>If so, what exactly is the HT mod? Couldn't find much on the Rolls website.
Right, something I can answer.
Early/mid 1980's, Boeing were developing the 747-400, and RR wanted a piece of the action. They developed the RB211-524G-19 engine, with 56,870lb MTO thrust, for the 744 airframe (the -19 signifies the 744 aiframe.)
British Airways order this, but also wanted the same engine for the 767-300 to ease maintenance and repair.
So, RR used exactly the same engine, but pushed the throttle to 60,600lb MTO to suit the 763. This became the RB211-524H-36 (-36 = 763 airframe.)
Having dome the increased thrust for the 763, why not do it for the 744. Thus, they also did the RB211-524H-19.
Fast Forward to Mid 90's The Trent 700 had been designed, and certified for the Airbus A330, and showed better fuel consumption than the RB211. Operators ALWAYS want lower fuel costs, so RR looked at using the Trent 700 HP system in the 524G/H.
Physically, it fitted unaltered, and I think there were only a few minor changes to the neighbouring hardware. The new engine configuration was certified as a new engine mark number, and customers bought the new 04 module kit if they wanted. It had the same thrust and operating limits, so intermix wasn't an issue.
With the new heart, sorry, HP System,
the RB211-524G-19 became the RB211-524G-T-19,
the RB211-524H-36 became the RB211-524H-T-36, and
the RB211-524H-19 became the RB211-524H-T-19.