Spool Performance's FX-RT clutch upgrade kit is designed for the Mercedes-AMG 725 Series 9G-Tronic transmission, with the aim of significantly increasing torque delivery in high-performance M177 engine projects. The kit is based on Spool's own FX-RT friction material and an increased number of clutch plates in all six clutch packs. The end result is that the transmission's torque delivery is increased to around 1,700 Nm , while maintaining a controlled shift feel and OEM compatibility.
Performance and torque endurance
The Mercedes 725 9G transmission is strong as standard, but at torques above ~700 Nm, the OEM clutches start to slip, especially under repeated heavy loads and high oil temperatures.
The Spool FX-RT series solves this:
- +20% higher friction coefficient
- Increased number of plates in all clutch packages
- Torque endurance up to ~1,700 Nm
Increase in torque duration depending on the gear:
- 1–4 gears: approx. +68%
- 5–9 gears: approximately +71%
Factory estimate for 725 transmission is ~1000 Nm → with FX-RT upgrade approximately 1680–1714 Nm clutch limited capacity .
Clutch kit updates
| Switch | Standard | Updated | Increase |
| A | 4 | 6 | +2 |
| B | 4 | 7 | +3 |
| C | 5 | 7 | +2 |
| D | 6 | 9 | +3 |
| E | 6 | 9 | +3 |
| F | 7 | 10 | +3 |
The increased friction surface distributes the load more evenly and improves thermal management.
Advantages of FX-RT friction material
FX-RT is designed specifically for high torque and fast shifting automatic transmissions.
- 20% higher μ → better grip
- Better thermal stability during hard driving
- Less slippage and shift flare
- Smoother and more controlled engagement
- Proven performance in racing use of the 8HP series
Series content
Complete drop-in kit, no machining.
- Clutch A pack (6-disc)
- Clutch B pack (7-disc)
- Clutch C pack (7-disc)
- Clutch D pack (9-disc)
- Clutch E pack (9-disc)
- Clutch F pack (10-disc)
- FX-RT friction plates and steel plates
- Custom pressure plates
- Pre-sized switch packages
Compatible with OEM drums, pistons and snap-ring solutions.
Who is this series for?
- M177 engine projects over 1000 hp
- AMG track cars
- High-boost setups
- Racing and drag builds
When engine power increases significantly, the transmission is no longer a limiting factor.
Technical benefits
- Torque endurance up to ~1,700 Nm
- Better temperature resistance
- Less slippage and wear
- OEM compatible installation
- Smooth and controlled shifting feel
Fitment
Mercedes-AMG 725-series 9G-Tronic-gearbox mm:
- AMG C63 / C63 S
- AMG E63 / E63 S
- AMG CLS63
- AMG GLE63 / GLS63
- AMG S63
and other M177-engine equipped AMG models.