Street economy without Cats

Basic theory question here.

I live in an area that has no inspections. Catalytic convertors are considered (by many) throw away items. Think of them as weight reduction.

So, on an older OBDII car with pre and post Cat O2 sensors, what are the performance and economy issues raised with no Cats? We are doing every tweak we can to improve fuel economy. Street driven basic people mover.

With both O2 sensors giving similar feedback, would you expect a positive or negative impact on fuel economy? My gut is telling me less exhaust restriction is a good thing, but the computer may default to a limp home type setting which might dump fuel. We have found a nearly 10% improvement in fuel economy when we use fuel without ethanol, so that's what we try to burn.

I'm open to discussion about the emissions impact, but I'm mostly interested in the fuel economy.

The car in question is a 1st Gen Honda Odyssey

I'm not sure about Honda, but I know that Toyota will use the rear O2 sensor to adjust fuel trim up to 2%.

It may also constantly try to run the catalyst monitor test which dumps in a little extra fuel waiting for the catalyst and O2 sensors to react. This might not give any noticeable MPG difference though.

Won't the constant MIL drive you nuts? Plus you won't be aware of any new problems if you just get used to ignoring the light. I can't see the lack of cats giving any noticeable power difference on a motor that doesn't spend a lot of time in the upper RPM range.

The MIL is always on now... kind of a special accessory feature.

RPM... sits at 60-65 mph for 80% of it's life. That should be under 3K.

Did you mean power difference, or economy difference?

I should have noted...

The Cat is failing. It's probably partially clogged, so I would expect a difference in flow.

The real question I whether I should pony up and replace the Cat or cut it out.

This makes my head hurt. You're driving a 1st gen Honda Odyssey and you're trying to sell catalytic converter removal as a way to increase operating efficiency?

I'm going to skip the really opinionated part of this response, but suffice to say, it wasn't positive.

do the rear o2 sensor in a spark plug non fouler to get it out of the direct exhaust stream trick and the computer should think a cat is present and functioning properly.

pres589 wrote: This makes my head hurt. You're driving a 1st gen Honda Odyssey and you're trying to sell catalytic converter removal as a way to increase operating efficiency? I'm going to skip the really opinionated part of this response, but suffice to say, it wasn't positive.

Yes, over the current operating efficiency which consists of a clogged cat.

So a new catalyst will be an improvement in flow anyway. Luckily you are not stuck with CARB only approved catalyst like here in CA. I'm sure you can get a replacement cat for cheap. The car will run properly and you won't be pushing large amounts of NOx , HC, and CO out your tailpipe. Think of the children...

Straight pipe will only get you noise with debatable power/mpg difference.

If nothing else, in lieu of a definitive answer about how Honda will react to possibly-weird O2 signals, it seems unlikely that straight pipe will cause the computer to run the engine in way resulting in better mileage than a working cat.

patgizz, how/why does the non-fouler trick work, and does it always work? Seems like a weird coincidence of physics that sitting in an exhaust tidepool/eddy should result in the same readings as being downstream of a properly functioning cat...

EDIT: I'll go on record as being anti-cat-removal, just in case there's any doubt...

In reply to clutchsmoke:

First off, I wrote my response before he had added the "oh and it's clogged" bit. Secondly, this really seems like "how can I not maintain my vehicle to factory standards and rationalize doing so in the process?" and less of a "hey I saw this on an ecomods website and it's getting rave reviews, do you guys think we could consider some science and get a definitive yes/no on this efficiency mod?" issue.

ransom wrote: If nothing else, in lieu of a definitive answer about how Honda will react to possibly-weird O2 signals, it seems unlikely that straight pipe will cause the computer to run the engine in way resulting in *better* mileage than a working cat. patgizz, how/why does the non-fouler trick work, and does it always work? Seems like a weird coincidence of physics that sitting in an exhaust tidepool/eddy should result in the same readings as being downstream of a properly functioning cat... EDIT: I'll go on record as being anti-cat-removal, just in case there's any doubt...

I can tell you that on Nissan's, the anti-fouler trick works everytime. Also, on most (all???) OBDII system, the second O2 is only there to confirm that the cat is working properly. It does it only by comparing its reading to the front O2, and making sure its reading is different. That's why the anti-fouler trick works. Althought I'm sure the newer systems might trim fuel slightly from the reading of the second O2, on an older system, it's doubtful.

And removing your cat will make your van extremely loud. It's surprizing how much sounds that things absorbs. If you have no CARB requirements, I would just shove a new aftermarket cat (100$ to 150$), and have a clean conscience.

A certain magazine did a test on a Miata of various catalyst set ups, including no catalyst. On a dyno, IIRC, there was no real improvement for no cat, and the performance cats were not much, either.

fanfoy wrote: I can tell you that on Nissan's, the anti-fouler trick works everytime. Also, on most (all???) OBDII system, the second O2 is only there to confirm that the cat is working properly.

If by that you mean almost no modern cars, then you would be correct. The O2 sensor is easy to use as a fine tuner to the closed loop fuel, and I fully expect that a Honda built in this century would be fully taking advantage of that.

Too bad you can't tune it to just ignore the rear O2's....

If you go catless, just move the rear o2 sensor farther downstream about a foot.....should keep it from messing with the ecu.

I would take the cat off, gut it with an air hammer, and bolt it back in. I have a friend running a Mustang GT that put the spark plug anti-fouler in with no cat, and it worked. Put the spark plug anti-fouler in, clear the trouble codes and see what happens. Run a couple of tanks of gas out and see what your MPG is. I'm interested in this as well. I have a Civic VX and would like to put a header on it removing the cat in the exhaust manifold. This would cut a bunch of weight off my car too, but didn't want to effect MPG. Yes OBD0 one 5 wire o2 sensor.

fanfoy wrote: Also, on most (all???) OBDII system, the second O2 is only there to confirm that the cat is working properly. It does it only by comparing its reading to the front O2, and making sure its reading is different. That's why the anti-fouler trick works. Althought I'm sure the newer systems might trim fuel slightly from the reading of the second O2, on an older system, it's doubtful.

The computers watch the rear O2 before the cat has lit off, to gauge how much the front O2 has drifted. Front O2s drift much more quickly than the rears because it is in a harsher environment.

When monitoring catalyst efficiency, the short version is that they want to see the rear O2 switching much more slowly than the front, if at all. If they switch close to the same rate, then that means the catalyst isn't actually doing anything to the exhaust anymore. Of course, it's way more complicated than that, with about eight or twelve different ways that it tests.

alfadriver wrote: A certain magazine did a test on a Miata of various catalyst set ups, including no catalyst. On a dyno, IIRC, there was no real improvement for no cat, and the performance cats were not much, either.

Modern cats are not flow restrictions the way the old pellet-bed cats were. If they were a flow restriction, the OEMs would put better ones on. They want every last shred of horsepower and fuel economy possible. It's not like the old days where they said "screw it, if they want more power then they'll get the next engine size up."

Modern exhaust systems in total are usually fine pieces of work, with aftermarket exhausts mainly making more noise, not power.

I've seen cars that made 1000 crank HP blowing through a pair of OEM sized cats. Which, I might add, were smaller than the monstrosities put on a Odyssey.

In reply to Knurled:

I was trying to illustrate that GRM did a dyno test of the exact subject. No cat wasn't an improvement at WOT, which ends up being less of an improvement below that.

And I'm providing supporting material.

Wait, crap, that's not how it's supposed to work. "GRAH! Angry sounds! Somebody on the internet is WRONG!"

Knurled wrote: And I'm providing supporting material. Wait, crap, that's not how it's supposed to work. "GRAH! Angry sounds! Somebody on the internet is WRONG!"

Now I just wish I remembered what issue that was.

alfadriver wrote:

fanfoy wrote: I can tell you that on Nissan's, the anti-fouler trick works everytime. Also, on most (all???) OBDII system, the second O2 is only there to confirm that the cat is working properly.

If by that you mean almost no modern cars, then you would be correct. The O2 sensor is easy to use as a fine tuner to the closed loop fuel, and I fully expect that a Honda built in this century would be fully taking advantage of that.

I know you have more experience than me on that subject, so you'll have to explain to me how the rear (after cat) O2 sensor can be used to trim the fuel table in any significant way. After all it isn't seeing the actual output from the engine, but rather the "treated" exhaust gases. Seeing as the efficiency of a cat is very temperature dependent, and that most (all?) injection systems do not monitor exhaust (or cat) temperature in any significant way, how would that work?

Knurled wrote:

fanfoy wrote: Also, on most (all???) OBDII system, the second O2 is only there to confirm that the cat is working properly. It does it only by comparing its reading to the front O2, and making sure its reading is different. That's why the anti-fouler trick works. Althought I'm sure the newer systems might trim fuel slightly from the reading of the second O2, on an older system, it's doubtful.

The computers watch the rear O2 before the cat has lit off, to gauge how much the front O2 has drifted. Front O2s drift much more quickly than the rears because it is in a harsher environment. When monitoring catalyst efficiency, the short version is that they want to see the rear O2 switching much more slowly than the front, if at all. If they switch close to the same rate, then that means the catalyst isn't actually doing anything to the exhaust anymore. Of course, it's way more complicated than that, with about eight or twelve different ways that it tests.

I know that's also why the rear O2 is usually the heated (three wire) kind. What are the other tests that are performed? Just curious to find out.

In reply to fanfoy:

One thing to remember- the sensor is basically a battery that nominally uses the difference of O2 across it to generate a voltage, it's not too hard to use that information. While the amount of O2 and NOx are much much lower downstream of the cats, it's still in realtive balance that you can use that information to determine the state of the catalyst that it's behind. And by state, I'm not meaning dead or alive, but is it rich or lean.

If there's a catalyst behind it, you can use that rich or lean signal to control the a/f going into the catalyst behind it- modulate it much like you do the front one- enough that the second brick is more useful, but not so much that it's upsetting the front one. It helps to visulaize that you can layer multiple a/f control patterns on top of each other- one fast, one slow, one high modulation, one low.

If there's not a cat behind it, the information can be used finely, to keep the state of the front brick as constant as possible. Can't really be perfect, but you can fine trim small amounts to keep the catalyst in a state that it's most efficient. Same kind of pattern layering as above- just different goals.

Does that make sense?

Oh, and rear O2 sensors are 4 wire. Just like the front ones. Unless it's a wide band, then it's 5 or 6.

pres589 wrote: This makes my head hurt. You're driving a 1st gen Honda Odyssey and you're trying to sell catalytic converter removal as a way to increase operating efficiency? I'm going to skip the really opinionated part of this response, but suffice to say, it wasn't positive.

I'm not trying to sell anything. Read more carefully.

I've got a busted Cat on a crappy old car I put 1000+ miles per week on.

I am asking if there is any point in trying to bother replacing it, or just cut the darned thing out.

An extra 1 mpg is worth it to me with the amount of miles I drive.

Why the attitude?