What is the stock exhaust size for our piping. I was thinking of getting cat-back exhaust and want to know what size yall think i should get.( i was thinkin' 2.5", what do yall say?)

 

Bigger does not equal better (in this case, anyway)
Lose all your back pressure and your torque goes out the window. You just want to relieve excessive back pressure.
2.25" is as large as I would go. With a high flow cat and muffler, I would venture to guess 2.0" might even be ideal for the mighty 1.5 liter motor.

 

I double that. Without the correct amount of back-pressure your engine will not run!

 

But will the 2.5" take away that much back pressure.

 

Go look at diameters of tuned exhaust systems for other comparable sized cars. Most are around 2.25"...why didn't they use 2.5"? I'll bet the loss of torque vs tenths of horsepower at redline made the tradeoff not worth it.
Sure, you'll see a 2.5 on a larger car or forced induction car...apples and oranges.

I think Even Mazdaspeed uses the same catback unit for both 1.5 and 1.8. If it's good enough for a 1.8, it'll definately work for the 1.5.

and mandrel bends...not crush bends

 

thanx fellas I'll probably just go with the 2.25" exhaust.