hyperion

iTrader: (2)

Let's not forget that the airpump actually injects air into TWO places:

1) the main catalyst (in the mid pipe)
2) the exhaust port (for the pre-cat, presumably)

The air pump is attached to the ACV which is hidden behind the lower intake manifold, right between the injectors. There are passages that run down the intake manifold that actually meet up with the exhaust. These are the same passages that the EGR valve uses to introduce exhaust gas into the intake. The ACV also shunts some of the flow to the pipe that runs to the main cat.

Here's a related question: why does the air pump affect the idle during warmup but not after? You can verify this by unplugging the car while the AWS is running. Is the AWS somehow linked to the air pump? After warmup the idle remains constant regardless of whether the air pump is plugged in or not...

-ch

Jonesboro

All of this catalytic converter overkill stuff on the FD is in response to the fact that it is tuned by the factory to run very rich. My understanding is that the precat was added to the FD because the air injected main cat alone was not enough to meet the cold start emission limits that apply to new vehicles. Apparently the precat being so close to the heat from the exhaust manifold didn't need the assistance of air injection.

The annual smog tests many of us have to contend with do not test for emissions during cold start conditions so that is why we can get away with a downpipe instead of the precat. Although, technically the car should flunk the "visual inspection" without the precat because it was originally required. I've never heard of anyone being flunked because of a downpipe.

David Beale

I have been told the newer "three way" cats do not need an air pump. When I first removed my airpump (no cats were on my car when I bought it) it messed up the idle, cold or warm. The A'pexi PFC fixed that! Its' removal did not clog any cats ;-).

As for the cat failures, the older style ones (90s) are supposed to be replaced at around 80k miles anyway. I would expect the honeycomb to collapse at 100k miles, just due to the heating cooling cycles it endures.

Matt Hey

I believe the PFC does NOT operate the split air pipe but I don't here PFC owners complaining of clogged cats. I also agree that newer 3 way cats shouldn't need more air. Here is what Random Technology has on their web site...

Although "three-way plus oxidation" type converters were prevalent during the 80s, that's no longer the case. With improvements in washcoat technology, and improved control of air/fuel ratios, the need for additional oxygen has been eliminated. Some vehicle manufacturers have continued to use oxidation converters on some models, but typically that has been done to use up inventory. As an example, the Corvette and Camaro Z/28 were equipped with oxidation converters through 1991 and 1992 respectively. But when the LT1 engine replaced the L98 (1992 in Corvette, 1993 in Camaro) three-way converters with no air tubes were incorporated.

I concluded that the no air pump kills cats rumor was unlikely true a while ago. I took my air pump off and run a higher flow cat (with PFC). I haven't had the setup for long enough to draw any conclusions.

kwikrx7

Here in PA (in the Harrisburg area anyway) they don't do emissions testing but do a visual...so you have to have a catylitic converter to pass inspection.

Even if midpipes were "legal" in this state I would go with a HF cat simply because you have more low-midrange torque, it's a lot quieter, and it doesn't stink like a truck stop.

My HF cat didn't have a nipple for the airpump because it was cheap - Dave at KDR told me the cat would eventually fail without the airpump but it would take years for it to happen.

Tom93R1

iTrader: (2)

I have a Shane Racing high flow cat. It came with no air pump inlet. I ran it like that for a few months, the previous owner ran it like that for who knows how long. When it came time for emissions I couldnt pass. I had Meineke weld up an inlet and I hooked up the airpump and passed with flying colors.

rotorbrain

thank you rynberg. . . if you are trying to pass emissions. . . you NEED an airpump. . . if you are worried about emissions that much. . . keep it all. . . cause you WILL NOT pass w/o it. . . unless you have portable airtanks in your trunk area and are injecting it into the exhaust. most normally tuned rotarys WILL NOT pass w/o an airpump. . .

paul

Tim McCreary

Here is the lowdown of why a catalytic converter is installed in a car. Threeway cats are to take care of the three harmful compounds. The reason that the air is injected in the middle of the cat is to provide oxygen to finish the catalyst conversion of Carbon Monoxide to Carbon Dioxide and to "burn" unburned hydrocarbons safely without an actual flame or explosion. Without the air pump, you will not complete this process properly. Eventually, you will generate heat or buildup that will either overheat and deteriorate the ceramic honeycomb creating failure and blockage or will generate buildup creating blockage and failure. The following is a description of what a catalytic converter does:

A catalytic converter is a device that uses a catalyst to convert three harmful compounds in car exhaust into harmless compounds.

The three harmful compounds are:

Hydrocarbons (in the form of unburned gasoline)
Carbon monoxide (formed by the combustion of gasoline)
Nitrogen oxides (created when the heat in the engine forces nitrogen in the air to combine with oxygen)

Carbon monoxide is a poison for any air-breathing animal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons produce smog.
In a catalytic converter, the catalyst (in the form of platinum and palladium) is coated onto a ceramic honeycomb or ceramic beads that are housed in a muffler-like package attached to the exhaust pipe. The catalyst helps to convert carbon monoxide into carbon dioxide (NEEDS OXYGEN). It converts the hydrocarbons into carbon dioxide and water (NEEDS OXYGEN). It also converts the nitrogen oxides back into nitrogen and oxygen(GENERATES SMALL AMOUNTS OF OXYGEN).

My comments are in capital letters in parentheses. As you can see, without an air pump, you cannot complete the reactions necessary to clean the air. Also, if you had enough Oxygen to complete these without the aid of an airpump, you would be running lean and we all know what that means to a rotary.

The reason it is important to put a cat on a rotary engine is due to it's design. The reason they moved the exhaust port to the side in the RX8 is because the unburned hydrocarbons are just wiped out to the exhaust by the apex seal. The Renesis engine eliminated this from happening which increased fuel economy by 35% and decreased emissions by 50%. Someone else hit it on the head why the precat was installed (to meet emissions because the engine was an emissions hog).

Back to the original question: Does the HF Cat clog without an air pump. The answer would be YES. Defining the "Clog" in a cat would be anything that would help deteriorate the interior of the cat to the point that the ceramic would break down and eventually block the flow. Any Cat will clog over time without an air pump. How long? That depends. It may last a few months or a few years. The question I would ask is why spend the money on a HF Cat when you can put a straight pipe on for much less. If the answer is emissions, then fix it right and help save the environment a bit so we can continue to run our emission hogs.

Remember also, without the air injected into the cat, the Cat will achieve much higher temperatures than with the air. These higher temperatures will lead to premature failure of the ceramics and eventually clog the cat flow.

High Flow Cats are much less likely to clog compared to the older bead type (highly restrictive and usually melted the beads together due to high heat and close contact). There are HF Cats that produce the same back pressure as the straight pipe (Random Technologies had an independent study done on an RX7 with their HF Cat and a straight pipe and found the same back pressure).

Tim

DamonB

Excellent point.

SyderJL

Actually the Cat. will run cooler if the air is not injected into the Cat.(no air pump) This is becuase it doesn't have the extra oxygen from the air injection to burn, so it will burn less and run cooler.

Although since the cat is burning less without the air injection it cannot burn as much of the emmissions and like it was stated before that is why the air pump is there to help burn more emmisions.

No Air Pump, Cooler Cat.

Tim McCreary

Logical thinking, but not quite what happens. Actually the cat will heat up, not run cooler because there is more fuel to react with the available oxygen in the exhaust stream. After reading up a bit, there is sufficient evidence to describe two different cats: Three-way and Three-way plus injection. The ones most commonly used on american cars are three-way plus injection where additional oxygen is injected in the middle of the three-way cat after the first stage which breaks down NOx and released oxygen, second stage combines CO and O2 and burns unburned hydrocarbons by reacting with Oxygen. The last part of the cat, air is injected in the stream to allow more complete combustion of any unburned hydrocarbons and conversion of CO to CO2 that may be missed in the first two parts of the cat.

So to change my stand on the original question, yes you can run without the Air pump provided you have the correct cat (three-way that does not require air injection) and a proper working O2 sensor. If you remove the O2 sensor, then the density of oxygen in the exhaust stream is reduced (rich running) and the cat will eventually fail.

To understand why the cat heats up more with less oxygen, understand that the cat is a very efficient tool to make the chemical conversions asked of it. If you remove oxygen injection from the equation, the resulting fuel will still be consumed by whatever oxygen is in the airstream. Without the air injection (78% nitrogen which helps cool the cat), the cat cannot cool down as much, therefore will run much hotter and self destruct if temperatures reach high enough. As temperatures increase, the efficiency of the reaction increases, utilizing even more of the minimal oxygen in the exhaust stream, propagating the process.

Attached is a website that describes what happens and at what temperatures. This test was conducted with the air pump disconnected and simulating a misfire. Although not quite the same as described above, I believe you can get the picture. The Hydrocarbon molecule is made up of many carbon and hydrogen atoms requiring greater catalytic action and therefore much greater thermal output. So reducing the air injection will actually increase the temperature inside the cat based on the fact that all of the fuel will be converted whether the air pump is on or off, but as excess fuel is sent downstream, there is no air injection to cool the cat down, so it builds up heat quickly eventually failing.

Tim

http://www.asashop.org/autoinc/oct97/techtotech.htm

Tim McCreary

Logical thinking, but not quite what happens. Actually the cat will heat up, not run cooler because there is more fuel to react with the available oxygen in the exhaust stream. After reading up a bit, there is sufficient evidence to describe two different cats: Three-way and Three-way plus injection. The ones most commonly used on american cars are three-way plus injection where additional oxygen is injected in the middle of the three-way cat after the first stage which breaks down NOx and released oxygen, second stage combines CO and O2 and burns unburned hydrocarbons by reacting with Oxygen. The last part of the cat, air is injected in the stream to allow more complete combustion of any unburned hydrocarbons and conversion of CO to CO2 that may be missed in the first two parts of the cat. Basically clean up stray crap that the first two parts might miss.

So to change my stand on the original question, yes you can run without the Air pump provided you have the correct cat (three-way that does not require air injection) and a proper working O2 sensor. If you remove the O2 sensor, then the density of oxygen in the exhaust stream is reduced (rich running) and the cat will eventually fail.

To understand why the cat heats up more with less oxygen, understand that the cat is a very efficient tool to make the chemical conversions asked of it. If you remove oxygen injection from the equation, the resulting fuel will still be consumed by whatever oxygen is in the airstream. Without the air injection (78% nitrogen which helps cool the cat), the cat cannot cool down as much, therefore will run much hotter and self destruct if temperatures reach high enough. As temperatures increase, the efficiency of the reaction increases, utilizing even more of the minimal oxygen in the exhaust stream, propagating the process.

Attached is a website that describes what happens and at what temperatures. This test was conducted with the air pump disconnected and simulating a misfire. Although not quite the same as described above, I believe you can get the picture. The Hydrocarbon molecule is made up of many carbon and hydrogen atoms requiring greater catalytic action and therefore much greater thermal output. So reducing the air injection will actually increase the temperature inside the cat based on the fact that all of the fuel will be converted whether the air pump is on or off, but as excess fuel is sent downstream, there is no air injection to cool the cat down, so it builds up heat quickly eventually failing.

Tim

http://www.asashop.org/autoinc/oct97/techtotech.htm

XSTransAm

hmm, just switched out my cat for a catless midpipe.... guess what, my cat was all broken up :P

now time to determine if it broke up because of no air pump or because its 10 years old? you decide!

edit: 62k miles on cat

SPOautos

This is what I've always wondered too. Why do people get a high flow with no air pump??? Your not going to pass emmisions without the air pump so you might as well run a midpipe.

If all you have is visual inspection keep your stock cat and throw that on there once a year to pass visual, it only takes like 30 minutes to put the stocker back on.

Plus a mp is cheaper than a hf

STEPHEN

Nathan Kwok

Exactly. I'm not sure if oxidation is the correct term for that reaction but its the reaction between the fuel and air inside the cat that causes the heat. For maximum effective life, you need an airpump. This does not mean the cat will fail right away without one, nor does it mean the failure mode is necessarily clogging. However, once again, for maximum life, you need an airpump. If you don't want to run an airpump, buy a 2-way cat instead of a 3-way. Those are designed for use without an airpump, although they won't last as long as a 3-way with an airpump. For those of you who are using a cat just for "looks" and not to control emissions, a neat idea might be to use the heatshields from a stock cat and wrap them around a midpipe to create a fake cat. If you're going to pollute, might as well have the horsepower .

Fritz Flynn

iTrader: (263)

Simple I like being able to hear

rdahm

iTrader: (6)

I got a HF cat so i wouldn't be pushing the limit of the stock ECU too much cause it was either a HF cat or the stocker brand new $$ .



that was why. lol So first i had to fix the Cat without screwing up the back pressure too much (mind you this is all hear say) I now have a PowerFC and want to removel the air pump.

That is how i came to be with the HF cat and wanting to get rid of the airpump.

911GT2

Reasons not to get a midpipe

• Horribly loud
• smelly
• boost creep
• illegal in most states

Hows that for a reason?

WaachBack

iTrader: (21)

well im in a state where there are no emissions, loudness dosent matter, and i dont care if its smelly. But, I do care about boost creep. Wouldn't a boost controller solve the boost creep problem?

DamonB

Not unless you make some big changes at the wastegate...

David Beale

Hmmmm:
Horribly loud - well, somewhat loud, but not horribly.

smelly - You guys are wimps! "When I was a boy, cars smelled". So my eyes water when I stand at the back with the engine running - so what ;-).

boost creep - never seen it.

illegal in most states - well, yes it's illegal to remove the cats here in Alberta Canada. But I didn't remove them, they were gone when I bought the car. It's not illegal to operate the car without them, and no-one checks anyway.

No cats., no air pump. Maybe my HKS Superdragger is restrictive enough to keep boost creep away.
Would love to have a HF cat, but it's expensive, and there is no longer any room for the air pump.