Here are some facts about the engines used in the 996 Carrera, Turbo, GT2 and GT3.

1. The M96/01 engine used in the 996 Carrera and the M96/02 used in the Boxster were designed from scratch.
- The cylinders are in one-piece with each side of the split crankcase. Also the cylinder heads are in one-piece.
- No dry sump
- More cost-optimised

2. The engines in GT3, GT2 and 996 Turbo are closely related to the Le Mans winning GT1 engine!
- These engines use aluminum crankcase similar to the crankcase of the air cooled M64 engines and with water cooled cylinder blocks that are separate from the vertically split crankcase
- Same bore and stroke as in M64 engines resulting 3600cc
- Unlike in Carrera/Boxster engine a plasma-nitriding process is used to ensure a deeper hardened layer and higher torsional resistance of the crankshaft
- GT3's M96/76 use titanium con-rods as the racing version
- In the M96/70 engine of the 996 Turbo and the M96/70S of the GT2 use steel con-rods. Titanium is not needed because they're limited to 6750 rpm.
- These engines are about 30kg heavier the M96/01/02 engines

There are also differences in valves, cams, air intake, etc...

And where did that magical GT1 engine come from? It was designed from scratch and used inputs from 959, 962 and also then upcoming M96 engines.

I collected the facts from Paul Frere's "Porsche 911 Story".