i've not heard a unicorse or full supersprint system in real life - only on short clips like mine. i think unicorse would be a higher pitch sound, cos my friend Abyss (with 156GTA) is going to start with their backbox cos he likes the higher pitch sound (a la Ferrari). mine might be more like supersprint, with a fair bit of low grumble/bass. not really sure though! would be great if it was possible to hear them side by side.
the backbox has been fitted for about 12 months now, so is well run-in. alone, it produces a lot of low down rumble, but also drone. i still like it a lot though. the cats are only new - less than a week, and about 200km so far - and they have improved the sound. i don't know if the effect of the cats (in terms of sound or performance) will alter with time. do you think i can expect that? probably not like the rear muffler which has a lot of packing/steel wool/fibreglass (or whatever!) inside.
I wonder if you can feel any difference with throttle response? Would you expect any change of any characteristic over time?
i cannot feel any difference. or at least, nothing startling. i need to drive the car more, to try and appreciate if throttle response is a bit better or not.
from the dyno reading, and comparing 'before/after', i wouldn't expect much. there is improvement across the rev range, but small.
Our GTs' have got 4 cats I believe, I would like to know if it would be possible to significantly increase throttle sensitivity without sacrificing an MOT Pass (I presume some decatting would be needed)?!
indeed, the cats attached to the manifolds are meant to be quite restrictive to gas flow. another Oz Alfista - ALFA_001 on AO, with a 147GTA - has recently fitted GTV manifolds (phase 1, pre-cat versions), with aftermarket equal-length front pipes (that are between the manifolds and main cats). he reports considerable 'seat of the pants' improvements. so the gains are there to be had. yet they are a much bigger job to fit; 'recommended' fitment involves removing the engine (!), though he managed without doing that, but still dismantling much of the front suspension and aircon piping, etc. but he hasn't found the 'sound' too pleasing so far, and is trying to modify the exhaust further downstream to improve it.
i'm pretty sure it would pass MOT, particularly with the engine warm. the manifold cats cater for cold start ups, to reduce emissions at this time. but the main under-car cats take care of the rest once they are warmed up.
not sure if it's possible or worth just 'punching out' the guts of the cats on the manifolds. but ultimately, some new equal-length manifolds like unicorse or supersprint would be the way to go.
but for me, it's mostly about 'sound', less about power. so i think i have finished.
attached, is my dyno print out from yesterday (backbox + high flow cats fitted). it is impossible to draw much conclusion or do comparisons with other dynos though.