oh im not even after abarth sounds at this point, just making it fun but not intrusive.
the best abarth-like sound I've gotten so far was:
1. Stock system resonator delete (same type of sound but smoother, due to tuning, and a bit quieter)
2. 2.25" to a 2" resonator (the abarth actually necks down to 1 7/8"!!) to a T with 24" distance between them. I did not build this system, I put my car on jacks and clamped it all together an revved it. It sounded very abarth like, minus the tuning and multi-air sound difference. The fiat engine makes a really neat sound because of the multiair valvetrain; a dodge dart 1.4 makes the same sound, sounds good. If you bump the diameter of abarth exhaust too much it doesn't sound the same, i.e.
View: https://www.youtube.com/watch?v=HMyR0aMjfxw
(the first one is still small diameter, the second is 3" - the first one sounds like an abarth, the second sounds like a rice rocket. The third one is 2.5" I think, and sounds in between the two.
I do also think that the sound has something to do with the differential in exhaust pulses when the rear section is uneven (stock) vs. even (distance from tip to T where the exhaust splits). Even distance abarth exhausts sound smooth, uneven retains the goofy sound. In my testing a 30% difference in distance between the two tips gave the neat gravelly sound, while equidistant made it too smooth.
What I think would replicate abarth sound pretty well is below, 2.25" pipe, cat converter to a small resonator (or none!) to a split, with one leg of the split going straight back and the other leg going out to the side then back over to rejoin the straight tip adding 30% more distance for gases to travel. demonstrated below in shitty ascii art:
split
[cat converter]==================== || \\ //= (_helmholtz_) \\=====//
side loop to add distance and get to ~30% greater distance traveled.
Yes, its a sesquipedalian setup but I think it would accomplish the sound in question. I don't know if I'll bother building it, maybe exit 1 needs a neck down or a small resonator to encourage exhaust to take exit 2 or something.
I attached my ascii art as a pic because the forum keeps pooping on my formatting.
View attachment 20293
Here's what the abarths stock exhaust looks like, from the cat back