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Discussion Starter · #1 ·
I’m looking to remove the cat on my kappa, but can’t decide whether which one is better or if anyone has any experience with both. RPM has a custom downpipe that would need to be welded or band clamped. DDM appears to be just a bolt on.
Any advise or direction is appreciated!
 

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I believe DDM HFC is by Solo Performance.
Many here installed the Solo HFC to great satisfaction.
RPM's ceramic coated one looks snazzy.

IIRC they are both bolt ons, no welding needed.
 

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Discussion Starter · #6 ·
Oh I see. I think Solo offers a catless bolt-on pipe too. Or OP can just knock out the OEM cat's substrate. Though I'd hate to see good cats destroyed.
The old turbo fried it I’m pretty sure. I’ve never thought about about removing substrate🤔

Any advantages over doing this vs a manufactured catless?
 

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The old turbo fried it I’m pretty sure. I’ve never thought about about removing substrate🤔

Any advantages over doing this vs a manufactured catless?
Very good question. Is a single diameter smooth pipe better in terms of power than a larger empty section of pipe? You might ask that of Dave at DDM - they may well have dynoed exactly those situations. Just going on guestimate, I'd think that a smooth pipe is going to be better for flow, but.........
 

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Discussion Starter · #8 ·
Very good question. Is a single diameter smooth pipe better in terms of power than a larger empty section of pipe? You might ask that of Dave at DDM - they may well have dynoed exactly those situations. Just going on guestimate, I'd think that a smooth pipe is going to be better for flow, but.........
I’ve heard/read the same. I’ll email them, but maybe with some know how could chime in!
 

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There will be a decrease in velocity as air goes from a narrow channel to a large channel. With the neck down from the large OEM cat chamber into 2.5-3" downpipe, there might be some air turbulence. Similar to a water being drained into a smaller pipe.

My other weekender is an OG RX8. Catalytic converters have low life span in the rotary engine cars. EGTs for these run through the roof. Many owners with damaged stock cats would often break out the collapsed substrates to convert the stock unit into a catless midpipe. However the cat chamber is pretty large. I noticed a degree of smoothness going from a catless stock unit to a true catless midpipe (with resonators) at high rpms. The fireballs are fun!

Since a Solo catless downpipe is not too expensive, you can go that route as opposed to clearing out your OEM cat. Not to mention it's a real beotch to reinstall that chubby unit. I think going fully catless may not be a pleasant experience as you will get nasty fumes especially during open top. I'm very happy with my Solo HFC.
 

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When I started racing back in the 70s, there were a couple of Mazda sedans running rotaries. The vibrations cracked normal exhaust tubing and they had to go t quite heavy pipe to keep a system together. They were also the loudest cars on the track, big bore V8 Canam cars included. Nice we don't have their problems!

I agree that if your local laws don't bother you, installing the replacement pipe is a nice idea. If you are stuck with inspections, a high flow replacement is the only option.
 

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you might find that you cannot get a solo performance downpipe. Because of the crap with the EPA.
 

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When I started racing back in the 70s, there were a couple of Mazda sedans running rotaries. The vibrations cracked normal exhaust tubing and they had to go t quite heavy pipe to keep a system together. They were also the loudest cars on the track, big bore V8 Canam cars included. Nice we don't have their problems!
Uncorked rotaries sound terrible.
But that's just nothing like it to hear a bridge ported tri-rotor matched to a high quality exhaust system.

HFC is not as loud as a catless downpipe too.... If noise is a concern. Oh and you'll get P0420 that an O2 extender may not alleviate.
 

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Oh and you'll get P0420 that an O2 extender may not alleviate.
The reporting of that DTC can be turned off using HPTuners. Depending on where you live and how advanced their emissions are they might be able to see that the ECM performing that check has been disabled.
 

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They are still available.. I am surprised. I know that one of the major exhaust brands got fined for selling cat delete pipes and manifold back exhausts without having a catalyst in them.
 

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I wonder of one went from a high flow Cat to one of those when the Cat died, if you'd have to do a retune......
 

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I think the difference is negligible. IIRC some old threads indicated about 3hp difference. Not enough to deal with the smell and CEL.

BTW, the Trifecta app has the ability to suppress DTC codes too. There is a variety of functions in that app that one can utilize. In order for anything to take effect (apart from code clearing), one has to flash the Trifecta tune in.
 

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BTW, the Trifecta app has the ability to suppress DTC codes too. There is a variety of functions in that app that one can utilize. In order for anything to take effect (apart from code clearing), one has to flash the Trifecta tune in.

good to know!!
 
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