Ford Focus ST250 Engine Build (UK) – Planning a Reliable 450–550 bhp Setup
Written by a UK Focus ST owner documenting a real-world Ford Focus ST250 engine rebuild, including true costs, proven power limits, and the decisions required to build a reliable high-power R9DA EcoBoost on UK fuel.
This page documents my Ford Focus ST250 Mk3 engine build, planned to support 450–550 bhp reliably on UK 99 RON fuel.
It is not a dyno-figure chase or a rushed project. This is a properly planned engine build, based on real-world UK constraints, proven EcoBoost limits, and the fact that this engine requires rebuilding anyway.
This page will be updated as the build progresses and is intended to act as a central reference for anyone considering a high-power ST250 build in the UK.
What This Page Covers
- Why the ST250 engine needs rebuilding
- Power goals and realistic limits on UK fuel
- Engine internals required for reliability
- Turbo and fuel system strategy
- Cooling, drivetrain, and supporting modifications
- A transparent UK cost overview
- A staged build plan
The Starting Point
- Car: Ford Focus ST Mk3 (ST250)
- Engine: 2.0 EcoBoost (R9DA)
- Transmission: 6-speed manual
- Current state: Engine rebuild required
Because the engine needs to come apart regardless, this build focuses on doing things once and correctly, rather than repairing stock components only to upgrade them again later.
Build Philosophy
The ST250 EcoBoost engine is well known for failing when pushed without proper preparation. This build is based on a few simple principles:
- Reliability before peak numbers
- Conservative calibration for UK 99 RON fuel
- Adequate headroom in fueling and cooling
- Mechanical sympathy over marketing claims
The aim is repeatable performance, not a one-hit dyno number.
Power Target (Realistic UK Goals)
Planned Output
450–550 bhp (final target to be confirmed)
This range represents the upper end of what is realistically achievable on UK pump fuel when the engine, fuel system, and cooling are built correctly.
Final output will depend on:
- Turbo selection
- Fuel system headroom
- Heat management
- Drivetrain limitations
Fuel Strategy (UK Reality)
E85 is not readily available in the UK, so this build will be calibrated around:
- 99 RON pump fuel (Shell V-Power / Tesco Momentum)
The fuel system is designed with:
- Conservative ignition timing
- Strong knock resistance via cooling
- Stable fuel pressure under load
A return-style fuel system is planned to reduce fuel temperature and improve consistency at high power.
Engine Internals (Planned Specification)
The factory R9DA pistons are the primary weak point at elevated power levels, so forged internals are non-negotiable.
- Forged pistons (lower compression)
- Forged connecting rods
- Performance main and rod bearings
- ARP head studs
- ARP main studs
- MLS head gasket
The goal is an engine that is mechanically unstressed at the target power level.
Turbocharger Strategy
Hybrid turbos are limited to roughly 360 bhp on this platform, so a full big-turbo setup is required.
- Garrett G25-660
- Garrett GTX3076R Gen II
- Precision 5858
Final selection will prioritise efficient spool, thermal efficiency, and broad usable power delivery.
Fuel System Upgrades
- Uprated high-pressure fuel pump (HPFP)
- 1700cc injectors
- Return-style fuel rail and regulator
- High-flow in-tank fuel pump
- ECU fuel pressure monitoring
This configuration provides sufficient headroom for ~600 whp while maintaining safe injector duty cycles.
Cooling & Reliability Modifications
- Large front-mount intercooler
- Uprated radiator
- Thermostatic oil cooler
- Improved crankcase ventilation / catch can system
- Heat shielding where required
Mandatory at this power level.
Drivetrain & Supporting Mods
- Uprated or twin-plate clutch
- Limited-slip differential
- Strengthened driveshafts (if required)
- ECU torque management strategies
Mapping & Calibration Plan
- Full custom dyno calibration
- Conservative ignition timing
- Boost-by-gear strategies
- Comprehensive datalogging and knock monitoring
No off-the-shelf or generic maps will be used.
Estimated UK Budget
| Area | Estimated Cost |
|---|---|
| Forged engine build | £1,500–£2,500 |
| Turbo system | £2,000–£3,000 |
| Fuel system | £1,000–£2,000 |
| Cooling upgrades | £600–£1,200 |
| Drivetrain upgrades | £1,000–£2,000 |
| Mapping & setup | £500–£800 |
Estimated total: £6,500–£10,000+
Planned Build Stages
- Engine teardown and inspection
- Machine work and block preparation
- Bottom-end assembly
- Turbo and fuel system installation
- Cooling and supporting modifications
- Initial mapping and testing
- Final calibration and road validation
Why This Build Is Being Documented
- Create a reliable high-power ST250
- Show realistic UK costs and decisions
- Help other owners avoid common and expensive mistakes
Written by a UK Focus ST owner documenting a high-power EcoBoost engine build.