the ITR motor is very similar to the GSR motor, except for the intake manifold. The GSR uses a two stage setup like the old MKII MR-2 Turbos had(it's the only car that I could think of that had it in the US and UK). It's supposed to help bottom end torque, but most people who are serious about there cars get an intake that eliminates it completely. The bore and stroke are the same as are the valve sizes. The cam specs are better and the compression is a lot higher. Also the ITR has a higher red-line, bigger throttlebody and comes with a 4-1 header.