Runing an OBDII car without a cat?

Just wondering does the OBDII system do anything weird if you run without a catalytic converter?

Obviously, it will throw a CEL, but does it make the mixture too rich or too lean? Retard the timing? Anything that would be bad for power or bad for the motor?

The car in question is a 99 Miata, and it would be a temporary thing, and then the car would be put back to stock.

thanks

The rear sensor would see the rich condition, but I don't think it would do anything other than throw a code. You might be able to run sensor 2 outside the exhaust.

You can purchase a sensor emulator to lie to the OBDII system and keep the CEL from lighting up. It won't hurt anything to let the light come on, though - the engine will run fine.

yes, the second sensor is there just to make sure the cat is in place. The computer compares the two readings to make sure they are different

I had a 96 Miata track car. It had O2 sensors before and after the cat. I replaced the cat with a race pipe, which would normally give you a Check Engine light.

I plugged the hole in the race pipe with an oil pan plug and then bolted the downstream O2 sensor to a bracket on the OUTSIDE of the floorpan. I wrapped a hose clamp around it and bolted it to one of the heat shield brackets. It needs to be outside because it's heated and gets really hot (don't park in the tall grass). With it outside the exhaust system, it samples outside air and tricks the computer into thinking that you have the best catalytic converter in the world; No Check Engine light.

You can see the O2 sensor just downstream of the cat in the first photo.

I ran my car like that for about 5000 miles without a problem. I'd return it to stock whenever I had to go for an emission test, or, uh... any time I drove it on the street.

Or you can use a sparkplug defouler if you're concerned about keeping the O2 sensor outside of the exhaust. Pulls the sensor out of the exhaust flow just enough to trick the sensor into thinking the cat's present (or working). Also a great trick when your cat goes bad and you don't want to buy a new one quite yet, or just want to sell the car.

damn. wish I knew this a month ago. nice insight, though.

Cool, thanks for the replies.

I read somewhere that the downstream sensor was also used for fine adjustment of the fuel mixture on OBDII cars.

And now for the follow up question... on a 99 miata is there significant hp to be gained by switching over to a straight pipe and race muffler vs the stock system?

There's significant noise to be gained. I needed earplugs in mine if I was going any distance.

I might have to look into the antifouler.. putting out $1200 for a new centre pipe with cat for my 14 year old BMW does not make me a happy camper

I think it comes down to the car. Some will use the downstream sensor for fuel delivery. Some won't. Pre-2006 Miatas don't have terribly clever computers so in that case, you'll be fine without it. Just ignore the light while you have the cat out, then reset the codes when the new cat goes in.

sevenracer wrote: Cool, thanks for the replies. I read somewhere that the downstream sensor was also used for fine adjustment of the fuel mixture on OBDII cars.

Not directly, no. The computer uses the front O2 for fuel mixture adjustment, and it monitors the rear O2 to see how well the converter is working... if it switches at the same speed as the front O2, it knows the converter is no longer properly absorbing/releasing oxygen.

The only time the rear O2 will affect mixture is if the computer does a little error-checking and deliberately runs the engine rich for a period of time to make sure that the rear O2 is actually functioning.

Knurled wrote:

sevenracer wrote: Cool, thanks for the replies. I read somewhere that the downstream sensor was also used for fine adjustment of the fuel mixture on OBDII cars.

Not directly, no. The computer uses the front O2 for fuel mixture adjustment, and it monitors the rear O2 to see how well the converter is working... if it switches at the same speed as the front O2, it knows the converter is no longer properly absorbing/releasing oxygen. The only time the rear O2 will affect mixture is if the computer does a little error-checking and deliberately runs the engine rich for a period of time to make sure that the rear O2 is actually functioning.

Actually, it does. Not as obiously as the front sensor does, but most do active "trimming" (what I'll call it) based on the rear sensor. Some systems do it more than others.

But, active trimming is done with the rear sensor- not failure mode, not error checking, but trimming for optimum emissions.

Eric

Any idea how much hp this will free up? Running the car with no cat and a straight through race type muffler?

I dont know about it for miatas, but for my mustang I have things that plug in in-line to the rear O2 sensors for the off road H (no cats) that make the CEL go away. http://www.latemodelrestoration.com/item/LRS-MIL02/96-04-Mustang-46L-V8-38l-V6-Mil-Eliminators-O2-Simulators

cant vouch for it, but might look into this http://atomicinternet.homeip.net/teamneco/listHOW.asp?RecordID=8

We recently did some dyno testing, matching a completely stock 1999 Miata exhaust against one with an FM midpipe and hollow cat. For those who don't know the Miata exhaust system, the cat is part of the midpipe, which stretches from bellhousing to rear axle. The car showed a 3-4 hp gain across the range.

I don't know how much was due to the 2.25" midpipe and how much was the missing cat. Based on some other comparative tests, I'd say it's almost all due to the larger pipe.

A few years ago, I did some work with Johnson Matthey, one of the largest catalyst manufacturers. Unless the cat was clogged or broken, it did nothing to reduce backflow, especially any further than what the normal pipes did. What I gleaned from this is that removing catalytic converters is usually unnecessary.

Old thread, similar question.

Planning to put a full exhaust system in an old focus which has a cat integrated into the stock manifold. I happen to be lucky enough to live in an area without emissions checks and I want to put a header and stuff. 

So, getting rid of the cat, what is my best option?

Programmer that would allow me to disable cat efficiency codes (if such a thing exists)

Put aftermarket cat in after the collector?

Live with the CEL and check codes periodically to make sure nothing else is wrong?

Thanks

aftermarket cat after the collecter.   I changed the mustang to a catted X-pipe and I think I actually make more power that way. Also, they work pretty well as suppletory mufflers, took the mustang from noisy to pleasantly loud. 

If only we knew of a magazine which ran tests on a Miata with and without a cat, etc. https://grassrootsmotorsports.com/articles/catalytic-converter-face-/  Note the article is a bit older, the spec of the car different from the test Keith quoted, etc., etc.

sevenracer said:

Just wondering does the OBDII system do anything weird if you run without a catalytic converter?

Obviously, it will throw a CEL, but does it make the mixture too rich or too lean? Retard the timing? Anything that would be bad for power or bad for the motor?

The car in question is a 99 Miata, and it would be a temporary thing, and then the car would be put back to stock.

thanks

Modern cars are using the rear O2 to trim the fuel trims even finer, because the front O2s are more likely to skew, but I would think a '99's computer is too "dumb" to do anything like that.

Edit:  Wow, ancient post, but I find it funny the things I have learned in the past few years

Knurled. said:

sevenracer said:

Just wondering does the OBDII system do anything weird if you run without a catalytic converter?

Obviously, it will throw a CEL, but does it make the mixture too rich or too lean? Retard the timing? Anything that would be bad for power or bad for the motor?

The car in question is a 99 Miata, and it would be a temporary thing, and then the car would be put back to stock.

thanks

Modern cars are using the rear O2 to trim the fuel trims even finer, because the front O2s are more likely to skew, but I would think a '99's computer is too "dumb" to do anything like that.

 

Edit:  Wow, ancient post, but I find it funny that I made such a similar comment after 7-8 years

What is considered a modern car? The focus is an 05. Duratec 2.0. Not looking to delete the cat for gains bruh, but to replace the stock manifold. If it wasn't part of the manifold I would leave it alone probably.

Knurled. said:

sevenracer said:

Just wondering does the OBDII system do anything weird if you run without a catalytic converter?

Obviously, it will throw a CEL, but does it make the mixture too rich or too lean? Retard the timing? Anything that would be bad for power or bad for the motor?

The car in question is a 99 Miata, and it would be a temporary thing, and then the car would be put back to stock.

thanks

Modern cars are using the rear O2 to trim the fuel trims even finer, because the front O2s are more likely to skew, but I would think a '99's computer is too "dumb" to do anything like that.

 

Edit:  Wow, ancient post, but I find it funny the things I have learned in the past few years

And I will counter your point the same as I did 8 years ago. I’m sure that in ‘99 focuses were using the rear sensor for trimming, so the 05 focus is as well.

As has been said, I think it depends on the vehicle.  I know my 96 Chevy just trips a code, but some VAG cars and some British cars (and the Ford/Jag badge-swap cars) sometimes put the car in some sort of limp mode.  Memory is a little hazy on that since it has been many years since my repair shop days.

Move the front sensor down so its not plugged into just one cylinder, then get an antifouler to space the rear one out.  No codes, still sees what it needs to, kinda.  Check any one of the seventeen thousand B15 Sentra threads where they talk about disintegrating cats for the details.