E36 OBD2 to OBD1 Conversion: Unleashing Performance for Your BMW

The E36 BMW, particularly models like the M3, are celebrated for their driving dynamics and timeless appeal. However, for enthusiasts seeking to maximize performance, especially on OBD2 models (typically 1996+), an OBD1 conversion is a popular and effective modification. While often described as a simple “electronics swap,” converting your E36 from OBD2 to OBD1 is a detailed process that requires careful planning and execution. This guide, based on real-world experience and technical expertise, will walk you through the intricacies of the E36 Obd2 To Obd1 Conversion, ensuring a smoother and more successful upgrade.

Originally, I embarked on this e36 OBD2 to OBD1 conversion journey to enhance the performance of my ’99 M3 with an S52 engine. Like many, I spent considerable time on forums like bimmerforums.com and consulted with experienced individuals to gather insights. Coupled with countless hours studying wiring diagrams in the Bentley manual, I successfully navigated this complex modification. This guide is designed to share that knowledge and provide a comprehensive resource for anyone considering this upgrade.

Here’s a glimpse of my OBD2 S52 engine bay before the conversion, highlighting the starting point for this transformation:

Alt text: Before E36 OBD2 to OBD1 conversion: Engine bay of a 1999 BMW M3 S52, showcasing the original OBD2 engine management system.

My project car was a 1999 E36 M3, originally equipped with OBD2 engine management and the EWS (Electronic Immobilizer System). To perform the conversion, I sourced essential OBD1 components, including an engine harness from a 1994 325i and a red-label 413 ECU, a preferred choice for OBD1 conversions due to their origin in non-EWS vehicles. The performance chip I opted for was from Active Autowerkes, a reputable name in BMW tuning.

Essential Parts for Your E36 OBD2 to OBD1 Conversion

To successfully undertake an e36 OBD2 to OBD1 conversion, you’ll need to gather a specific set of parts. This list provides a clear overview of the components required for this upgrade:

1. OBD1 Engine Harness: This is the backbone of the conversion, providing the necessary connections for the OBD1 ECU and sensors.
2. OBD1 ECU (413 “Red Label” Preferred): The “red label” 413 ECU is highly sought after as it typically comes from non-EWS vehicles, simplifying the conversion process.
3. OBD1 Performance Chip: An OBD1 chip, like the Active Autowerkes chip I used, is crucial for optimizing engine performance with the OBD1 system.
4. OBD1 Intake Manifold: The M50 intake manifold, known for its superior flow characteristics compared to OBD2 manifolds, is a key component for performance gains.
5. OBD1 Fuel Rail: The OBD1 fuel rail is necessary to accommodate the different fuel line configurations of the OBD1 system.
6. OBD1 Crank Position Sensor: The crank position sensor differs in location and type between OBD1 and OBD2, necessitating an OBD1 sensor.
7. OBD1 Cam Position Sensor: Similar to the crank position sensor, the cam position sensor must also be switched to the OBD1 version.
8. OBD1 Knock Sensors (x2): OBD1 knock sensors are required and typically come in a pair.
9. OBD1 Oxygen Sensor: OBD1 systems utilize a single pre-catalyst oxygen sensor, unlike the multiple sensors in OBD2.
10. OBD1 HFM (Hot-Film Air Mass Meter): The OBD1 HFM is calibrated for the OBD1 system and is essential for accurate air mass measurement.
11. OBD1 Main Engine Coolant Hose: This larger coolant hose connects to various cooling system components and differs between OBD1 and OBD2 setups.
12. OBD1 Throttle Boot: The throttle boot connects the throttle body to the intake system and is specific to OBD1 configurations.
13. OBD1 Throttle Body (Optional): While optional, an OBD1 throttle body can simplify the conversion, avoiding the need for adapters.
14. OBD1 Valve Cover and Coil Packs (Optional): These are optional but can streamline the coil pack wiring setup and aesthetics.

While the parts list may seem straightforward, the e36 OBD2 to OBD1 conversion involves several intricate steps and considerations. Let’s delve into the details of each stage.

Step-by-Step E36 OBD2 to OBD1 Conversion Guide

This section details each step of the e36 OBD2 to OBD1 conversion, highlighting potential challenges and solutions encountered during the process.

1. Valve Cover and Coil Packs: OBD2 or OBD1 Configuration

The OBD2 valve cover lacks the necessary routing provisions for OBD1 coil pack connectors. This is because OBD2 wiring enters from the passenger side, while OBD1 wiring enters from the driver’s side. At this stage, you have a choice:

• Option 1: Retain OBD2 Valve Cover: This allows you to keep your OBD2 coil packs. However, it requires slight modification of the OBD2 valve cover using a Dremel or similar tool to accommodate the OBD1 wiring routing.

• Option 2: Switch to OBD1 Valve Cover: This necessitates using OBD1 coil packs. While it simplifies wiring routing, it requires sourcing OBD1 valve cover and coil packs.

Both OBD1 and OBD2 coil pack versions are compatible with the OBD1 harness, offering flexibility. I opted to keep my OBD2 valve cover and modify it slightly for wiring clearance.

Here’s a visual comparison of the OBD2 and OBD1 coil pack setups:

Alt text: E36 OBD2 coil pack setup: Close-up view of OBD2 valve cover and coil packs, demonstrating the wiring configuration.

Alt text: E36 OBD1 coil pack setup: Image showcasing the OBD1 valve cover and coil packs, highlighting the different wiring entry point compared to OBD2.

2. VANOS Solenoid Wiring: Extending for OBD1 Harness

The OBD2 VANOS solenoid wire connector is shorter than its OBD1 counterpart. To address this discrepancy, you have two primary solutions:

• Option 1: Use OBD1 VANOS Solenoid: Swapping to an OBD1 VANOS solenoid ensures direct compatibility with the OBD1 harness.

• Option 2: Extend OBD2 Wiring: Alternatively, you can retain your OBD2 solenoid and extend its wiring. A perfect solution for this is utilizing BMW part # 12-52-2-274-971, a transmission harness originally designed for connecting the engine harness to the backup light switch. This harness is ideal because it features identical connectors, simplifying the extension process.

I opted for the latter approach, utilizing the BMW transmission harness to seamlessly extend the OBD2 VANOS solenoid wiring for OBD1 compatibility.

This image shows the BMW transmission harness used for the VANOS solenoid extension:

Alt text: BMW transmission harness for E36 OBD2 to OBD1 VANOS solenoid extension: Image of the BMW part # 12-52-2-274-971, highlighting its connectors suitable for extending the VANOS solenoid wiring during an OBD1 conversion.

3. Coolant Pipe Adaptation: OBD1 Hose Compatibility

The main coolant pipe originating from the timing cover exhibits differences between OBD1 and OBD2 systems. OBD2 models employ a metal pipe fixed within the timing case cover using sealant, while OBD1 systems use a rubber hose connected to an aluminum neck protruding from the timing case cover. To bridge this gap, you have two options:

• Option 1: OBD1 Timing Cover Swap: Replacing the OBD2 timing cover with an OBD1 version ($100 approx.) allows for direct compatibility with the OBD1 coolant hose.

• Option 2: Coolant Pipe Adapter: A more cost-effective solution is to use a coolant pipe adapter ($20 approx.) available from BMW performance parts suppliers like Bimmerworld, Active Autowerkes, or Turner Motorsport. This adapter fits into the OBD2 timing case cover and is secured with JB Weld, creating an aluminum neck to which you can clamp the OBD1 coolant hose.

I chose the coolant pipe adapter for its cost-effectiveness. It’s a straightforward solution that avoids a more involved timing cover replacement.

Here’s a visual representation of the coolant pipe adapter in place:

Alt text: E36 OBD2 to OBD1 coolant pipe adapter location: Image pointing to the aluminum coolant pipe adapter (located below the oil filter housing) installed on an OBD2 timing case cover, facilitating OBD1 coolant hose connection.

4. Intake Manifold: Unlocking Performance Gains

The intake manifold is where a significant portion of the “performance gain” from an OBD1 conversion originates. The OBD1 M50 intake manifold is known for its superior airflow compared to the OBD2 (M52/S52) intake manifolds. Therefore, using the OBD1 manifold is crucial for this conversion.

Key features of the OBD1 manifold include an air temperature sensor and a vacuum port for the fuel pressure regulator located on the underside, near the firewall. Importantly, the M50 intake manifold directly bolts onto M52/S52 cylinder heads without requiring any modifications, simplifying the installation process.

5. Throttle Body Considerations: Adapters or OBD1 TB

You have flexibility in choosing your throttle body setup for the e36 OBD2 to OBD1 conversion:

• Option 1: Retain OBD2 Throttle Body with Adapter: You can use your existing OBD2 throttle body. However, it requires an adapter to ensure proper gasket sealing. OBD1 throttle bodies have a flat mating surface that clamps to a gasket on the OBD1 intake manifold, while OBD2 throttle bodies are the opposite, featuring a gasket within the throttle body that seals against a flat surface on the OBD2 intake manifold. Solutions include:

  • Adapter Plate: A $20 adapter plate positioned between the OBD2 throttle body and OBD1 manifold provides compatible mating surfaces for both gasket types.
  • Extended Gasket: Alternatively, a $15 extended gasket allows direct clamping of the OBD2 throttle body to the OBD1 intake manifold, eliminating the need for an adapter plate.

• Option 2: Switch to OBD1 Throttle Body: Using an OBD1 throttle body eliminates the need for any adapters or special gaskets, ensuring direct compatibility with the OBD1 intake manifold.

6. Coolant Temperature Sender: Adapting to OBD1 Setup

OBD2 engine management utilizes a single coolant temperature sender located on the cylinder head beneath intake runner #1. In contrast, OBD1 systems employ two temperature senders positioned on the cylinder head under intake runners #1 and #2. To accommodate this difference, you have options:

• Option 1: Wire Splicing and OBD2 Connector: You can splice the main engine harness wiring and reuse the OBD2 plug connector from your original harness to connect to your single OBD2 temperature sender.

• Option 2: Coolant Temp Sender Wiring Adapter: For a cleaner, plug-and-play solution, a coolant temperature sender wiring adapter ($50 approx.) from Turner Motorsport is available. This adapter simplifies the wiring adaptation process.

7. Crank Position Sensor: OBD1 Sensor is Mandatory

The location of the crank position sensor differs significantly between OBD2 and OBD1. OBD2 sensors are situated on the engine block in front of the starter motor, while OBD1 sensors are located on the timing cover, mounted on a circular tab with a 6mm Allen bolt.

For the e36 OBD2 to OBD1 conversion, using an OBD1 crank position sensor is mandatory. Simply install the OBD1 sensor in its designated location on the timing cover. You can leave the OBD2 sensor in place to plug the hole in the engine block.

8. Fuel Lines and Fuel Rail: Significant System Change

The fuel delivery system undergoes a substantial change during an OBD2 to OBD1 conversion. Key differences include:

• OBD2 Fuel Rail: Both fuel lines connect at the rear, near the firewall. The fuel pressure regulator is located under the car, forward of the fuel filter, on the driver’s side.

• OBD1 Fuel Rail: The supply line connects at the front of the rail, and the return line at the rear, near the firewall. The OBD1 fuel pressure regulator is integrated into the fuel rail itself, adjacent to the return line.

Using the OBD1 fuel rail is essential for this conversion, necessitating modifications to the fuel lines. This involves:

1. Removing the OBD2 Fuel Pressure Regulator: Disconnect and remove the OBD2 fuel pressure regulator from its undercarriage location.
2. Routing New Fuel Lines: Run new 8mm fuel lines to the OBD1 fuel rail. Locate the pair of hard lines mounted to the chassis between the OBD2 fuel filter and the engine bay. Bridge the gap created by removing the OBD2 fuel pressure regulator using new fuel line.
3. Connecting Fuel Lines: Connect the fuel feed line from the fuel filter to the front of the OBD1 fuel rail and the return line from the back of the OBD1 fuel rail to the return line under the car.
4. OBD1 Fuel Pressure Regulator Vacuum Line: Connect the vacuum line from the OBD1 fuel pressure regulator to the one-way valve located on the underside of the OBD1 intake manifold, at the rear corner closest to the firewall.

9. PCV (Positive Crankcase Ventilation) System: Adapting Venting

The OBD2 and OBD1 PCV systems differ in their venting setups. Several options exist to address this during the conversion:

• Option 1: Retain OBD2 PCV (with OBD2 Valve Cover): If you are using the OBD2 valve cover, you can maintain your OBD2 PCV setup. This requires finding a suitable mounting location for the breather valve (the cone-shaped plastic valve with a round breather on top) underneath the intake manifold.

• Option 2: OBD1 Breather Valve (with OBD1 Valve Cover): If using an OBD1 valve cover, utilize the OBD1 breather valve. This valve clips onto the crankcase vent port and features a vacuum line connecting to the plug joining the ICV to the intake manifold, along with a larger oil drain line running to the dipstick.

• Option 3: Breather Catch Can: An alternative is to connect a hose to the crankcase vent and route it to a breather catch can system.

In my conversion, I utilized a short length of 1” rubber hose to connect my OBD2 valve cover to the OBD1 breather valve. I then used a barbed connector to join the oil drain line to my dipstick, creating a functional hybrid PCV setup.

Here’s a visual representation of my PCV setup:

Alt text: E36 OBD2 to OBD1 PCV system setup: Image illustrating a hybrid PCV system configuration using OBD1 breather valve with OBD2 valve cover, showing hose connections and routing.

10. Idle Control Valve (ICV): Reusing OBD2 Component

The OBD1 and OBD2 Idle Control Valves (ICVs) are identical. You can reuse your existing OBD2 ICV for the conversion. You will need to acquire the correct connector and hose for connecting the ICV to the intake manifold, as well as the hose connecting the ICV to the throttle boot vacuum port.

11. Fuel Tank Breather Valve: Adapting Vacuum Connections

You can reuse your OBD2 fuel tank breather valve. However, you will need to obtain fittings to connect the vacuum hose to the vacuum port on the throttle boot. Standard hardware store barb fittings (3/8” and 5/8”) can be used to create a rigged connection to the throttle body vacuum port.

12. Oxygen Sensors and Secondary Air Pump: Emission System Changes

OBD2 systems incorporate secondary air pumps, an emission control system absent in OBD1. This system is completely removed during the OBD2 to OBD1 conversion. Furthermore, the conversion eliminates the need for both the two pre-catalyst OBD2 O2 sensors located in the OBD2 exhaust headers and the two post-catalyst OBD2 O2 sensors in the catalytic converter.

Remember to acquire M18 bolts to plug the O2 sensor ports in the exhaust system. Toyota Land Cruiser oil pan drain bolts are a readily available M18 plug option. OBD1 management only uses a single pre-catalyst OBD1 oxygen sensor.

13. Oil Pan & Dipstick: OBD2 Compatibility

Contrary to some misconceptions, you do not need to swap to an OBD1 oil pan and dipstick for the conversion. Your existing OBD2 oil pan and dipstick are fully compatible and can be used without any issues or modifications.

14. EWS (Electronic Immobilizer System): Wiring Modification

The presence of EWS varies across E36 models. While I used an ECU from a non-EWS vehicle, ignition issues can still arise after the conversion. A straightforward modification to the main engine harness addresses potential EWS-related problems.

To bypass EWS, locate the connector to the ECU and remove its protective rubber boot. Cut wire #66. While typically solid green, Bentley wiring diagrams indicate it may also be black/violet. Cut the wire and insulate both ends with electrical tape.

15. Power Distribution & Grounding: Careful Reconnection

Crucially, all power and grounding work should be performed with the negative battery post disconnected. Before disassembling the OBD2 wiring, meticulously label or photograph the power and ground connection points for accurate reconnection.

The OBD2 main battery positive post is situated on the passenger side, near the ECU compartment. The OBD2 distribution box is mounted parallel to the fender with two M10 bolts. Relocating this distribution box is necessary to access the power connections on the OBD1 harness.

The removal of the secondary air pump reveals two screw holes. These holes can be used to remount the distribution box, moving it closer to the motor and parallel to the firewall. This repositioning, though a tight fit, allows sufficient reach for the power connections on the OBD1 harness. This is essential to access the main power feed for the OBD1 harness.

A ground connection is located beneath the OBD diagnostics port. Carefully verify which wires connect to power distribution and which are grounds. Power feeds are typically RED, while grounds are BROWN or BLACK. If unsure, peel back the wiring sheath to confirm wire colors. There will also be a large power feed for the starter and a smaller power feed for the fuse box. Another ground connection is a small wire (with a round terminal connector) originating from the spark plug rail, which needs to be grounded to the bolt securing the engine hoist loop on the VANOS unit.

Here’s an image of the relocated power terminal after the conversion:

Alt text: E36 OBD2 to OBD1 relocated power terminal: Image showing the OBD2 power distribution box relocated after OBD1 conversion, highlighting the new mounting position and wiring connections.

16. General Wiring: Labeling is Paramount

Labeling connectors before installing the OBD1 harness is absolutely critical. Utilize Bentley wiring schematics to identify each plug on the harness. Label each connector with painter’s tape and a marker, clearly indicating its intended connection point. The engine bay can quickly become a confusing tangle of wires, and labeled connectors significantly simplify the installation process.

Fortunately, wire lengths are generally practical, and connectors are located in the approximate vicinity of their corresponding components. Taking photographs of power and ground connections during OBD2 wiring disassembly is also highly recommended. Additionally, photograph the starter wiring connections to avoid potential confusion later, as even a few wires can become perplexing during reassembly.

This image illustrates the OBD1 harness after labeling all connectors:

Alt text: E36 OBD2 to OBD1 labeled wiring harness: Image of the OBD1 engine wiring harness with all connectors meticulously labeled for easy identification and installation during the conversion process.

Conclusion: OBD1 Conversion Complete and Performance Unleashed

The e36 OBD2 to OBD1 conversion is a complex but rewarding undertaking for BMW enthusiasts seeking enhanced performance from their E36. While it’s more involved than a simple “electronics swap,” careful planning, attention to detail, and adherence to these guidelines will ensure a successful conversion.

My final OBD1 configuration on my 1999 M3 S52 included:

• AA Cam Chip
• Sunbelt Cams
• Euro 3.5″ HFM
• Conforti 3.5″ Intake
• 24lb Injectors
• Sunbelt Valve Springs
• ARP Headstuds
• AA Race Headers
• AA Race Exhaust
• Zionsville Competition Radiator
• Euro Oil Cooler

Here’s the final result – my OBD1 S52 engine bay after the conversion:

Alt text: After E36 OBD2 to OBD1 conversion: Engine bay of a 1999 BMW M3 S52 post-OBD1 conversion, showcasing the cleaner OBD1 engine management setup and performance modifications.

Post-conversion dyno testing confirmed the performance gains. Initial dyno results showed 253 RWHP and 228 TQ. After refining the chip tuning with Active Autowerkes to lean out the fuel mixture, subsequent dyno runs yielded 262 RWHP and 230 TQ, with optimized AFR. These results validated the effectiveness of the e36 OBD2 to OBD1 conversion and associated performance modifications.

If you have any questions regarding the e36 OBD2 to OBD1 conversion, feel free to ask. This guide is intended to be a helpful resource for fellow BMW enthusiasts embarking on this performance upgrade journey.