With the amount of power you'll be dealing with (considering future upgrades), 3" would be a better choice. 2.5" might be a bit restrictive. Most B series conversions would probably utilise 2.5".
2.5 if its daily driver. 3' for all out power/race.
what drugs are you smoking boy? 2.5 inch if you like breathing through a straw. if you are going to do a k swap and try to save $100 from a 2.5 inch to a 3 inch and its going to bust your budget then do it
Bookmarks