For Honda Accord enthusiasts seeking to maximize their vehicle’s performance, engine tuning is a crucial step. While modern OBDII systems offer diagnostic capabilities, older OBDI systems are often favored for their tuning flexibility. This guide, tailored for Autel France readers and the English-speaking market, delves into the process of converting your OBDII Honda Accord to OBDI, specifically for enhanced tuning capabilities.
The primary benefit of undertaking an OBDII to OBDI conversion lies in the realm of tuning. While options like AEM EMS or piggyback systems such as eManage exist for OBDII, many tuners and enthusiasts prefer the direct control and extensive support available with OBDI ECUs. This conversion opens up a wider array of tuning possibilities, allowing for more precise adjustments to fuel and ignition maps, ultimately unlocking your Accord’s hidden horsepower.
To embark on this modification, it’s essential to gather the necessary components. This comprehensive list ensures a smooth and successful conversion, addressing both electronic and mechanical aspects:
- OBDI ECU (P28 Recommended): The heart of the conversion, the ECU (Engine Control Unit) dictates engine management. The P28 ECU, commonly found in 92-95 Honda Civics and Del Sols with VTEC, is a popular and readily tunable choice. Its robust aftermarket support and proven performance make it ideal for this conversion.
- OBDII to OBDI Conversion Harness: This adapter is crucial for bridging the wiring differences between the OBDII vehicle harness and the OBDI ECU. It simplifies the wiring process, making it essentially plug-and-play and avoiding complex wire splicing.
- OBDI Honda Resistor Box: OBDI systems typically utilize a resistor box for the injectors. This component ensures proper injector impedance, which is vital for correct fuel delivery and engine performance.
- OBDI Injector Clips (Matching Injectors): OBDI and OBDII injectors often use different connectors. These clips ensure a direct and secure electrical connection between your chosen injectors and the conversion harness. Having clips that match your intended injectors (like DSM injectors as mentioned in the original article) is critical for compatibility.
- Injectors from an OBDI Car (DSM Injectors Suggested): While not strictly mandatory, using injectors from an OBDI vehicle, such as DSM (Diamond Star Motors) injectors, can simplify the process. They are known for their compatibility with OBDI systems and are often readily available and affordable.
- OBD1 F22 Intake Manifold (Complete with Sensors): This is a key mechanical component. The OBDII intake manifold on Accords (like the F23) is not directly compatible with OBDI Idle Air Control Valves (IACV). The OBD1 F22 intake manifold, commonly found on 90-93 Accords, provides the correct mounting and ports for an OBDI IACV. Ensure it comes with all necessary sensors to avoid sourcing them separately.
- OBDI IACV (Idle Air Control Valve – if not included with manifold): The IACV controls the engine’s idle speed. OBDII systems often use 3-wire IACVs, while OBDIs use 2-wire. The OBD1 intake manifold is designed for a 2-wire IACV. If your F22 manifold doesn’t include one, you’ll need to acquire an OBDI 2-wire IACV.
- F22 Fuel Rail for Manifold: The fuel rail on the F22 manifold is designed to fit properly. While potentially reusable from your original manifold, using the F22 fuel rail ensures correct fitment and fuel delivery.
- OBDI IACV Plug from Wiring Harness: Since you’re switching to a 2-wire IACV, you’ll need the corresponding 2-wire plug to connect it to your conversion harness. Sourcing this from an OBDI Honda wiring harness ensures compatibility.
- 98-02 Accord Haynes Manual & 92-97 Accord Haynes Manual (for Reference): These manuals are invaluable resources. The 98-02 manual provides wiring diagrams for your OBDII Accord, while the 92-97 manual offers diagrams for the OBD1 components, specifically the F22 engine and its associated wiring. Having both allows for cross-referencing and accurate wiring identification, particularly for the IACV.
The necessity of the F22 intake manifold stems from the incompatibility of IACV mounting. Currently, adapter plates for fitting OBD1 IACVs to the F23 manifold are not readily available or reliably functional. While the F22 manifold bolts directly onto your Accord’s engine block, ensuring proper connection of all vacuum lines, sensors, and the throttle cable is paramount. If you have an EGR block-off plate, this conversion is also an opportune time to install it, simplifying the vacuum line routing.
The OBDII to OBDI conversion process is often described as “plug and play” due to the conversion harness. However, meticulous attention to detail is crucial:
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Wiring Identification: Utilize both Haynes manuals to carefully identify the wiring for the IACV. You’re transitioning from a 3-wire OBDII IACV to a 2-wire OBDI IACV. The manuals’ wiring diagrams and color codes will be essential for correctly connecting the 2-wire IACV plug to the appropriate wires on your conversion harness. Remember, the third wire on the OBDII IACV is for resistance measurement, while the OBDI system uses only two wires for operation.
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Sensor Connections: Ensure all sensors on the F22 intake manifold are correctly connected to the conversion harness. Double-check each connection against the wiring diagrams to prevent any miswiring, which could lead to engine malfunctions.
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Fuel System: Properly install the F22 fuel rail and connect it to your fuel lines. Ensure there are no leaks and that the fuel pressure is within specification after the conversion.
Dispelling Myths: Distributor Conversion Not Required
Contrary to some misconceptions, converting to an OBDI distributor is not necessary for this conversion to function correctly. Your OBDII distributor will work seamlessly with the OBDI ECU in this setup. This simplifies the process and reduces unnecessary expenses.
Crucial Warning: Emissions Non-Compliance
It is imperative to understand that converting your OBDII Accord to OBDI will likely result in failure to pass emissions testing. OBDII systems are designed with sophisticated emissions monitoring and control. This conversion bypasses those systems, making your vehicle technically non-compliant with emissions regulations. Consider this carefully, especially if emissions testing is mandatory in your region. This modification is intended for off-road or closed-course use and is not legal for street-driven vehicles in many areas. The responsibility for understanding and adhering to local regulations rests solely with the vehicle owner.
ECU Chipping and Tuning: Unleashing the Potential
To fully realize the benefits of this conversion, your chosen OBDI ECU (like the P28) must be chipped and socketed. This modification allows for the installation of a programmable chip (EPROM), enabling custom tuning of your engine management system.
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Basemap Installation: Before starting your car with the OBDI ECU, you must have a tuner burn a base engine map (basemap) onto an EPROM chip and install it in your chipped ECU. A basemap provides initial fuel and ignition settings that allow the engine to start and run safely. Without a basemap, the ECU will likely not allow the car to start.
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Professional Tuning: For optimal performance and engine safety, it is strongly recommended to seek a professional tuner experienced with Honda OBD1 systems. A tuner can fine-tune your engine’s parameters on a dynamometer, optimizing for your specific modifications and ensuring peak power and reliability. If you intend to use VTEC, ensure your P28 ECU is appropriately configured for VTEC engagement and that your basemap and tuning account for it. Some ECUs may have VTEC disabled by default.
Conclusion: Embracing OBDI Tuning for Your Accord
Converting your OBDII Honda Accord to OBDI for tuning purposes is a popular modification for enthusiasts seeking greater control over their engine’s performance. By following this comprehensive guide, gathering the necessary components, and understanding the implications, you can successfully perform this conversion and unlock the tuning potential of your Accord. Remember to prioritize professional tuning to maximize the benefits and ensure the longevity of your engine. While this modification offers significant tuning advantages, be mindful of the emissions implications and legality in your region. This conversion is a gateway to enhanced performance, placing you firmly in control of your Accord’s engine management system.
