It's not underdrive- it's OVERdrive. That is why you go from 2.5" down to 2.3" for greater boost. The SC will spin faster.

Talith- I would not go any farther than a 2.3" without some type of fuel system change. I saw that the Skyline fuel pump is a possibility, but that does nothing if the fuel rail is limited to a certain amount of delivery. Larger injectors are also a good way to go, but may cause driveability issues when not in boost or while idling. We need a piggyback computer for the stock ECU. I am trying to get Venom to produce one, but I don't think there is enough interest. I just ordered the Service Manual, which provides a complete wiring diagram. I am going to try and install an A'pexi Super AFC (which changes fuel delivery at programmable rpms, not across the whole rev range).