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The more restrictions you put in the exhaust system, the less likely you are going to shoot flames. With the Davesport cat, you'll probably shoot the core out earlier than the flames.
If you are running any type of cat, you will not be able to shoot flames.
Known problems with cats is the fact that they burn out quiet easily if they aren't necessarily the OEM one. So other high flow cats will probably burn out relatively fast and will be costly due to the high exhaust temps of the rotary.
So just built a custom mid pipe, I would have started a new thread but i cant do that yet so I put it here. I used the BHR resonated mid-pipe as the base but any 3" diameter mid-pipe will work. I found a company that sells metallic core cats with very low core density 100 or 50 CPSI, BUT have a 5" outer diameter. This 5 inch outer diameter is important, it slows the velocity of the flow threw the cat and increases catalyst efficiency while also providing greater flow capacity. so you get all the advantages of a full mid-pipe but will still pass emissions. I chose to clamp my cat in place after carefully measuring lengths and flange orientation. i did this to make the cat easily replaceable. (longevity of this cat is unknown at this point, but i will keep everyone updated) See the Midpipe i built below, and just for fun i will show the full exhaust that i installed at the same time, HKS Legamax.......AWESOME SOUND just FYI.
This is the cat
Below is a pic of the mid pipe after the cat is installed and clamped in-place. Yes i was cringing when i had to cut a $450 part lol. If you do this mod, be sure to measure carefully. I took my time and everything bolted up perfectly.
Final install with the HKS Legamax catback. This thing is a work of art then you first look at it lol. The final product sounds awesome, gave me good butt dyno power gains and no check engine. I putt 200 miles on it in one day after the install no check engine light.
^is there any particular reason why you wrapped the cat? They already run incredibly hot from rotary engines and wrapping it just exacerbates problems including premature wear from heat...
I would advise in removing that ASAP unless you have a really good reason...
^is there any particular reason why you wrapped the cat? They already run incredibly hot from rotary engines and wrapping it just exacerbates problems including premature wear from heat...
I would advise in removing that ASAP unless you have a really good reason...
Good observation, but its part of the greater design so to speak . I wanted to test the robustness of the cat, and my failure hypothesis. Below is my reasoning if you feel like reading it..... I didn't include any of this in my initial post due it to being boring and most people not going to read it any ways.
I specialize in failure analysis for my job as a reliability engineer. I have been, in my free time, looking into failure methods for catalytic converters. specifically on rotaries. Exhaust gas temps for the Renasis are high, but nothing a performance big-block V8 cant generate; and no abnormal failures are reported with that application. So absolute exhaust gas temp is most likely not the root cause of the failures like most people believe. There is some other mechanism that causes catalytic converters to fail on rotaries. I feel that the root cause for premature cat failure is the amount of un-burnt fuel in the exhaust stream. (this is a known failure method even for the original OEM cat) The un-burnt fuel reacts in the cat and raises its temps; basic cat operation 101, too much un-burnt fuel actually melts the substrate. This is what happens when the OEM ignition system becomes weak. I noticed that every aftermarket cat that people have used with the Renisis is 4"OD. I looked at the total size of the stock cat and saw the problem. Aftermarket cats are just too small; even if their flow rated for the amount of exhaust gas produced by rotaries. Because even being high flow, the cat still must react the same amount of unburnt fuel as before, but now in a much smaller volume. This means the root cause has become a thermal density issue. The 5" outer diameter cat I chose not only reduces flow velocity, and increases catalyst eff, it also decreases the thermal density. Its very close to the size of the stock cat, but much higher flow, being 50 CPSI. Total volume occupied by the catalyst material in my cat is 50% more than most of the 4" OD cats. This cat by MBS is actually the only one I was able to find that had a 5" OD, and 3" inlet/outlet. The manufacture claims it has the only cat with these specs. Therefor, if am correct wrapping the cat in heat wrap wont cause a failure. AND I can lower the temps felt in the car; my floor pan above the cat is MUCH cooler now. Once I can get some measurements with a my infrared thermometer at work I can get a better sense of if I am correct or not. Also I am planning on dropping the mid pipe at my next oil change to see how its doing. I Hope the wall of text wasn't to much, does it qualify a a really good reason? lol
Awesome work. ^^
That sounds like a fun job you have. What kind of devices do you investigate?
Just to throw this in, I've read that the way many aftermarket cats are built, the cylindrically wound pattern of the reactive element, makes it susceptible to bulging out toward the rear under exhaust pressure. Not sure how exhaust gas velocity/flow rate from a Renny compares to a V8, but suspected this may play into the high failure rate. A deformed element could buckle further under heat expansion and lose its expected flow characteristics. This is in contrast to the OEM grid.
The cat appears to be a simple spiral wrap. It will likely fail in time just like all the other cheap metallic cats. There's a reason that a quality metallic cat like HJS cost so much more, but don't fail while also easily outflowing any ceramic cat.
Awesome work. ^^
That sounds like a fun job you have. What kind of devices do you investigate?
Just to throw this in, I've read that the way many aftermarket cats are built, the cylindrical wound pattern of the reactive element, makes it susceptible to bulging out toward the rear under exhaust pressure. Not sure how exhaust gas velocity/flow rate from a Renny compares to a V8, but suspected this may play into the high failure rate. A deformed element could buckle further under heat expansion and lose its expected flow characteristics. This is in contrast to the OEM grid.
I work in the Oil field up in PA on the directional drilling side. when things break down hole i have to figure out the root cause.
Yeah i was thinking the same thing as well, looking at some pictures of failed metallic wound cats. My determination (educated guess lol) was that that type of failure occurred after the cat over heated and the substrate lost it's rigidity and basically turned to jelly then deformed. wont know for sure till i get some good hard miles on my test case
The cat appears to be a simple spiral wrap. It will likely fail in time just like all the other cheap metallic cats. There's a reason that a quality metallic cat like HJS cost so much more, but don't fail while also easily outflowing any ceramic cat.
Well i cant sit here and tell you that you are wrong and i am right, I have no data at this point. I wish i had a good FEA model to plug in to my work copy of Ansys but i don't have enough information to even begin to build one from scratch. However like I said before if it is a thermal density issue (i really think i am on to something here) the cat will be fine even if it is as you so eloquently put it "just like all the other cheap metallic cats". Time will tell in this case.
Worst case: @4k when i change my oil the cat is melted and blown out, i highly doubt it will clog up. Then I buy an HJS cat, yes i know about the HJS cats i can use the search function..... do you see what i did there i kid i kid.
Best case: I found a cheap alternative for the RX8 community and everyone who has to deal with emissions testing lives in peace and harmony, except for California because there hella tough hahaha
Thanks!
Last edited by 556Magpul; 06-01-2015 at 11:38 PM.
Reason: piss pore typing at 12:30 am
You may be onto something here, as rotaries are far less efficient than piston engines (especially these days), and all I can add is that I have had FAR fewer issues with the ceramic cats I provide as opposed to the metallic cats I used to use.
Good luck and thank you for your efforts.
Thanks for the Input, i am honored you commented on my humble post I will post an update in a month or 2 once i get some good miles, or a check engine light lol. I am thinking that the fact i haven't gotten a check-engine light while using such a low density (50cpsi) cat means I am moving in the right direction...
P.S loving the BHR ignition upgrade!
also should have just responded to everyone in one reply and multi quote..... oh well
There are multiple reasons why the cat converter you are using only costs $120 and the 5.7" OD HJS cat I used cost $1000 and it doesn't take an FEA analysis to figure it out (BSME myself), but good luck and best wishes just the same ....
just out of curiosity, did you inquire about what the maximum sustainable temperature rating for that cat converter?
Sorry for how long it took me to reply been busy with work,
I spoke with mbs before ordering and their tech said they haven't seen any problems with temps up to 2000 deg F but didn't know the official number. I actually just got off the phone with them, I asked for a full data sheet or whatever detailed specks they have on the substrate.
So i have about 1k on the cat now, still no check engine light, or other problems to report. However the temperature of the floor above the cat is much lower now. Also the temp in the center console is much cooler. I didn't think to get a before and after measurement. its just by feel.
True, and I have installed a few of them for customers who sent them to me, but most people (especially these days) cannot afford the HJS installed on ANY midpipe.
$$$$$
If the cat clogs and doesn't get caught in time then the compared to the cost of a new engine an HJS cat converter is not that expensive
The specific one I used also flows as well as a catless 3" mid pipe and after 4 years of competition use the internal metallic substrate still looks the same as when it was new. It's suitable to 400 hp, so even FI worthy.
But everyone gets to decide on their own ... it is expensive, it does deliver max power without failure ...
So I have an update on the custom cated mid pipe made form the the BHR mid pipe.
Conclusions.....
1st, The MBS 50CPSI 5 inch DIA cat works fine on the road and does not throw a CEL. Will need to replace and try another without tracking it to get longevity test but initial test run was good.
2nd, The cat is not quite up to the task of the track. This really isn't a surprise but I wanted to try.
The Test; Not exactly Scientific but hey it was fun.
So I have been running this cat on my car since around June and it was running fine, it even survived the first track night at Pit Race. The second track night later in the year apparently cooked the catalyst. I had a total of about 6K on the cat and one track night before i cooked it at the second track night. I am pretty sure the second track night is what cooked the catalyst because I started to get an intermittent CEL for catalyst eff about half way threw the second session on the track. Also I started to have some raw fuel smell when the engine is warming up afterwards. So I am pretty sure the cat isn't working any more. I have the car in storage for now and stopped driving it back in October. Being I am in Pittsburgh and its d**n cold I wont be pulling the mid pipe down to get pictures till I get moved into my new garage later in the spring. Then i will see if i melted the substrate or just cooked the catalyst.
My plans are to just buy another one of the inexpensive cats to use for standard road driving and then just switch out for the open cat for Track Nights. Then i can get a better idea of the standard road reliability.
I had it welded into my BHR midpipe at a local shop a couple of weeks ago, but I won't be able to test it at the track until April, unfortunately.
Initial impressions are good on the street. My CEL went away almost immediately, and my tailpipe smells better than it did with my OEM cat. Time will tell, of course.
]Being I am in Pittsburgh and its d**n cold I wont be pulling the mid pipe down to get pictures till I get moved into my new garage later in the spring. Then i will see if i melted the substrate or just cooked the catalyst.
Any update on whether the substrate melted or cooked catalyst?
08-18-2013 | 06:59 PM
. I wanted to test the robustness of the cat, and my failure hypothesis. Below is my reasoning if you feel like reading it..... I didn't include any of this in my initial post due it to being boring and most people not going to read it any ways. 
i kid i kid.
