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The 425 horsepower, normally aspirated (no supercharger) 6.1-liter Hemi matches the gross horsepower rating of the 426 Hemi engine exactly, albeit with lower torque (420 versus 490 lb-ft). It seems to beat the Elephant Engine's net horsepower, which has various sources have pegged at between 330 and (again) 425. In short, the 6.1 Hemi is the closest any V8 has come to the most successful stock racing V8 to come out of Detroit - and it gets better gas (and oil) mileage.

The SRT group uses traditional HEMI engine cues such as an orange-painted cylinder block and black valve covers. Greater power was achieved by boring the cylinders (increasing their diameter) by 3.5 mm each; they also increased the compression ratio from 9.6:1 to 10.3:1, and made other changes as detailed below.

Engine breathing was increased with new high-flow cylinder heads, a specially designed intake manifold, and exhaust “headers” with individual tubes encased in a stainless steel shell, all unique to the 6.1-liter Hemi. Larger diameter valves and reshaped cylinder ports in the heads maximize air flow. The intake manifold was designed with larger diameter runners for higher-speed tuning. Exhaust is routed through a larger-diameter (2.75-inch vs. 2.5-inch) exhaust system with 3.5-inch chrome tips.

Performance-oriented camshaft profiles were developed to balance total vehicle requirements, allowing more air in and out of cylinders. This increases performance and allows a higher top engine speed, 6,200 revolutions per minute (rpm) rather than the stock Hemi's 5,400 rpm. Intake and exhaust valve stems are hollow, and the exhaust valve stems are filled with sodium to help dissipate heat more efficiently.

The 6.1-liter Hemi V8 engine is further strengthened with redesigned components, including a reinforced engine block with increased coolant flow, forged steel crankshaft, high-strength powdered-metal connecting rods, floating-pin pistons (cooled by oil squirters), and an oil pan modified for reduced oil foaming.

http://www.allpar.com/cars/lx/srt8-chrysler.html

The new racing only Hemi:

The just-announced-at-SEMA 6.4 liter version, at a historic 392 cubic inches, due in 2008 with the Dodge Challenger, does the SRT-8 one better, and we'd like to know how since the extra size isn't all that much. It generates a full 505 horsepower. Because it’s a crate engine for racing only, it’s quite possible that some sacrifices made for gas mileage and emissions reduction in even the SRT-8 version have been removed.

http://www.allpar.com/mopar/new-mopar-hemi.html