It's a question of degree. Exhaust upgrades are largely why the mid-'85 CFI GTs had about 15 HP more
than the '84s. The engines were otherwise the same. The '85 CFI engine used the same shorty headers
as the 4V engines, and while the Y-pipe was different, it was the same diameter. Both used the same
main catalyst bed, but with a different connection to the Y-pipe.
I don't know that the '85-style headers and Y-pipe, with a larger diameter single exhaust wouldn't get you
most, if not all, of the improvement, vs running dual mufflers and tailpipe.
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