RX-8 Owners Club UK

Hey All,

Asking a bit of advice from anyone who has experience of comparing the 100 cell and 200 cell sports cats.

I'm going to be building a custom mid-pipe and wanted to know whether anyone has any feedback on 100 and 200 cell sports cats. I've got a 200 cell at the moment, but wanted to know how well a 100 cell cuts down the fumes and smell? I'm not fussed about road legal-ness and ability to pass an MOT at this point, just the smell and eye-melting ability... lol

The Irish guys don't have to worry about cats for their NCT, as rotaries are excluded from the emissions test, so many (most) of them have switched to decats.

However, a few have fitted 100 cel sports cats for the reasons you mention and the results are favourable.

Certainly from following behind in a convey, I could tell the difference between a decat and a sports cat.

Cheers Dave.

Still in 2 minds about whether I'd be happy with a 100 cel or whether to go for a 200...

I suspect that there's not that much difference in real world engine performance between the two and a 200 stands a fighting chance of getting through an MOT

a 100 cell unit will struggle / fail to pass an MOT as I've got a 200 cell unit on mine and that just about passes.....

I 'd say both will clean up the exhaust to point of stopping your eyes from watering but if you want something that is emissions compliant as well then 200 cell all the way

I know it won't get through an MOT, just after something to soften the nasal assault

Dr. FrankenRex wrote: ↑

Mon Feb 22, 2021 10:06 pm

I know it won't get through an MOT, just after something to soften the nasal assault

Just kill your sense of smell and never worry about it again cheaper.

Yorindesarin wrote: ↑

Mon Feb 22, 2021 10:07 pm

Dr. FrankenRex wrote: ↑

Mon Feb 22, 2021 10:06 pm

I know it won't get through an MOT, just after something to soften the nasal assault

Just kill your sense of smell and never worry about it again cheaper.

The sole positive of 'rona for an 8 owner or something? ain't no smell getting you down

I have the Milltek sport cat and that is 200 cell. It will pass an MOT.

I wonder what number of cells a standard OEM cat has or where it fits on the 100/200 scale?

V8 Power wrote: ↑

Tue Feb 23, 2021 4:17 am

I wonder what number of cells a standard OEM cat has or where it fits on the 100/200 scale?

I thought 400 but I don't know for sure.

v-rex wrote: ↑

Tue Feb 23, 2021 8:49 am

V8 Power wrote: ↑

Tue Feb 23, 2021 4:17 am

I wonder what number of cells a standard OEM cat has or where it fits on the 100/200 scale?

I thought 400 but I don't know for sure.

I think 400 cells per square inch is pretty standard but don’t know the specific one used in the stock RX8 cat.

400 would make sense.
I wonder how they compare in terms of actual restriction to exhaust gas flow. I would not think it is linear so a 400 won’t be twice as restrictive as a 200 etc?

Interesting discussion. I know that a lot of people with 200 cel are able to pass an MOT as long as it's up to temp. I guess the differentiating factor is the uber-daft porting I have.

I have RRP's 200CEL cat in my R3 with RB catback. I have no issues whatsoever with smells.

I haven't put it through an MOT yet though.

The OEM cat is not only more dense but it's multi-stage, a lot more sophisticated than any of the aftermarket replacements.

This is why a tatty used OEM cat is worth more scrap than most of the shiny new replacements.

also cell count is just 1 factor in how it flows as there are different foil shapes and sizes as well as the different coatings and the thickness level of the coating

As Dan is just looking for something to take the edge off I'd say to go for a 200 cell cat with at least a 150mm diameter as that shoudl cover all the bases

I can't say I know as much about Catalytic converters as some people, but I do know about flow and aerodynamics. I read about this all in the past, and still find it fascinating. I do know that it isn't just a case of sticking the biggest pipe on as the backpressure may be wrong, and also a well designed higher cell system can out perform a 100 CEL as you need more length to achieve the right surface area to catalyse the gases.

I know the distribution of precious metals (Rhodium, Platinum etc) affects the ability to catalyse the various type noxious chemicals from the exhaust. I also know alot is talked about in terms of metal vs ceramic and temps on a rotary but regardless, I think "sintering" which happens at 800-900C must affect the the substrate, whether it's metal or ceramic, so I don't understand how a metal CAT will last longer than a ceramic one unless it contains alot more substrate. I know metal tends to have a smaller cross sectional area which is often a big selling point.

viewtopic.php?f=25&t=77699&p=1165957&hi ... g#p1165957

At the end of the day I know Rotaries run hot and benefit from freer exhaust at high speed and especially if ported. I question if those high flow cats will work as well whilst idling on the M25 in traffic as I often am ( pre lockdown ). I understand why hi-flow CATS often will fail the MOT, but I wonder how hi-flow cats that can pass MOT's would not have to be more restrictive, so as to achieve enough catalysing surface area to achieve the required emissions.

Fascinating conversation and I still may try a high flow CAT one day, as so many people rate them, but so far for my purposes I am happy for now to stick with my standard "type approved" OEM replacement, and it hasn't let me down so far. If Mazda did the OEM one at a more reasonable price then I might consider but it seems they really pack it with precious metals so sadly is a thief magnet these days.

Here were some articles I read on this before, and found were quite good.

http://speed.academy/how-catalytic-converters-work/

https://www.klarius.eu/understanding-back-pressure/

Anyway I am probably detracting from the thread, but I'll be interested Dr FrankenRex what you end up with, and how you find the performance of the 100/200 CEL affects power/smells/noise etc when you put it on the car.

Metal cats tend to be more robust in terms of vibration and shock loads and ceramic tend to have a higher temp rating
however the temp rating of a metal cat tends to be at least 950 degC these days if not 1050 so the higher temp rating of a ceramic 1100 degC probably isn't worth the added fragilty

most OEM's are now tending toward the metal cat route these days as the current legislation now requires tests for 'aged' cats and the metal lasts longer

350matt wrote: ↑

Tue Feb 23, 2021 8:25 pm

most OEM's are now tending toward the metal cat route these days as the current legislation now requires tests for 'aged' cats and the metal lasts longer

Interesting. I always thought ceramic was what every one used by default. How long does the legislation say they need to last now?

120k miles

350matt wrote: ↑

Tue Feb 23, 2021 11:09 pm

120k miles

They expect a cat to last up to 15~ years plus for some people? Thats actually mad.. Is that even achieveable at all?

My OEM cat is 17 years old and passes the emissions test no problem, but my car hasn't reached 120k miles yet: though it has passed 120k kms.

I mean I did account for those with low mileage uses. Like yourself Dave, it could take you a lot longer to hit that target (and your car wouldn't be held to the current 120k standard anyway). But it begs the question, 120k miles for some people can easily be a year (taxi's etc) or 10 or so years of 10k~ average miles a year.. Of which the cat will have to function all the way to 120k miles... Without premature failure. I'd find it hard to believe a manufacturer is going to hit a success rate above 90% on such a claim... Especially with how much stricter new cars emissions are. Do you warrant a catalytic converter that dies pre 120k miles? Or does it fall into the usual consumables...

High mileage cars usually mean lots of long distance driving, and a fewer relative number of cold starts.

The problem with trying to engineer for an extended lifetime is the different factors that impact stress.

An obvious example is when you look at aircraft and find that those used on short-haul routes, especially internal flights and island hopping, have failures within a a quarter of the flying hours of those used on long haul flights, even for the same model of aircraft.

It's the number of take-offs and landings, pressurisation cycles, etc, that are the biggest factors, not so much the flying hours at cruise.