Is it the engine size? And/or other things?

 

Depends on the version and things. XJ8 is the series, with different levels in it, the XJR is the trim level of the Supercharged model with fancy wheels and goodies. But they DO offer that engine in other versions of the XJ8 model, so it really depends on exactly what you're looking to compare.

https://en.wikipedia.org/wiki/Jaguar_XJ_(X308)

 

Thanks, I'll check out your link.

 

XJR has many mechanical performance upgrades over the standard XJ8.

engine, suspension, transmission, later model comes with Brembo brakes

it is more sporty and aggressive while XJ8 has a more traditional elegant appearance like chrome trims.