These had higher compression ratios, greater RPM limits, and more effective supercharging, and they led to the "B series". The first B series model was the Wasp Junior TB, which could maintain 420 hp (313 kW) at sea level and could reach 440 hp (328 kW) for takeoff.[4][6] The TB was tuned for best performance at sea level; it was soon joined by the Wasp Junior SB, which was tuned for best performance at altitude and could sustain 400 hp (298 kW) at altitudes up to 5,000 ft (1,500 m), with 450 hp (336 kW) available for takeoff.[4][7] A still later model, the Wasp Junior T1B2, had improved performance at low level, being able to sustain 450 hp (336 kW) up to 1,500 ft (460 m)[4][7] while still matching the SB's power at high altitudes.[8] The SB and T1B2, and later versions of these with similar performance, were the most popular Wasp Junior models. One later development of the T1B2, the Wasp Junior B4, was especially designed for vertical mounting in helicopters.[7][9]