The 5.3 V8 was an upgrade from the 4.8, not to be confuse with the 4.2 IIRC. The Vortec 4200 (er, 4.2) is an all aluminum 24 valve I6 that has been available in the Trailblazer for a few years now. Part of the story with the good power number is in the variable timing exhaust cam. Lots of great info on inliners. http://www.inliners.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=3;t=000538
The bellhousing bolt pattern is unique to the engine family which includes an I4 and I5 engine.
It is an impressive six.
mv
