Is it because the back pressure is out, How do i know what it should be.
Yep: When you installed that straight pipe you destroyed the engineering and programming that the factory used to make Your car run it's best. And if you start messing with aftermarket stuff to try to make it better you will probably be wasting your money as well as making the performance worse. Have a factory exhaust put back on it. you'll be a LOT better off.
Some cars are designed to work with back pressure. Yours is one so it seems. Giving it a straight through exhaust system may make it sound faster or deeper or throaty but did nothing for performance. So, in your case sticking with factory exhaust system is the best bet. Besides, when it comes to emmission inspections - they know you modified the exhaust (and they will probably ding you right there and then)
any time you make a engine breath better ex. performance filters exhaust intake ect you need to ad more fuel to the mixture because there is more air going threw get a chip is my suggestion can also try a higher fuel pressure regulator no need to rebuild injector unless your pushin 600hp with a turbo
