can anyone explain where is the difference ?

in the engine or the ecu ?

 

If you are talking the 2.7T biturbo, then the same engine is in all the C5 cars. It puts out 250hp, except for the 2004 S-line which put out 265. It was a slightly different ECU tune. Chipping any of these engines using GIAC (and I think APR was the same), raised it to 318hp. When I did mine in 2006 I had to send in my ECU. Now I think it may be just a flash (new code) of the ECU.

 

Where would i send my 2.7t ecu to?

I have the money to do this and want it done

 

I gopt my s4 tuned by jet performance but had to send it to california, this was a few years ago though and now you can get a programmer shipped to you, just plug it in and switch it to performance, i believe 'revo' has a really good programer.