How much horsepower can a 4L60E handle?

Per Speedway Motors: a stock 4L60E is comfortable around 300–360 lb-ft of torque (roughly 300–400 hp in a typical street vehicle). With a stronger 3–4 clutch pack, hardened input shaft, beefed sun shell and better valve body calibration, street builds reliably handle about 450–500 hp; fully built units have survived 800+ hp. Heavier vehicles, towing and aggressive launches shorten its life. The '60' in the name is a GVWR class, not a torque rating.