The nominal size of the stock exhaust system is 2.25in outside diameter. This image shows the area of the stock system where the pipe is 'flattened', or 'ovalled', to maintain adequate clearance from a floorboard crossbrace. The red lines show where I cut the pipe during the engine replacement procedure. The red lines are at the end of the flattened section of pipe, where the pipe turns up to send the exhaust gases to the front muffler.