I bought some eBay pipes, and I made my own CAI. it goes through the stock airbox because of the sensor. Though the pipes i bought were 2.5" Which is narrower than what the stock or even aftermarket intakes are. So i had to use a reducer. My question is would this cause any problems/performance loss as the pipe is lower in diameter?

Making your pipes smaller, would cause air flow problem and easier to heat up quicker under the bonnet. good luck.

AEM uses a 3.5-3.0 reducer coupler on their intake setup.
I wouldn't restrict airflow anymore than that.

bad idea dude.

won't that mess with the MAF too?

What is the formula for Mass Flow rate? What does Diameter do to it? Once you figure it out you'll have your answer.

ok guys, now i ave another problem, i was DIY another CAI just right now with 3.5" pipe. also i made a short ram intake. made my own fire wall or heat shield, drilled hole for a sensor in the pipe and using epoxy atached sensor mount.when i put everything together i start the car and runing after couple seconds turns off. and when i try to step on gas it dies right a way.does anyone knows what is wrong this time? is the pipe too big?thanks

You could potentially throw off your MAF sensor readings, and loss of power would be the least of your problems.

You could potentially throw off your MAF sensor readings, and loss of power would be the least of your problems.
Throw off the sensor by what? By having a 3.5" pipe? is that too wide in diameter? Please explain.

Throw off the sensor by what? By having a 3.5" pipe? is that too wide in diameter? Please explain.

Any pipe size different than OEM will need to be tuned for the larger/smaller size.
This will throw-off the MAF sensor.
Easier to just keep it the same.