Firma 3900Kv 4-Pole Brushless Motor

The Spektrum Firma 3900 Kv 4‑Pole Brushless Motor is a high‑RPM sensorless upgrade for 1/10‑scale 4WD vehicles like the ARRMA GROM BLX. Featuring a low‑cog 4‑pole rotor, CNC‑machined aluminium housing, pre‑installed bullet connectors and a durable 5 mm steel shaft, it delivers smooth, efficient performance at 3S voltage levels.

Features

• Sensorless brushless inrunner with high‑tolerance 4‑pole rotor design
• CNC‑machined aluminium can, insulated high‑purity copper windings
• 5 mm hardened alloy steel shaft (supports SafeD‑5 pinion)
• Pre‑installed 4 mm gold‑plated bullet connectors
• Ideal for 1/10 scale models running up to 3S LiPo

What’s in the box

• Spektrum Firma 3900 Kv brushless motor with connectors
• Instruction manual

Needed to complete

• Spektrum Firma Smart ESC (e.g. 130) or compatible standard ESC rated ≥ 45 A
• 2S–3S LiPo battery (7.4–12.6 V)
• Full‑range transmitter and receiver for your radio system

Specifications

• KV rating: 3 900 Kv
• Application: 1/10‑scale surface vehicles
• Input voltage: 2‑3S LiPo (up to 12.6 V)
• Maximum continuous current: 45 A
• Approximate output power: 1 420 W @ 3S
• Efficiency: 80 %
• Outer diameter: 36 mm (1.4 in)
• Length: 60 mm (2.35 in)
• Shaft diameter: 5 mm (0.2 in)
• Weight: approx. 253 g (8.9 oz)
• Connector type: 4 mm gold‑plated bullet
• Technology: Sensorless 4‑pole brushless motor