Engineering the 725 9G Upgrade: How Spool Performance’s FX-RT Clutch System Achieves ~1,700 Nm Capacity

The Mercedes 725 9G is one of the most advanced automatic transmissions in the OEM world. Smooth, fast, compact—and surprisingly capable even in high-output AMG applications. But once torque levels pass the mid-700 Nm range, the factory clutch packs begin to operate at the edge of their thermal and friction capacity.

For modern turbocharged AMG platforms—especially M177 builds pushing well beyond factory boost—clutch slip becomes the limiting factor long before gearset strength or shafts are at risk.

Spool Performance set out to engineer a drop-in clutch upgrade capable of handling the torque levels demanded by today’s 1000–1200+ whp builds. The result is our new FX-RT 725 9G upgraded clutch kit, rated for up to ~1,700 Nm, based on the weakest gear’s capacity. This is a +68–71% increase over stock, achieved without any machining or complicated installation requirements.

This article breaks down the engineering behind the upgrade, why it works, and how each design change contributes to higher torque capacity and improved thermal robustness.

Understanding Clutch Torque Capacity: The Physics

The torque a multi-plate clutch pack can hold is governed by:

$$T = \mu \cdot N \cdot F \cdot R$$

Where:

• T = torque capacity

• μ = coefficient of friction

• N = number of friction interfaces

• F = apply (clamp) force from the piston

• R = mean effective radius

In a sealed, factory-style drum like the 725 9G, apply force (F) and radius (R) are fixed.
The only two variables we can change—without redesigning the transmission—are:

1. The friction material’s coefficient of friction (μ)

2. The number of friction interfaces (N)

Our upgrade increases both.

Increased Plate Counts: Why More Interfaces Matter

One of the most powerful ways to increase torque capacity is to increase the number of friction interfaces inside each drum.

Factory vs Upgraded Clutch Counts

Each added interface proportionally increases torque capacity, because:

$$T \propto N$$

For example:

• Stock B clutch = 4 plates

• Upgraded B clutch = 7 plates

That alone is a 75% increase in the number of interfaces, before even considering friction improvements.

This is why plate count increases are the single most impactful modification for any automatic transmission clutch upgrade.

FX-RT Friction Material: +20% Higher μ

After years of development on the 8HP series, we created our proprietary FX-RT friction material—a high-energy, high-μ compound designed specifically for rapid-shift, high-torque modern automatics.

FX-RT provides:

• +20% higher coefficient of friction

• Stronger static holding force

• Higher dynamic friction for better shift performance

• Superior heat resistance and reduced glazing

• More consistent apply characteristics under repeated thermal cycles

Increasing μ by 20% has a linear effect on torque capacity:

$$T_{\text{new}} = 1.20 \cdot T_{\text{old}}$$

This multiplier stacks with the additional clutch plates, creating a compound gain.

Combined Gains: The Real Torque Increase

When we combine:

• +20% friction coefficient

• +2 to +3 plates per clutch

• Optimized steel thickness

• OEM-calibrated piston travel

We get the following torque-capacity multipliers:

These gains multiply into the gearset based on which clutches are applied in each gear.

Torque Capacity Increase Per Gear

Using the 725's clutch engagement map:

Maximum holding capacity in each gear is limited to the weakest clutch pack that is engaged.

Final Torque Rating: ~1,700 Nm Capability

The highest factory rating for the 725 9G is 1000 Nm.

With Spool Performance’s upgraded clutch packs:

• 1st–4th gear: ~1680 Nm

• 5th–9th gear: ~1714 Nm

• System-limited torque rating: ≈1,700 Nm

This is a massive jump in drivetrain headroom, especially for:

• Hybrid-turbo AMG builds

• Built M177 engines

• High-load track driving

• Heavy SUVs like GLE63 / GLS63 under elevated boost

• 900–1200+ whp applications

Thermal Considerations: Why These Upgrades Matter Even at Stock Power

Torque capacity isn’t the only benefit.

Higher plate count = higher heat rejection

Every friction engagement generates heat.
More steels and frictions means:

• More thermal mass

• More surface area

• More uniform temperature distribution

• Lower friction energy density per plate

This significantly improves long-term durability and reduces glazing during aggressive launches or repeated high-torque shifts.

FX-RT friction = higher thermal ceiling

FX-RT resists:

• Glazing

• Hot spotting

• Shear breakdown

• Viscosity-dependent friction loss

All common failure modes in high-powered automatics.

This means even stock-power cars benefit from:

• Smoother shifts

• Less slip

• Less transmission fluid breakdown

• Longer clutch life

True Drop-In Installation

A major design requirement of this kit was ease of installation:

• No machining

• No basket cutting

• No custom drums

• No piston re-spacing

• Compatible with factory snap-ring locations

• Uses factory steels or our included precision steels

This allows performance shops—and even advanced DIY builders—to install the kit quickly without the complexity of a full custom build.

Engineering Headroom Into the 725 9G

The Spool Performance FX-RT 725 9G upgraded clutch kit transforms the transmission from a 1000 Nm unit into a ~1700 Nm-capable performance transmission, with:

• Increased plate counts

• Higher friction coefficient

• Better heat handling

• Smooth and consistent shift quality

• True drop-in installation

• Proven FX-RT technology from the 8HP series

In high-powered AMG cars, drivetrain headroom isn’t optional—it’s essential.
This upgrade ensures your 725 9G has the margin it needs to support modern power levels reliably and repeatedly.