AP Racing by Essex 2024-2026 Ford Mustang S650 GT Dark Horse Radi-CAL Competition Brake Kit (Rear CP9451/365mm)
or as low as $429.08/mo with Affirm
AP Racing by Essex
Radi-CAL Competition Brake Kit Rear CP9451/365mm · Hard Anodized
Purpose-built rear brake kit for the 2024+ S650 Mustang. Saves approximately 15 lbs of unsprung weight versus OEM while retaining full electronic parking brake functionality. Hard anodized finish for maximum track durability.
Rear Axle Upgrade
Complete the Package With a Purpose-Built Rear Kit
The OEM Dark Horse and GT Performance Pack rear brakes were designed around street driving compromises. On track, they overheat, fade, and hold back the potential of your front brake upgrade. The AP Racing by Essex CP9451 rear kit eliminates that weak link.
This kit saves approximately 15 lbs of unsprung weight from the rear axle versus OEM while delivering consistent, fade-free stopping power through extended track sessions. Every component is rated for sustained high-temperature operation. Piston sizes are carefully calibrated to integrate seamlessly with the OE master cylinder and ABS system.
The hard anodized finish grows an extremely hard aluminum oxide layer directly from the caliper body. Unlike paint or powder coat, it cannot chip, flake, or degrade under sustained track heat. Designed to pair with the AP Racing by Essex CP9668/390mm front kit for a balanced, championship-proven brake system front to rear.
Key Advantages
Engineered Without Compromise
Eight reasons this rear kit dominates extended track sessions
Radi-CAL Design
AP Racing's patented asymmetric caliper architecture delivers 30%+ greater stiffness than conventional designs while reducing weight and improving airflow around the disc.
Hard Anodized Finish
Hard anodizing grows an aluminum oxide layer directly from the caliper body via electrical current through an acid bath. Far more durable than paint or powder coat. Will not chip, flake, or degrade at track temperatures.
~15 lbs Lighter
Saves approximately 15 lbs of unsprung weight on the rear axle versus OEM Dark Horse and GT PP brakes. Less unsprung mass means faster response, better grip, and reduced fatigue on suspension components.
EPB Compatible
Retains full OE electronic parking brake functionality. Precision-machined caliper spacers for EPB included in every kit. No loss of street drivability or parking brake function.
72-Vane J Hook Discs
Superior airflow and heat evacuation versus the OEM 53-vane design. J Hook slot pattern creates uniform heat distribution with more leading edges for consistent bite lap after lap.
Stainless Steel Pistons
Ventilated, domed-back stainless steel pistons (not aluminum) provide a superior thermal barrier, keeping heat out of the brake fluid and maintaining firm pedal feel through long sessions.
Universal Pad Shape
D42 pad shape (132.4 x 42 x 16.7mm) is commonly available from Ferodo, Hawk, CL Brakes, Pagid, PFC, Carbotech, Cobalt, and more. Never locked into a single pad supplier.
Quick Pad Change
Bridge bolts pop out with a 6mm hex wrench. Change pads trackside in minutes — faster than you can remove the wheel. No need to remove the caliper from the car.
Complete Kit
What's In The Box
- AP Racing CP9451 Radi-CAL Pro5000R Calipers (Pair)4-piston, 4.85 lbs each (no pads) — hard anodized finish with ventilated domed-back stainless steel pistons and anti-knockback springs
- AP Racing 365x30mm J Hook Racing Discs (Pair)CP5775-214/215GA — 72 curved directional vanes, 2-piece fully floating with 6061 hard-anodized aluminum hats and 10-bolt attachment hardware
- EPB Caliper SpacersPrecision-machined spacers to retain full OE electronic parking brake functionality
- Mounting HardwareAircraft-quality hardware for secure caliper and disc installation
- Spiegler Stainless Steel Brake Lines (Pair)DOT-approved with abrasion-resistant coating and lifetime warranty for improved pedal feel
- Detailed Installation InstructionsStep-by-step guide for proper fitment and torque specifications
- Not Included: Brake PadsSelect your preferred compound at checkout — Ferodo DS2500, DS1.11, DSUNO, or DS3.12 (+$175 each)
- Not Included: Pad Tension KitOptional CP9451 pad tension kit available (+$140) — eliminates brake noise with billet aluminum blocks and stainless springs
Technical Data
Specifications
Caliper (CP9451)
4-Piston Radi-CAL Pro5000RDisc (365x30mm)
72-Vane J Hook Racing DiscEngineering
Technology Deep-Dive
Radi-CAL Technology
AP Racing's patented asymmetric caliper design, developed using advanced CAD and FEA analysis. During braking, massive tangential friction force pulls the caliper in the disc rotation direction (parallelogram effect). The Radi-CAL design distributes body mass diagonally to oppose this force.
The result: 30%+ more stiffness, less weight, better airflow around the disc, and superior packaging compared to conventional symmetric calipers.
Hard Anodized Finish
Paint and powder coat degrade under sustained track heat. Hard anodizing passes electrical current through the aluminum caliper body in an acid bath, growing an extremely hard aluminum oxide layer directly from the base material. The result is an integral protective surface that cannot chip, flake, or separate.
Hard anodized surfaces are significantly harder than the base aluminum and provide excellent corrosion resistance, thermal stability, and abrasion protection through thousands of track miles.
Two-Piece Floating Discs
One-piece discs cone when heated unevenly — the center heats faster than the outer edge, causing the disc to warp and create pedal pulsation. Two-piece floating design uses the 10-bolt hat mounting system that allows radial expansion without distortion.
The floating design accommodates thermal growth, reducing stress cracks, pad taper wear, and the vibration that comes from distorted one-piece discs. The 6061 billet aluminum hat saves significant weight compared to a one-piece iron rotor.
J Hook Slot Pattern
Straight slots create localized cool spots and stress risers that can lead to cracking. AP Racing's J Hook pattern creates a constant pathway of evenly distorted material across the entire disc face.
The result is uniform heat distribution, cleaner pad transfer film, and more leading edges per rotation for consistent bite from the first lap to the last.
Stainless Steel Pistons
Ventilated, domed-back stainless steel construction (not aluminum). Stainless steel provides a superior thermal barrier that keeps heat out of the brake fluid, maintaining consistent pedal feel through extended sessions.
The domed back adds structural stiffness under load. Anti-knockback springs maintain piston position through corners, eliminating the pedal drop common during aggressive cornering and disc knockback.
EPB & ABS Compatibility
Unlike many aftermarket rear brake kits, this system retains full OE electronic parking brake functionality through precision-machined caliper spacers included in every kit. No loss of street drivability.
Piston sizes (28.58mm x2 and 30.0mm x2) are carefully calibrated to integrate seamlessly with the OE master cylinder and ABS system, maintaining proper brake balance and pedal feel without any modification to the hydraulic system.
How It Compares
Essex/AP Racing vs. OEM S650
| Specification | Essex CP9451 | OE S650 Rear |
|---|---|---|
| Caliper Type | Radi-CAL Pro5000R Fixed Superior | 4-Piston Fixed |
| Weight (no pads) | 4.85 lbs 46% Lighter | ~9 lbs |
| Finish | Hard Anodized Track Grade | Paint |
| Pistons | Stainless Steel, Domed-Back | Standard |
| Anti-Knockback | Yes Included | No |
| Specification | Essex/AP Disc | OE S650 Rear |
|---|---|---|
| Type | 2-Piece Floating J Hook Superior | 1-Piece Iron |
| Weight | 15.4 lbs 28% Lighter | 21.3 lbs |
| Diameter | 365mm Larger | 355mm |
| Thickness | 30mm | 28mm |
| Internal Vanes | 72 Curved Directional 36% More | 53 Straight |
| Weight Savings | ~15 lbs saved on rear axle | |
Pad Compatibility
D42 Pad Cross-Reference
D42 pad shape (132.4 x 42 x 16.7mm) — commonly available from every major manufacturer
| Manufacturer | Part Number | Available Compounds |
|---|---|---|
| Ferodo | FRP3018 | DS2500, DS1.11, DSUNO, DS3.12 |
| Hawk | HB718.654 | HPS, DTC-60, DTC-70, DTC-80 |
| CL Brakes | 5004W42T17 | RC5+, RC6E, RC6, RC8 |
| Pagid | 2126 | RSL1, RSL29, RSC1 |
| PFC | 7768.XX.16 | 01, 08, 11 |
| Carbotech | CTCP9449 | XP8, XP10, XP12, AX6 |
| Cobalt | AP13 | XR1, XR2, XR3, XR4 |
| Alcon | PNF0084X276.4 | Various |
| AP Racing | CP3215D42 | Various |
| Mintex | 1970 | Various |
Fitment
Compatible Vehicles
Wheel Fitment
Confirmed Clearance
Gallery
Product & Install Images
Spare Parts
Replacement Components
Individual replacement parts available for long-term serviceability
Critical Cautions
Competition use: This kit is designed for track and competition use. Intended for off-road use only.
Front kit pairing: This rear kit is designed to pair with the AP Racing by Essex CP9668/390mm front kit. If upgrading rear only, use lower friction front pads to maintain proper brake balance.
Mounting: Do NOT bolt anything between the caliper bracket and suspension upright. Use only original AP Racing bleed screws.
Disc sides are handed: CP5775-214GA is the right disc and CP5775-215GA is the left disc. They cannot be swapped side-to-side.
Pro Tips
Brake fluid: High-quality racing brake fluid recommended (Motul RBF 660, Castrol SRF, or equivalent). Flush system completely during install.
New disc prep: Discs have a rust-prevention coating — wash thoroughly with soap and water before installation.
Mushy pedal fix: If pedal is mushy after install, cycle ABS via scan tool then re-bleed the system.
Pad bedding: Follow proper bed-in procedure for your chosen pad compound before any hard track use.
Professional Installation
Function Factory Performance offers professional brake service and installation for all AP Racing by Essex brake systems.
Located in Thousand Oaks, California, serving Mustangs, Corvettes, McLarens, and more.
Phone: (805) 206-2523
Lifetime Support
Caliper Reconditioning Program
Built to Last — Backed for Life
Essex is AP Racing's official North American caliper reconditioning center. When your calipers need servicing after years of hard use, Essex can restore them to factory specification for approximately $160 per caliper.
This is not a warranty service — it is a testament to the quality of AP Racing's manufacturing that calipers can be rebuilt and returned to active competition rather than discarded.
Expert Guidance
Questions About Big Brake Kits?
Our brake specialists can help you choose the right kit, compounds, and components for your setup. Give us a call or schedule a consultation.
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