The XK8 is a normally aspirated 4.2 liter V8 engine that produces 300 horsepower at peak output. It is mated to a ZF 5-speed gearbox in the 1997-2003 models. The XKR is a supercharged version of that same engine; it makes about 400 horsepower. It is mated to a Mercedes 6-speed transmission. The XKR is outfitted with upgraded components: electronically controlled shock absorbers ("CATS" suspension), a different power steering system, re-tuned engine management systems, and beefier suspension components. Jaguar also offers an optional Brembo big brake kit specifically for the "R" line. So while the bodies are basically the same, there are noticable differences between the two trim lines. You get a lot more performance out of the XKR.