This is the more aggresive of turbo upgrades. This is fitting a very large compressor wheel into the compressor housing. Usually this also involves modifying the exhaust wheel by clipping. Clipping the exhaust wheel is removing some of the material (at an angle) which will allow more exhaust to bypass the turbine in extreme conditions. The reason this is done is that when u are using your big upgraded turbo at a high psi and RPM, the exhaust cannot get out fast enough. So the clipping procedure will allow a little more flow at the expense of lag. The large compressor wheel also has it's share of lag, which most don't mind for the absolute kick in the pants it offers once spooled up. If you do go with this upgrade it is suggested that you upgraded your downpipe and exhaust system to help the turbo spool up earlier. This upgrade usually contains a TO4 "E" upgrade wheel. Speaking in trim this would 54 and 60. Some companies even offer a 60-1 wheel which is even larger. Most consider this overkill and would never operate at full potential. These turbos flow well at up to 25 psi, and are limited basically by the exhaust side. The usual price for this upgrade is $650, but you can find it for less at some companies. An example picture is below. Keep in mind that this a supra turbo, note that the supra compressor wheel is even bigger than an all trac. Also notice the clipped turbine on the bottom right picture. They also clean up your turbo nice.