I agree with Benny, I've been running a 2.25" exhaust w/turbo muffler for quite a few years on my A15 with headers (2.5" collector). Before that it was 2" with straight through glasspack, and the larger one felt stronger and wasn't as obnoxious. Anything less than 2" on the A series is garbage, mostly because 90% of exhaust shops are going to use "crush bends" instead of mandrel bends. This means that any bend in your system is going to be crushed down to a smaller diameter. Your exhaust is only as big as it's smallest point after the primary tubes connect, so if you have a 2" crush bent exhaust, it'll neck way less than that at any bends. If it's a mandrel bent system, a 2" exhaust would be plenty big for an A series, but I personally like to run as big as I can so I know that the exhaust is able to flow great if I continue to upgrade.