The way the system works is to use the 02 to give readings to the computer on emissions. When you gut a cat the computer does not know it is not there. In turn the 02 gets higher emissions readings cutting fuel slightly to try and correct the issue causing the engine to run slightly lean. Which in turn over time causes extra heat which damages rotors , housings and seals. And yes running an engine slightly lean will actually make it run a bit better until it starts melting stuff.

 

For all this time I thought having a gutted cat is harmless. I've never seen anyone say that having a gutted cat makes the car run lean. Everyone said it didn't affect the front wideband O2 sensor. Only the rear O2 sensor. From what I know, the rear O2 sensor is a narrowband sensor and cannot change the fuel trims or AFRs to make the car run richer or leaner. It's only there to compare the results with the front O2 sensor and check if the cat is doing it's job. Correct me if I'm wrong. I am not an expert on this. I read this somewhere. I can't seem to find it now.

I did however, find this post by RIWWP.

And this

 

Haven't done it personally on an rx8 but have had cars come in the shop where people have done it and seen the end result of burnt valves. Have heard stories from rotary shops though that it has the same effect over a period of time except since there aren't valves the apex seals take the blow.

 

Man this is scary stuff I have a gutted cat! A lot of people I know have gutted cats! Since everyone on this club said there aren't any bad effects, I was cool with it. RIWWP even said it's better to run with a gutted cat than a working cat, because that's one less part to fail and destroy the engine. I didn't know that gutting the cat itself can destroy the engine! Can someone else confirm this?

 

I don't remember anyone else saying that.

 

no rotary should ever have a cat, there is no downside to not having one (performance and engine health wise)

 

Then what's with all the engine running lean stories? Does gutting the cat actually make the car run lean?

 

You may ask around. Like I said not sure about it on the rotary but I've heard from the local rotary shops it's not the best plan. I know it's definitely not good on a valve train piston engine. I believe if you buy an ap tuner you can do it and it will be OK. I still prefer to do it by buying the correct parts are designed to all work together so that it's done correctly. The renesis engine really isn't going to make any noticeable power by gutting the cat at all. Maybe 3 hp which you won't notice. So other than sound that's all it really does.

 

When you gut a cat basically what happens is that the burnt fuel passing the first 02 sensor is operating as it should. The 02 reads that and sends it to the ecm. When the second 02 reads the ecm is expecting to get a reading as if the mixture has passed through a functioning cat but instead the reading is showing much higher volumes of fuel in the mixture which it sends back to the ecm so the ecm tries to correct the problem by keeping the air intake volume the same but while cutting fuel trim which creates a lean condition unless you have a tune to correct it. That's why these all of these cars have these sensors and computers in place is to attempt to make the car run like it should. I'd you start modifying performance you need to have the correct tune to accommodate the upgrade to make the car run like it should with what parts or mods you have done. If you add intake and exhaust the car still thinks it's stock and runs accordingly. It's doesn't know you have added or removed anything.

 

Let me stop you right there rxgt04, the rear 02 sensor has NOTHING to do with fuel trims. The AFR has ZERO change with or without a cat in place. All the rear 02 does is let you know that the cat is there doing what its supposed to do or not.

 

Thanks sonic.

 

If you are not sure about something, don't make declarative statements without proof.

Hearing from the local rotary shop doesn't cut it.
Identify said rotary shop, name and city please.

 

Been gutted for over a year. My 8 never runs lean. I monitor my afr through obd2. Readings are same as before gutting my cat.