http://dsmtuner.com/forums/showthread.php?t=108755&highlight=t25+t28+difference

The SEARCH button is a wonderful thing

Hope that helps

I have tried the search but nothing. their isn't really an answer to my quesion?

Then check the link I posted.

The easiest way would be by counting the number of blades on the turbine wheel.

T25 turbine wheel has 11 blades
T28 turbine wheel has 10 blades
Big28 turbine wheel has 9 blades

Every T25, T28 and Big28 that I've seen has dual fins on the compressor wheel. Obviously, the best way to determine the type of turbo you have would be to remove the housings and measure the wheels. There are several wheel trim variations out there.

When I did my big28 install I noticed the blades on the big28 looked allot diffrent then the blades on the T-25. They were much bigger, dual blades.