Quoted from Dave Shadel in the YS forums on RC Universe:
"The engine has several new and noteworthy features, the most noticeable being the regulator placement on the left pushrod housing. The YS engineers felt that the former placement of the regulator caused problems in high heat conditions, so it is now in a much cooler place.
The lower crankcase has been strengthened to help prevent distortion during both high air and HP loads.
The cylinder is now "linerless" for better heat transfer, another aid in high heat conditions.
we have been flying prototypes of this engine for several months, and the results have been excellent. YS has made the engine better and more user friendly at no increase in price.
We hope you enjoy running it!!