Heh, replying to myself here.
I started with installing the HKS dual resonator front pipe between the stock header and HKS HiPower Spec L catback. The HKS front pipe is 2 pounds lighter than stock front pipe. It was nice to replace rusty stock pipe with this shiny stainless steel pipe and have HKS all the way after over pipe. Here are the highlights.
- After several days (1 tank 91 octane gas emptied and filled), JR factory tune is working; no problems, no CEL
- Power increased, engine definitely revs up quicker. Maybe gain is more noticeable when combined with my catback. Short of racing exhausts, this is one of the best flowing catbacks, just like N1. Now, I have all the way straight through from stock header to the end.
- Sound changed, not necessarily louder (maaaybe a bit), but much better sound tone. I like it. There are four resonators and two mufflers in this HKS exhaust system (and still ~20lbs lighter than stock)
- MPG increased 10% !!! That was surprising to me but it is true. I have never seen 33 mpg in the city before. When I drive normally, I enjoy gas savings. So, efficiency and power both increased.
- I don't notice any exhaust smell without second cat.
Next is replacing the header with 2017+ header which I have received already. I am not expecting much/any change with that but I will post another update when I do that ... in a couple of weeks. This will be interesting to know because I have not seen anybody installing the new stock header on a 2014 JRSC/AT before. Let's see how JR factory tune takes it. I think it will just work normally.