The early model Stelvios had A5 as the engine prefix number, they shared the common engine with the Griso. These models had quite a distinctive power step about 5000rpm, but it was still a torquey motor.
In 2010 the engines came with an A8 prefix number, these had revised cam profiles and a change in the ecu mapping, giving different engine characteristics, they lost the power step and were a bit smoother at lower revs.
The 2012 models (came out in 2011) have the 32 litre fuel tank, different cams and followers, and a different fuel map, the engine prefix is AC.
Some of the 2008/9 models (Griso and Stelvio) were subject to a recall for cam and followers to be replaced.
The Guzzi 8v engine is very hot running, it is essential to run it on full synthetic 10/60 oil, and change it regularly.
I had a 2009 model for two years and 10,000mls, I swapped it last October for the 2012 model. Both are great motorcycles, and for some reason under rated by the press. Perhaps Moto Guzzi should spend more on press advertising

I love 'em.
