems attempting to run a single main cat in a high boost environment. I have never heard of a single cat that flows in excess of 560 CFM. They may exist, but I have never come across one. That being the case, if your 13b flows more than 560 CFM, you will need two main cats in parallel to handle the flow. That is where the problem comes in. Regardless of what your local or state emissions test requirements are, according to the Federal Clean Air Act it is illegal to change the number of factory catalytic converters, move them from the stock location, or change converter types. This means any type of true dual exhaust with parallel cats in a 13b application is not legal. This also means if the original cat left the factory with air injection, the replacement cat must have it as well. The good news is, for most cases, a rotary owner can find a high flow cat that is legal, and meets their air4flow requirements. Besides, now you have another reason to contact and join a car club.