Cat removal .... mandatory for rotary health???
 

Just picked up an 04 RX8 with 72K miles. At 69K the previous owner bought a reman engine from the Mazda dealer. The receipts he provided do not seem to indicate what the point of failure was, just that the original engine had low compression.

I would like to do what I can to keep this new (ish) engine healthy. I assume I should at least remove and inspect what appears to be the original equipment cat mid-pipe. If it looks good, should I keep using it? Or should I replace it with a HFC or test pipe? All else equal I prefer the exhaust tone and odor with a cat. However I do want to preserve the life of the engine if possible.

I plan to street drive the car and maybe 4-5 HPDE a year. Thanks,

It will shred through cats eventually. HJS high flow and motorsport large catalytic converters can take the abuse but at a price.
Going decat is not so civilized on the other hand.
​There's a ton of stuff you must watch and adress to keep it healthy,
https://www.rx8club.com/new-member-f...t-here-202454/
A lot of info on this forum. A lot! I enjoy reading through.
Take care and be safe.

Welcome to the group.

I’m still learning from the longtime members. What I have found is you want to check the existing cat isn’t failing/plugged. I dropped a small led flashlight down the back of the mid pipe. If there’s clear light shining through you’re probably good ( worked for me for 1 year). Could also put a small camera down the back end. You need to drop the mid pipe to check this.

If you’re contemplating gutting your cat DON’T. I replaced my mid pipe with a BHR mid pipe ( high flow cat coming at a later date). When I took the original mid pipe to the recycling depot they gave me $1502 USD or $1911 CAD for the catalytic converter.

Thanks,

If he cat is still good, I won’t be gutting it or throwing it out.

How do do you like the bhr pipe? Did you consider buying it with a hfc installed? I thought about that but did not want to waste money on a hfc that will get destroyed and/or hurt the motor.

I bought the mid pipe used and had the HJS cat removed since the vendor wasn’t sure of it’s condition. Will probably get another one since the smell isn’t great. There’s lots of information on the forum about cats and what will survive in our cars. Check the link from the post above my first reply.

Thanks for the reply. I started this by searching. From what I can tell here are some of the potential answers to my original question:

1). Yes, mandatory.
2). Maybe
3). No, if it’s stock
4). Yes, if it is a hfc.
5). No, if it’s a high dollar hfc from Germany.
6). Yes, if you track
7). No, if you just street drive

and of course the most common answer to any question on the rx8 forums...

8). Yes, your rotary engine needs to be replaced.

More seriously, I do appreciate the responses. Personalized feedback on a specific question is helpful.

I have the BHR catless midpipe as well, and while I love it for its tone and low drone at highway speed - it stinks. Any decatted car is going to STINK. It's not something you get used to, and its not good for you to breathe in. If I had to do it again I would have gotten a more expensive high flow cat from a reputable source, but that's just me. My RX8 is only a weekend toy at this point so its tolerable, but I wouldn't go with a decatted car daily.

im all for decatting, they rob power, and can harm your engine. grew up on 2 strokes so the smell doesnt bother me at all and shooting flames is cool af

Would you have gone with the BHR catted pipe instead?

Bad smell is subjective. I also happen to like how it smells decatted and premixed. Other people can't stand it. I've gone through 3 cats including the oem. Last one lasted 10 years, did not crumble to pieces.
if it were me and I lived in a rural area and occasionally raced it, I would go decat. But if I planned to use it daily in populated areas I would go hfc, the one from Germany. In fact I plan to.
That's why there are different views on the subject.
Do you plan on keeping and maintaining the car? Spend the money for something that lasts.
Do you plan on using it hard? Will you race it? And so on.
If stock is in good condition, you could drive it, have fun, maybe see its needs and go from there.
Be well.

I dont know enough about the BHR catted pipe to say for sure, but probably. From the interactions I have had with Charles at BHR in the past, he makes an excellent product and stands behind it.

Personally, I kept a cat in my system for three reasons:

1) Emissions - wanted to minimize pollution
2) Smell - gas smell is too strong for me and my SO
3) Performance / driveability - OEM cat was starting to fail, which led to some engine hesitation

I ended up replacing the OEM cat with a BHR midpipe and Gesi Gen2 HO+ (350-500hp rated) high-flow cat welded in. Went with the Gen2 as that claimed to have a higher EGT threshold than the Gen1, and I read on the RX7club forums that people were using it with good results. So far I've put about ~1500 miles on mine and have been pleased so far. Improved the driveability and butt dyno slightly and has held up so far to a trackday and DGRR drive. Hopefully it lasts!

More great feedback, thanks. I looked into the BHR catted pipe and they are using a ceramic cat and do not recommend it for track use. I did see the GESi cats which seemed of decent quality. It would be nice if someone offered a high quality, high flow cat on a RX8 midpipe.

Generally it seems that using any cat does increase failure risk and potential engine damage. However the risk is not substantial enough to consider cat removal mandatory. Does that sound about right?

Yeah I'd agree with that, the downsides for me of going catless outweigh the upsides. Just can't justify the emissions on a daily driver 8 as a fairly environmentally conscious person, even if the flames look cool and removes one potential fail point. There's lots of "sports" catalytic converters offered in Japan (from sard, wedsport, leg, re amemiya, etc) and people track their cars heavily there, so not like getting a cat is a death sentence like some people here make it out to be.

the HJS metallic cat converters from Germany are what you want, going to be a universal type that you have to install in a midpipe, recommend it be sized for at least 3.0 liter engine capacity

Where can you buy the HJS cat?

not the car for you

https://cimg1.ibsrv.net/gimg/www.rx8...43e108cbfb.png

If you are in Europe, there are several eshops. You can look up official dealers in HJS's site.
I don't know about the US market though.
In Greece I can get the 3 inch Motorsport for around 800 euros delivered. The 200cpsi ones have gone way up in price.

To the original question: cats can be the victim of poorly maintained components upstream, rather than their own root cause of failure. Poor ignition kills cats, poor cats kill engines. It doesn't have its own expiration date.

On my car I inspect it once a year along with ignition and other major components. So far so good. Tracking and everything. I had an aftermarket cat for a couple of years which failed shortly after I sold it. Your mileage may vary, inspect and go from there.

as much generic smoke as you blow out your tailpipe I suppose you were lucky that it lasted that long 🤔

​​​​​​.

Thank you..

So basically the answer is no. No, assuming peripheral equipment is in good working order and proactive regular cat inspections. I am premixing, but from my understanding burning a JASO FD oil is no worse for the cat than burning 4T oil.

I think if you are dailying the car and your car is in good shape, the factory cat will be fine. If you are heading to a track, though, you probably want a midpipe.

I always get concerned with people who like the gas smell, because that stuff is actually addicting. Some people actually get bad enough that they need to sniff tailpipes to get a hit.

where’d you get that value for the factory cat?

At a recycling facility (edit - Canadian Auto Recycling) in Abbotsford BC.

that’s wild, I knew they were worth a considerable amount but I didn’t think it was nearly that high! I’m in Winnipeg btw.

I feel sorry for convertibles, motocyclists, bicyclist, EV drivers stuck behind a decatted rotary. The irony with the latter category is especially hilarious.

Actually i think i've found the solution to cat failures on the RX-8. I recently experienced the glowing 02 sensor, thinking it might be clogged catalytic converter, also had the "marbles in a can sound". It turns out it was the type of gasoline i was using. Here in Europe its normal to fill with 95 octane, which is also what is recommended for the RX-8. After i switchted to 98 octane from Shell, the cat stopped glowing!! I tried to abuse the care many times, but cat still not glowing. Try some proper fuel, i bet it fixes the issue!

Have you actually had a look inside the cat? It can be causing damage without glowing.

Cat temps???
 

Continuing this discussion. I started data logging obd2 cat temps and am seeing temps as high as 1730f just driving around. I understand this may not be the actual temp of the cat as the ecu uses multiple sensors to “calculate” the temp data.

Can this data point be used to determine cat function or condition? From what I can tell the functional max range seems to be around 1600-1800f. Interestingly I noticed reports of even catless pipes registering similar temps.

So does this data tell us anything usable?

Took my midpipe with working cat off a few years ago just after purchase. Lives on shelf in shed just in case its needed.
Every time i drive the car i see EGT temps via OBD up to 1750 (950C plus for me), this was the same for both the old tired engine and fresh engine.
This car does not see any tracfic at the 100km speed limit that i always do! 😉
Can't help you with data of those temps being detrimental to cats as i only drove a few times with cat installed. Im more of a dog person! Very likely though.

If your cat were to clog you might see higher temps than that. By itself the temperature doesn't say much, although I did find it interesting that the temperature climbs when you're in low regimes and decreases when you open up. At least that looked like the pattern. Possible it was shedding heat better with speed.

Interesting that you mentioned that. I noticed the cat temps can be maxed out by driving at medium/low load on the highway. 80mph in 6 gear about a 1/4 throttle. Max temps will be around 1750. If I got WOT, the temps actually drop to about 1680 and stay there until I let off.

I guess at least I know the current functional range of temps under light duty driving. I know if temps spike from there on track to shut it down.

Exhaust temperatures are highest near the stoichiometric mixture ratio. At high load, the ECU will switch to open loop enrichment (you will see that in the AFR from the wideband O2), which cools the exhaust. It is commonly noted that the stock tune is very rich near the max rpm - I think this is done to prolong the life of the cat.

The logic follows since temps seem to drop when going to WOT.