Catalytic converter options
 

I have a FC with a G35 turbo and 3" exhaust. Currently I only have a presilencer, but I want to add a cat. It's decided on a bonez, but the 3"universal is out of stock. Any other recommendations that won't break the bank? I've seen mixed things on magna flow, such as it's only functionally 2.75" and might not hold up to the heat.

Thanks

that won’t do much for emissions

but you get what you pay for, HJS in Germany makes the best metallic cat converters rated for 1050°C and 100 cpi for the European motorsports racing use

part# 90 95 0069 is the largest they offer; 3.5” in/out and rated for 500 hp on a 3.0L piston engine. I know you’re not wanting this, just posting it for information purposes


https://cimg4.ibsrv.net/gimg/www.rx7...503f58eeb.jpeg
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If you are looking to add a cat to decrease the nasty stink of the exhaust and / or add a little restriction to make boost control easier, look for a metallic core cat. 15ish years ago I welded one to my midpipe for the reasons mentioned and it worked very well. Doesn't filter enough to pass emissions though.

Magnaflow seems to offer a few different options. Vibrant seems to have one too. Be careful which one you choose regardless of brand. You want the metallic core substrate, not ceramic or others that don't have the heat capabilities needed for the rotary. Take note of the max EGTs supported. The one Team mentioned is probably a good option.

From my personal experience, and after previously promoting the Vibrant brand catalytic converters, I can say unequivocally that these units are not up to snuff regarding typical rotary exhaust temps. Specifically, the Vibrant Performance GESI Ultra High Output Catalytic Converter, 7830 model, didn't last long before clogging internally. The clogged cat actually would not allow me to achieve greater than 10-11 psi on my newly installed EFR 7670 turbo at the time but since the cat was brand new, we didn't bother to check for any exhaust restrictions.

I eventually discovered the clogged cat when I suffered similar boost limitations when upgrading to the EFR 8374 unit; performance at 16 psi was about 80% of expectations, w limited ability to boost further. Only after experimenting w a different catback did I discover the clogged cat at my midpipe. Note: I only have this one data point. And perhaps my Vibrant unit was simply a lemon.

Over the 30 yrs of FD ownership, I have never been wout at least some form of a functioning catalytic converter. And I was prepared to go the HJS route myself when chunking the clogged Vibrant unit. But before upgrading to a higher end HJS model, I was pleasantly surprised to find the noxious exhaust fumes quite subdued at traffic stops in the absence of anything to scrub the gasses further; presumably, due to my dolphin tip catback sending exhaust fumes downward to the ground and or running lean-ish AFRs in those same low-rpm moments. Something to consider unless you truly need to meet emissions limits.

Air Pump and aftermarket sportcats
 

Bringing back from the dead. I got curious and thought to myself, "How does the air pump even work in conjunction with the cat?" I am also looking at universal German sport cats because I need to pass emissions, and the previous owner absolutely obliterated the cat. On the Japanese Sard sportcats for fc/fd you can clearly see the little neck for the air pump. Here's the link: https://www.ebay.com/itm/194262880681

Now if I build my own catted downpipe rated for up to Euro 4, will I pass emissions? Anyone from the EU or smog states to elaborate on this?
I am just wondering if the air pump has to be connected to a new-edge cat to pass emissions or not. Needless to say, I need to keep the EGR valve because I believe it cuts down a different greenhouse gas than what the cat does.

Cheers,
Amael

As I understand, the air pump is a dilution assist. In theory, a more efficient core than the stock one would negate the need for the air pump. Keep in mind, the air pump doesn't work ALL the time. Only at idle and below some super low rpm, I forget.

So it adds additional air at the lowest air flow condition and then shuts off above that. I'm not sure how one would gauge efficiency without a sniffer the government uses AND both cats to test back to back.

G-Sport high flow cat: https://www.gsportbygesi.com/g-sport...stems-products
EPA certified, no air pump needed.
Had a good chat with the owner before buying mine and the Gen 1 - UHO is exactly what they recommend for the rotary. Build quality is excellent, only had it on since last year but no issues so far.
We don't have SMOG anymore, but the car smells a lot better with it on!

The cost of defeating of a smell is wild to me. Idk if I'm just used to it or doing something wrong but my cars don't stink 😅😅

Now that there is less sulphur in fuels and people are used to oxides of nitrogen and some HC from diesels, i don't think a rotary with moderate porting and decent efi setup is that bad. Following a Japanese import skyline with an old school chip tune under load is pretty horrendous though, and when that rich the catylist isn't doing that much, maybe taking the edge off NOX.

there’s the pre-mix/injection oil residue to consider as well.

nobody ever thinks their own tail pipe stinks … :fart:
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seems I'm no exception :lol: damn

Converters only work when the exhaust is at a stoichiometric ratio. The air pump allows the engine to run richer than stoich, and still have a working (as opposed to sleeping) converter.

^That's my understanding as well, and it got me curious about a few things WRT using catalytic converters with modified rotaries....

If you run a cat, but no air pump, your cat won't be doing much to clean up emissions when the engine is running outside of stoich conditions, which for most of us would only be in low load/vacuum cruise conditions (e.g., RPMs < 4~5K, MAP < 80 KPa). Idle is typically set richer than stoich, mine seems to idle happy at around 0.97 lambda when at full operating temperature. So for the rest of the RPM/MAP table, its running considerably richer, cat does nothing (or almost nothing?) for emissions and is possibly being degraded over time by the exposure to the richer exhaust gasses, and by burning pre-mix. So bottom line is if you're adding a cat w/o and air pump just to deduce stink while cruising and perhaps help keep your rear bumper cleaner on long road trips, it should work fine. Just don't expect the cat to last forever.

If you want the cat to work 100% of the time for emissions then, you would need to add an air pump that meters some air into the cat only at idle and everywhere in your map else OUTSIDE of stoich cruise conditions (all those richer regions per my example above). That presents a couple of problems as I see it - how do you know how much air to meter in so the cat "sees" an exhaust stream at stoich, and more importantly, wouldn't the added air in the exhaust system dilute your WBO2 sensor reading, making it artificially lean, and rendering it useless for things like closed loop lambda control?

You don't want it to work 100% of the time, it would melt down under heavy loads. The OEs only use power enrichment to keep the converter from overheating, and newer thermal management strategies (mainly integrated exhaust manifolds but also steel substrate cats) allow the engines to run stoich under boost or WOT, but this is in 2023 not 1993.

In the FD era all we cared about was passing the test, so to speak, so the cats only had to work during the test cycle that whichever regulatory body used (Federal, and CARB, in the US). This is why, say, FCs ignored the O2 sensor and went full rich over 3000 or 3800 or something - it was outside the parameters of the test, and a rich enough exhaust has no free oxygen so the converter goes to sleep, keeping it cool and prolonging its life.

The neat thing to remember is that you only care about running stoich to make the cat work, not the engine. So you trim fuel based on what makes the CONVERTER happy. So when the air pump is active, the computer just trims fuel so the O2 reads stoich, which is rich in the engine but who cares, the engine isn't why we have feedback fuel control.