Ford Focus ST Mk3 Diesel Tuning Guide

Ford Focus ST Diesel Mk3.5 Tuning Guide

The Ford Focus ST Diesel Mk3.5 (2015-2018) is a popular choice for those seeking a blend of performance and efficiency. With its 2.0-litre EcoBlue engine, the ST Diesel offers a good platform for tuning. Many owners look to increase power and performance through aftermarket modifications. In this guide, we will outline the stages of tuning for the Focus ST Diesel Mk3.5, what modifications are needed at each level, and some considerations to keep in mind when upgrading your vehicle.

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Stage 1 Tuning

Stage 1 tuning is typically the entry-level upgrade for the Focus ST Diesel Mk3.5. At this stage, the primary modification is an ECU remap. This optimises the engine's performance by adjusting the software that controls fuel delivery, boost pressure, and other critical parameters. The result is a noticeable increase in power and torque without the need for any additional hardware changes.

• Power gains: Typically, a Stage 1 tune will increase the engine's power by 20-30 bhp and torque by around 40-60 Nm.
• Benefits: Better throttle response, improved acceleration, and enhanced overall performance.
• Considerations: It is important to ensure the vehicle is in good mechanical condition before opting for a Stage 1 tune, as the stock components need to be reliable to handle the extra power.

At this level, no physical modifications are necessary—just the ECU tune. It’s a cost-effective and relatively simple way to improve performance while maintaining the car’s reliability. If your clutch is worn, you could experience a slipping clutch even at this level, consider the age of your clutch and the mileage covered before proceeding. 

Stage 2 Tuning

Stage 2 tuning builds upon the Stage 1 ECU remap by adding supporting modifications to improve airflow, cooling, and exhaust efficiency. At this stage, the following upgrades are generally recommended:

• Aftermarket intercooler: Upgrading to a larger, more efficient intercooler is important at Stage 2. This helps to lower intake temperatures and improves the performance of the turbocharger by reducing heat soak during hard driving. JXL Performance recommends the AIRTEC Motorsport Intercooler Upgrade - An OEM upgrade is available from the Focus RS Mk3, this is a popular choice, as it fits the ST Diesel Mk3.5 and offers improved cooling compared to the stock unit.
• DPF delete: The diesel particulate filter (DPF) is a common bottleneck in diesel tuning, especially when increasing performance. A DPF delete allows for improved exhaust flow and can increase power and throttle response. However, it is important to note that removing the DPF is illegal in some regions and will affect the vehicle's emissions compliance. This can cause issues with MOTs and road legality, so be sure to check local regulations before going down this route.
• Induction hose upgrade: Replacing the stock induction hose with an upgraded, larger diameter hose can improve airflow to the turbo, further enhancing the gains from Stage 2 tuning.
• Power gains: A Stage 2 tune, with all the hardware upgrades, can typically increase power by around 40-60 bhp and torque by 100-150 Nm.
• Considerations: A DPF delete will render the vehicle non-compliant with emissions laws in certain regions, which can lead to fines and failed MOTs. Additionally, modifying the exhaust system may affect the vehicle's warranty.

Stage 3 Tuning

Stage 3 tuning involves significant upgrades and modifications, requiring a more substantial investment in performance parts. At this stage, the following components are typically upgraded:

• Upgraded turbocharger: The standard turbocharger on the ST Diesel Mk3.5 is good for Stage 1 and Stage 2 power levels, but for substantial gains in power and torque, an upgraded turbo is essential. The larger turbo will allow for increased airflow and provide a stronger power band at higher revs.
• Single mass flywheel: The standard dual mass flywheel (DMF) is designed to absorb vibrations from the engine, but it can be prone to failure when subjected to high torque from Stage 3 modifications. A single mass flywheel is a more robust solution that is better suited to handling the increased power output and providing more direct throttle response.
• Uprated clutch: With the increase in power and torque, the stock clutch will struggle to handle the additional load. An uprated clutch is necessary to prevent slipping and ensure the drivetrain can transfer the power efficiently to the wheels.
• Carbon synchromesh upgrade for the gearbox: The added torque from Stage 3 tuning can put additional strain on the gearbox, leading to potential issues with shifting. A carbon synchromesh upgrade will allow for smoother shifts under heavy load and help preserve the longevity of the transmission.
• Other supporting mods: At Stage 3, further upgrades such as high-flow fuel injectors, larger fuel pumps, and stronger internal engine components may also be necessary to support the higher power levels. The car will also benefit from a more aggressive ECU map to accommodate the increased airflow and fuel delivery.

Power gains: At Stage 3, the Focus ST Diesel Mk3.5 can see power gains of up to 100-130 bhp over the stock setup, with torque figures increasing by 200-250 Nm.

Considerations: Stage 3 tuning requires significant investment in both parts and labour. Additionally, expect to spend more time on maintenance and monitoring as the modifications put greater strain on the vehicle's components. Be prepared for increased fuel consumption and the possibility of further upgrades down the line.

Other Considerations

• Cooling and Brake Upgrades: Given the increased performance at Stage 2 and Stage 3, upgrading the cooling system and brakes may be necessary for consistent performance. The Focus RS Mk3 brake kit is a popular upgrade for the ST Diesel Mk3.5, offering larger discs and more powerful calipers. This helps to manage the increased heat generated during hard driving and track use.
• Reliability: As you increase the performance of your Focus ST Diesel Mk3.5, be aware that certain components will be under more strain. Regular maintenance is essential, especially for parts such as the turbocharger, clutch, and suspension, which may require more frequent attention as power levels increase.

Conclusion

Tuning the Ford Focus ST Diesel Mk3.5 can provide significant improvements in power, performance, and driving enjoyment. Starting with a Stage 1 ECU remap is a simple and cost-effective way to improve performance. If you’re after more power, Stage 2 tuning with upgrades like a larger intercooler and DPF delete is a logical next step. For those looking for substantial power increases, Stage 3 tuning involves major modifications, including a larger turbo, upgraded clutch, and supporting components to handle the additional torque.

While tuning can unlock the full potential of the ST Diesel Mk3.5, it’s essential to understand the impact on reliability, legality (especially regarding the DPF delete), and maintenance. Be sure to balance performance upgrades with the necessary supporting modifications to maintain a reliable and enjoyable driving experience.