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Posted by Magnum
Trim - Trim, simply put, is the size of the turbine and compressor wheels. Sometimes trim is expressed with letters or stage numbers but they actually represent a decimal number that can be calculated with these equations:
compressor trim = (inducer ^2 / exducer^2) x 100
turbine trim = (exducer^2 / inducer^2) x 100
All other things being equal the smaller the trim the less lag there will be, but the turbo may not be able to produce power at higher RPMs. A larger trim will produce more lag but have a lot of top end power.
http://www.beesandgoats.com/boostfaq/trim.jpg