im getting custom exhaust, should i get 2 1/2 or 2 3/4 piping
 

whats 2 1/2inch piping rated up to.i know ill never be pushing more than 240hp cuz im tipronic and im planning on going with a 2 1/2 inch dp. figured i'd ask. also if i removed the cat and hooked the sensor correctly on the test pipe will i get any check engine lights, i mean not that i would do that or anything. Thanks

2.75 piping not too common but could help a bit in the top end dept. ...
 

you would get the check engine light unless there is some sort of 02 sensor simulator involved

If you plan on doing a 2.5" dp then I would say go 2.5" all the way back.

2 ways that I know you can figure this out (or I can do it for you)
 

1.. Find out the size of your turbine outlet and add 10% for the size of your tailpipes and downpipe..
or
2..I recommend using both methods just so you can check yourself.. Now method #2 bases its answer on how much air is going into your engine, and think it' will yield you a more accurate answer because I don't think turbine outlet sizes are based solely on the amount of air you can push into your engine and I know your cars air induction system is restricted by audi .. Now, this part requires some calculations, but don't worry Ive set up an excel spreadsheet so you don't have to worry about that..
Here is how I did it..
It was a little tricky because the value you get from your MAF sensor is obviously a mass flow reading and when you use this formula you need a volumetric value.. So you have to convert your answer over to a volumetric flow value using the volumetric flow rate equation.. There are still a few variables that I would like to confirm such as the density and temperature of air (i am not 100% sure from where in the engine you would want your density and temperature readings from ..Ive been using the temp and density after your turbo)..
So whats the maximum MAF reading your getting from your car ??