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Rotary Tech: Catalytic Converters
Submitted by SuperUser on Thursday, March 13, 2003 - 6:17pm

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.

Partially clogged converter

Partially clogged converter

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.

In a high performance application, it's common for the fuel mixtures to be
off at some point. Too much fuel will cause the cat to overheat. Do you
have a set of oversized injectors, additional injectors, or an aftermarket
ECU? Usually the catalytic converter is in place while the engine is tuned
for the additional fuel. That being the case, the converter could have
been ruined while the fuel system was being dialed in. There can be also
be problems if you haven't modified the car. Has your car ever had dirty
fuel injectors with bad spray patterns? What about leaky injectors - the
kind that bleed 40 PSI of line pressure directly into the intake port
every time the engine is turned off? Has your car ever had weak coils,
dead plugs, or near-dead spark plug wires? Any one of those problems can
make the car run rich. Here's one most rotary owners know all too well:
Has your rotary ever had a problem with flooding? If a rotary floods,
excessive fuel will wash into the catalytic converter. All of these things
cause the converter to overheat. If the converter overheats, the catalyst
material can burn out. If the converter gets hot enough, the inner core
will melt. You usually have some warning before the core melts down. The
"OVERHEAT EXH SYSTEM" warning light will typically be on for some time
before your exhaust system does it's impersonation of Chernobyl. Of
course, if you've loaned your car to a "friend", they will no doubt
conveniently ignore the light and continue to drive the car until the
engine can't rev higher than 2000 RPM. In those cases, try to look on the
bright side - at least it wasn't the temperature gauge they were ignoring.

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.

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cat clogged?
No Rotor
December 18, 2004 - 8:21am
is there any additive to clean a maybe clogged cat?
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No Rotor
December 31, 1969 - 4:00pm
I was wondering about the back pressure to induse operation of the secondary injectors. Can we have too much free flow.
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No Rotor
December 31, 1969 - 4:00pm
Thanks for this and future technical articles!
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Re: Injectors
No Rotor
December 31, 1969 - 4:00pm
You were wondering about having too much free flow and how that may effect secondary injector performance. The secondary injectors are not effected by exhaust back-pressure. The only things that are affected by exhaust back pressure are the six port actuators on an 86-88 NA (the 89-92 NA's use the air pump rather than the exhaust) and the amount of boost produced in the turbo models. A high flow cat will produce enough back pressure in either instance so as not to cause any problems.
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