The collector necks down to 2" then back out to 2.5". It's used and the PO may have done that (it came off of an IT racecar). 2.5" should work well both in cost and fitment.
I had a 3" single on my ported 13B FC (all custom as usual) and it ran great (mid-13's at 104 in the 1/4 with a bad 3rd gear so I had to short-shift into 3rd). It did make quite a bit more power then the 12A, but the exhaust is not the only mod that's going to happen.
The basic 12A will remain totally stock but I'm ditching the fan for an e-fan, losing the air pump/emissions, rebuilding the carb, installing a custom intake up to the carb (cone filter in a ram air box in the nose, carb hat), the header/exhaust, and a bunch of cooling mods (radiator/coolant/fans/t-stat). I'm going to try and hit 150rwhp on a stock port before an eventual on purpose rebuild with port work to go for 200rwhp NA.
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