i have a borla headers ,hks down pipe with a 3" pipe to the STi muffler !Do you think i can get more hp with a greddy/turbox rear muffler?Or does the sti muffler work great?
Thank you
Adam

You will probably get a few more ponies, but not enough to justify the expense really. I would dyno-guess an aftermarket muffler would give you 1-3 HP over what you are currently making. It's a judgement call on wether you like the looks/sound. A Greddy Evolution is one of the quieter ones you can get so it will give you more HP and still be stealthy. It does have a big, honking tip on it that many think is too ricey though.

I think the power addition will be closer to 5 or 8HP, if not more. The STi muffler is a cork!

I the STi Muffler over to my Stage 4 WRX once just for fun... the car almost wouldn't run! CEL in 3 blocks and strange puffing sounds during spooldown.

If you have the rest of the exhaust freed up, pull that cork off. However, if you want to keep your car quiet and don't mind leaving some power on the table, leave it on.

Russ