Overview
Great choice for 1/10 Scale 4WD models looking for a high RPM motor upgrade running up to 3S * (12.6v) power. Low cog 4-pole rotor design, CNC machined aluminum housing, insulated high purity copper windings, high-quality alloy steel 5mm output shaft, and high-precision bearings for high durability and smooth rotation. Preinstalled with 4mm gold plated bullet connectors for easy installation. Perfectly matched for the Firma 130 Smart ESC.
*Will work with the 85A Firma ESC, but only for 2S power.
Features
- Perfect upgrade for brushed 540- and 550-size brushed motors with the same diameter and mounting pattern (vehicle must accept pinions with a 5mm bore)
- High-quality 4-pole sintered rotors
- Includes 4mm gold plated bullet connectors
- Precision ball bearings and hardened steel 5mm shaft
- SafeD-5 Pinion compatible
Overview
The SPM Firma 3900Kv 4-Pole Brushless Motor is an exceptional upgrade choice for 1/10 scale 4WD RC models seeking high-performance, high-RPM capability. Whether you're looking to breathe new life into an existing brushed motor setup or building a competitive RC car from scratch, this Spektrum brushless motor delivers the speed, reliability and precision engineering that serious RC enthusiasts demand. With support for up to 3S LiPo battery power (12.6V), this motor represents a significant performance upgrade over traditional brushed 540- and 550-size motors, whilst maintaining compatibility with existing mounting patterns and pinion bore specifications.
Designed and manufactured by Spektrum, the global leader in spread spectrum technology, the Firma 3900Kv motor combines decades of expertise in RC propulsion with cutting-edge engineering. This isn't just a replacement motor—it's a complete performance solution that transforms how your RC car accelerates, corners and responds to control inputs. The 3900KV rating means rapid spool-up and aggressive acceleration that will thrill drivers of all skill levels, from weekend bashers to competitive racers exploring options for brushless RC cars UK-wide.
Every aspect of this motor has been engineered for durability and smooth, predictable operation. From the precision-machined aluminium housing to the high-purity copper windings and hardened steel shaft, the Firma 3900Kv represents genuine quality that translates into consistent performance lap after lap, run after run. It's the perfect match for the Firma 130A Smart ESC, creating a powertrain combination that delivers both control and raw speed.
Key Features
- Low Cog 4-Pole Rotor Design — The sintered rotor construction minimises cogging and vibration, delivering smooth power delivery and reduced mechanical stress on your drivetrain components, which means longer service life and more predictable handling characteristics.
- CNC Machined Aluminium Housing — Every motor body is precision-machined from high-quality aluminium, ensuring perfect tolerances, superior heat dissipation and a professional finish that protects internal components from dust and moisture.
- Insulated High Purity Copper Windings — The motor uses premium copper windings with insulation that maximises electrical efficiency, reduces heat generation and provides excellent longevity even under continuous high-load operation.
- Hardened Steel 5mm Output Shaft — A robust, hardened steel shaft rated for high torque loads provides exceptional durability and maintains perfect concentricity with the motor bell, ensuring smooth rotation and minimal runout even under stress.
- High-Precision Ball Bearings — Quality roller bearings throughout the motor assembly support smooth, efficient rotation with minimal friction, contributing to longer runtime and cooler operating temperatures.
- Preinstalled 4mm Gold Plated Bullet Connectors — No soldering required—the motor arrives with professional-grade, gold-plated connectors already fitted, enabling quick installation and reliable electrical contact with your ESC without any hassle.
- Perfect Drop-In Upgrade — With the same diameter and mounting pattern as traditional 540- and 550-size brushed motors, this brushless motor fits directly into existing motor mounts on compatible 1/10 scale vehicles, making it an ideal brushless RC upgrade.
- D-5 Pinion Compatible — The 5mm bore is compatible with standard D-5 pinions, giving you flexibility in gearing ratios to fine-tune performance for your specific racing conditions or driving style.
- Matched for Firma 130A Smart ESC — This motor is specifically calibrated to work optimally with Spektrum's Firma 130A Smart ESC, creating a perfectly integrated powertrain that maximises efficiency and performance whilst offering Smart ESC programming and telemetry features.
Technical Performance
The Firma 3900Kv brushless motor represents a significant performance leap over brushed alternatives. The 3900 kilovolt (KV) rating indicates that the motor will spin at 3900 revolutions per minute for every volt applied to the battery, translating to exceptional acceleration and top-speed capability when powered by a 3S LiPo battery (up to 12.6 volts). The low-cog, four-pole sintered rotor design is engineered to minimise electrical noise and vibration whilst delivering smooth, predictable power across the entire throttle range. The CNC-machined aluminium housing provides superior structural rigidity compared to plastic alternatives, ensuring that all internal components remain perfectly aligned even during aggressive cornering, hard landings or high-speed runs. Precision ball bearings throughout the motor assembly reduce friction and heat generation, allowing the motor to run cooler and more efficiently than many competing designs. The hardened steel 5mm output shaft can handle the sustained torque loads of competitive driving whilst maintaining perfect concentricity, and the high-purity copper windings maximise electrical conductivity and minimise resistive heating. For 1/10 scale 4WD models, this motor delivers the performance characteristics needed for competitive racing, high-speed bashing, and technical track driving. Whilst the motor is rated for 3S battery operation (up to 12.6V), it will also function with 2S battery power when used with the Firma 85A ESC, though performance will be reduced compared to 3S operation. The preinstalled gold-plated bullet connectors ensure reliable electrical contact with your chosen ESC, and the standard 5mm pinion bore accepts D-5 compatible pinions for flexible gearing options. This motor is not sensored, meaning it requires a compatible sensorless ESC for reliable operation, and the Firma 130A Smart ESC is the perfect match, offering telemetry, programmable performance curves and comprehensive protection features.
What's in the Box
- SPM Firma 3900Kv 4-Pole Brushless Motor with preinstalled 4mm gold-plated bullet connectors
What You Need to Complete
- Brushless ESC (Firma 130A Smart ESC recommended; Firma 85A ESC also compatible for 2S operation)
- 2S or 3S LiPo battery (3S recommended for optimal performance)
- LiPo battery charger
- 1/10 scale RC car with compatible motor mount and 5mm pinion bore
- D-5 compatible pinion gear
Frequently Asked Questions
- Is the Firma 3900Kv motor suitable for beginners learning about brushless RC cars? — Yes, absolutely. Whilst the 3900KV rating delivers impressive performance, the motor itself is straightforward to install and use. The real consideration is choosing an appropriate ESC and battery setup. If you're new to brushless RC, pairing this motor with the Firma 130A Smart ESC and a quality 3S LiPo battery is ideal because the Smart ESC offers multiple safety features, programmable performance curves and telemetry feedback to help you learn how the system responds. The preinstalled connectors mean there's no soldering involved, making installation straightforward even for beginners.
- What battery does the Firma 3900Kv motor need? — The motor is designed for 2S and 3S LiPo battery operation. A 3S LiPo battery (11.1V nominal, up to 12.6V fully charged) delivers the full 3900KV performance and is the recommended setup. A 2S LiPo battery (7.4V nominal) will work but will provide reduced performance. For 2S operation, use with the Firma 85A ESC; for 3S operation, the Firma 130A Smart ESC is the ideal match. Ensure your battery has a capacity suited to your vehicle weight and driving style—typically 25C discharge rating or higher is recommended for this performance level.
- How does the Firma 3900Kv compare to other Spektrum Firma motors? — Spektrum manufactures Firma motors across a wide range of KV ratings to suit different applications and scales. The 3900KV is positioned as a high-performance, high-RPM motor ideal for 1/10 scale racing and bashing. Spektrum also produces lower-KV variants such as the 2800Kv, 2200Kv and 1900Kv motors for users who prefer different performance characteristics or are using larger scales or gearing setups. Lower-KV motors provide more torque and lower top-speed, whilst higher-KV motors like this 3900Kv deliver more top speed but slightly less torque. Your choice should depend on your vehicle, intended use and personal driving preferences.
- Does the motor come with sensoring? — No, the Firma 3900Kv is a sensorless brushless motor, which is appropriate for most 1/10 scale racing and recreational driving applications. Sensored motors, which are available in other Firma product lines, typically offer smoother operation at very low speeds and can provide additional telemetry data, but for 1/10 scale car racing, sensorless operation is standard and fully adequate. You will need a compatible sensorless brushless ESC to operate this motor.
- Is this motor waterproof? — The motor housing is designed to protect internal components from dust and light moisture, but it is not fully submersible and should not be dunked in water. The motor can handle typical RC driving conditions including wet grass and light rain, but avoid deep water crossings or submerged operation. If your motor does get wet, allow it to dry completely before operating.
- What is the 5mm shaft and pinion bore compatibility? — The motor features a hardened steel 5mm diameter output shaft that accepts D-5 compatible pinions. This is the standard pinion interface for most 1/10 scale RC cars. D-5 simply refers to a pinion gear with a 5mm bore hole. You can choose from a range of pinion sizes to fine-tune your gearing ratio depending on your track layout, battery choice and desired performance balance between acceleration and top speed.
- Can I use this motor with my existing RC car? — Very possibly. If your 1/10 scale 4WD vehicle currently uses a 540- or 550-size brushed motor and has a motor mount that accommodates those sizes, the Firma 3900Kv will fit directly thanks to its identical diameter and mounting pattern. You will need to check that your vehicle has a 5mm pinion bore (standard on most 1/10 cars) and that it can accept a suitable pinion gear. You will also need to upgrade to a brushless ESC if your vehicle is currently running a brushed ESC. Consult your vehicle manual or contact RC Model Shop for compatibility advice specific to your model.
- How long will the motor last? — With proper care and maintenance, Spektrum Firma motors deliver thousands of runs of reliable service. The use of precision ball bearings, hardened steel components and quality windings means these motors are built to last. Lifespan depends on operating conditions (ambient temperature, battery voltage, driving intensity), maintenance (ensuring dirt doesn't accumulate in vents) and thermal management (not consistently overheating the motor). Competitive users often get a full season of regular racing from a single motor. For recreational bashing, a Firma motor can easily outlast multiple ESC and battery cycles.
Why Buy from RC Model Shop?
RC Model Shop is a trusted UK RC retailer and authorised dealer for all the brands we stock, including Traxxas, ARRMA, Losi, FTX, Hobbywing, Spektrum and more. Based in Walsall, we have a physical store where customers are welcome to come in, view products and purchase in store. Whether you are a seasoned competitor, a weekend basher, or just discovering the world of RC for the first time, our team of experienced RC enthusiasts is here to help you find the perfect setup and provide ongoing support. Order the SPM Firma 3900Kv 4-Pole Brushless Motor M-SPMXSM3300 today with confidence, knowing you are buying from an authorised retailer with genuine products, expert knowledge and a commitment to customer satisfaction. Shop online or visit us in Walsall — we look forward to helping you get the most out of your RC experience.
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