Hey, i have 2008 mustang gt 4.6l i was wondering if i have to get a tuner kit or could i just flash the ecu?

 

What do mean by tuner kit? 90% of the power gain is from just a tune alone on a stock car.

 

How would you "flash the ecu" without a tuner? The tuner is what connects/communicates to the ecu and writes the tune (i.e. flashes the ecu)

 

You need a handheld tuner that plugs into your underdash OBD 2 diagnostic port with a preloaded tune to reflash your car's ECU.

 

You need a device... these days it can be a Bluetooth/OBDII dongle, and the software for uploading the tune on your phone (licensed, so not free), so I guess in that scenario the phone would be the "handheld tuner".
OP needs to research which tuning software solution will be used to find out what is compatible with it. The entry point to writing your own tunes is quite high, which is why most just buy tuning device and either use the canned tunes or get mail order tunes to load to that device... dyno tuning still requires you have a device.
Bottom line you need a tuning device.

 

call a performance shop. There is a ton to pick from that are reputable. Everyone has who they like and claims to be the best. With that said even the best have unhappy customers at times. With a stock car and a tune. You should NOT have any problems.