Pulled from MAPA:
Early -GB Engine Problems
The original 210 horsepower Continental TSIO-360-GB engine ran hot, both in climb and cruise. Extremely high temperatures are hard on cylinders and the -GB did experience premature cylinder replacements. The engine also had a fixed wastegate in the exhaust system, meaning careful pilot technique was required to keep from over boosting the engine on takeoff. If the pilot inadvertently added full throttle for takeoff or a go-around, the engine would over boost, with only a mechanical pop-off valve in the induction system to save the engine from literally coming apart on the runway. Fixed wastegates also mean high turbocharger speeds (RPM) at altitude, which reduces turbocharger life.
Compared to the absolutely bulletproof Lycoming IO-360 engine installed in the M20J, the TSIO-360-GB engine in the M20K got a bad reputation. The engine did run too hot (shame on Mooney's flight test department during those days). But added maintenance costs are just a fact of life with a turbocharged engine compared to a normally aspirated one.
The TSIO-360-LB is a Big Improvement
My very first job in 1983 as Mooney's newly hired engineering test pilot was to certify the 1984 model changes to the M20K. These changes included the installation of a newer variant of the TSIO-360 engine, the -LB, which replaced the -GB. The changes between the two engines were minor, but one of those minor changes proved to be a major improvement in engine life and longevity. The throttle body on the -LB engine was a larger diameter than the one on the -GB. This allowed more induction air into the cylinders, which helped to cool the engine. In flight testing the -LB, we found a decrease of approximately 20 degree F on the cylinder and the oil temperature with the -LB installation. This was huge.
Continental recognized the improvement, so much so that any -GB engine that goes to the TCM factory for an overhaul gets changed to the -LB configuration automatically and the engine's data plate gets changed to reflect the upgraded -LB configuration. If you're shopping for a 231, look closely at the engine's data plate. If it says TSIO-360-LB, great. If it says TSIO-360-GB, you've got an original hot running engine that will simply give you more trouble. Move on to another one with the -LB engine.