My Mongoose down pipe has a 3" internal bore and I know that the PDUK version does. Not to sure about the blueflame but I would imagine it would also be 3"
Edit: Not sure how easy the DS CAIS is to fit as I have a code:red one fitted to my car
. All I'll say is the standard air box is a right pain in the rear end to remove
. You might need to lower the radiator out the way to help ease the removal of the poxy thing!

Edit: Not sure how easy the DS CAIS is to fit as I have a code:red one fitted to my car

