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The GLT turbo models (turbos introduced on GLTs in 97) use
a smaller turbo. It builds less boost but does build boost
faster. This is called a Low Pressure Turbo. The High
Pressure Turbos (T5s) obvously build more boost and flow more
air. Note, the torque curves I have seen for LPT and HPT show
the HPT has more torque at all engine speeds (many think the
LPT has more -- it probably just has less turbo lag).
The HPT engine has thicker cylinder walls (and thus displaces 2.3
liters vs. 2.4 for the LPT). The thicker cylinder walls can handle
more heat stress. Along the same lines of handling heat are
changes to the pistons and other engine internals (including
sodium filled valves) on the HPT engine. The HPT also uses oil
cooling for the crank (a stream of oil is sprayed on the crank).
Sometime around the 2000 model oil cooling was introduced on the LPT
(and maybe the base engine, not sure). So, the HPT is basically
beefed-up to produce lots of power while the LPT does not use all
of the same expensive parts; but, the LPT is not as stressed so
it does not need all of the expensive parts.
The 2001 LPT produces 210 lb/ft torque wich compares quite well
to the 222 lb/ft of the 850 high pressure turbo models.
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