I’ve been considering options with IMR releaseing their down pipes for the type S. I do not want any codes so my hope is that by having a hi flow cat in one of the positions i might be alright. They’ve got options for hi-flow cats and catless for both the primary and secondary downpipes.

IMR Downpipes


after watching this video below my understanding of how the cat system actually works has become blurred.

https://m.youtube.com/watch?time_con...ture=emb_title


It was my understanding that in the type S the sensor is in the primary downpipe cat whereas the secondary downpipe has no sensor so the secondary downpipe would have been an easy item to straight pipe. After watching this im questioning everything i thought i knew. If the cat in the primary down pipe is the precat, which is only really useful on startup/cold engine situations and the cat in the secondary downpipe is the primary cat, Would it be possible to eliminate the pre-cat in the primary downpipe and keep the secondary downpipe (or upgrade to hi flow cat) and move the sensor to the secondary downpipe which is the primary cat? Obviously you would have to add a bung and make a harness, but i guess i never really thought about all this before and wanted to see if anyone else had any thoughts.

 

Looks like the sensors are tied to the primary cat. Moving the sensors to the downstream cat might throw a code due simply to the fact that the readings at the O2 sensors may be off. Looking at the pics on IMR's website, it looks like the primary cat is the main cat, not a pre-cat. The secondary cat looks significantly smaller and may be Honda's way of doing this. I would just leave it as is and stick with your initial set-up.

That being said, looking at the location of the secondary cat, all you Type-S owners should be weary of parking your vehicles out in the open. With their location, it looks like you're susceptible to the degenerates running around stealing cats from under cars. Once these scumbags get wind of the Type-S having easily accessible cats, you might start becoming victims of this crime wave. Perfect reason to straight pipe the secondary downpipe IMO. I would be less concerned with the theft and more so these lowlifes damaging the car while jacking it in the air ... since their primary objective is speed rather than the condition of the car.

 

I am not a SME on cats, but from my limited understanding the upstream O2 sensor is critical to "light" off the primary cats (two for V6 one for 4 cylinder) so they start working ASAP to minimize emissions. IIRC there are two control loops for the cats that the ECU monitors, the short loop gets the cats lit then the much slower loop maintains the overall cats and "walks" the air/fuel mixture all the time to various levels. I remember reading a good article about it, so if I find it I'll post it. Moving the upstream O2 sensors after the primary cats would definitely affect the short control loop probably throwing codes/CEL.

 

The second cat just seems so useless if its not being monitored. I guess i had always believed that the sensor was for emissions purposes more then just air/fuel monitoring. If it was for emissions then you would think it would be on the secondary after as much nitrogen oxide was converted as possible.

my next concern is is noise level, im really concerned with how much louder it’s going to make things, im quite happy with the exhaust note as is and i dont need a super loud exhaust in my life, im past that. At this point im either looking at doing both cats as hi flow or leaving the primary alone and deleting the secondary. Hoping someone else will be a guiney pig before me so I can get an idea of sound.

 

I'm in the same boat. I already have to upgrade my exhaust to make a trailer hitch fit, so while I'm at it I think I will add the ATLP A pipe and get rid of the secondary cat. However, I am a little worried that adding that at the same time as the exhaust that it will make the car too loud. If it's roughly as loud as the stock exhaust in sport/sport plus mode, I'm totally fine with that. But, too much louder I think it would get annoying to myself and my neighbors.

 

The 2nd or 3rd cat is not useless even though it is not being monitored. The primary cats do the heavy lifting, the 2ndary ones are there to ensure that the leftovers are broken down to meet emissions standards or beat them.

Primary cat is monitored first and foremost to get the air/fuel ratio right. The sensor after the cat is simply to let the driver know of an emissions system problem. Putting another sensor after the 2nd/3rd cat would just cost automakers more money to build and sell the car with no real added benefit.

Removing the primary cat will make the car louder, high-flow will not really make it louder. Removing the 2nd/3rd cat won't make a large difference in power or sound either since the restriction would be the primary cat.

 

in the type S’ case, the ONLY sensor is in the primary and since the system splits immediatly after the secondary (and no 3rd cat)it seems like the easiest and cheapest way to make a legit difference in the exhaust flow of the vehicle, without any codes. I dont see how removing a section of 2.25 pipe with a honeycomb core and replacing it with a section of straight 3” pipe cant make a significant difference. It will likely be more beneficial then doing the rest of the catback.

it also feels a bit like Acura designed it with a bit of thought given to the aftermarket. Its extremely accessible and should be a quick and easy swap

 

There's a guy on YouTube (and I think he was on here briefly) named hondamobilemechanic who replaced the downpipe to the stock exhaust and made pretty noticeable performance gains.


From what I've seen, most catbacks are really more for sound than anything else, until you start pushing much higher power than stock.