Overclocking
Overclocking the Core i5-9600K on air was clearly limited by thermals. We could only increase voltage by +0.080 V (while increasing Turbo limits at the same time), or the CPU would thermally throttle and drop (slightly) below our targeted clocks.
Using this voltage increase, our maximum stable overclock was 4.8 GHz.
With 240 mm AIO watercooling, we could dial in much higher voltage without even getting close to throttling. We used +0.15 V in this test and reached 5.0 GHz stable. Any higher wasn't be stable, no matter the voltage.