Confused...
I'm used to cars having a downpipe, midpipe, and then catback to the muffler. Those setups usually hav a precat in the downpipe, then a main cat in the midpipe, and then straight pipe back to the muffler. This is how the RX7 and others I've looked at function.
Stock, how many cats does a Supra have? I thought 2. If so, can you simply remove the precat with a downpipe and run a highflow cat at the midpipe position? I've looked on MKIV.com and their pics of the exaust seem to jive with what I've seen on other cars but this thread makes it sound as if replacing the downpipe replaces ALL of the cats, yes? If there really are two cats on the Supra I'd remove the precat in the downpipe and put a highflow in the middle if roadside inspection is a concern.
My RX7 has a pile of bolt-ons and with just a midpipe cat passed a rolling road emissions test with flying colors - no AIR hose connected to the cat either! I almost passed out when I saw the numbers they were so low. Anyone who looks under the car see the midpipe cat and it blows clean so no worries, flows well too. The exhaust tone completely changed with the new midpipe and the way it crackles occasionally it may even be blowing flame
Do the Supra have trouble maintaining boost without spiking when the exhaust is freed from cats and replaced with better pipe? RX7 have trouble with this but I've not noticed any comments about this problem with the Supra. Is it not an issue?