OK I forgot for a minute it's a GT-500 were talking about, BUT same principle applies. Wonder what "open headers" would reveal on THIS car?
24 horse aint bad at all on your sidepipes! I was astonished my loss was 96 and it's not like the sidepipes are "quiet" either!
Perhaps more loss is incurred with muffler restrictions based on cam, heads, cubic inch, overall horse power, etc. Engine dynoed at 667 horse with open headers on mine.