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The wifes XJ has a performance chip, seems to make it a little stronger in the mid range. Believe it´s a modified ignition advance curve. It also did away with the RPM limiter, at the top end. Raised top speed about 15-20 MPH (really the last thing she needed, she drives like a Grandma anyway).
But like the man said, the only real way to get more power, is more fuel, the only way to get more fuel is more air. A low restriction air intake and the 02 sensor will adjust the mix accordingly. All a chip is really gonna do is move peak horsepower and/or torque around in the RPM band a bit, don´t really think it makes many, if any more power.
I´ve had mixed results with after market exhausts. Most stock exhausts are restrictive, but the restriction actually helps low end torque some (good for big tires). While and open exhaust, will allow it to rev a little faster and get into the higher RPM band a little quicker (good for four bangers).[addsig]