On the left is the stock pipe that I cut, to show the size of the tubing where the flattening is. It is NOT 2.25in diameter there.

On the right is a piece of 2.5in OD aluminized tubing...the size that I installed in the wagon.

2.5in tubing will flow considerably better than 2.25in flattened tubing; and all bends used are mandrel bends, to maintain the diameter of the tubing.

For the intended use of the SP wagon, 2.5in diameter exhaust tubing will provide sufficient flow for the engine. It is a DD: daily driver.

For high performance use, 3in diameter tubing would be better; but 3in is also louder than 2.5in, and is harder to fit and maintain sufficient body and ground clearances.