Socketing or chipping a 1990–1995 Honda OBD0 or OBD1 ECU requires installing a 32 KB (256-kilobit) parallel memory chip. While several chip types fit the physical DIP-28 socket footprint, they have different access speeds, erase procedures, and pinouts.

1. Supported Chip Technologies#

Tuners commonly choose between three distinct chip technologies:

• SST 27SF256 (Flash Memory - Recommended): This is the industry standard for Honda tuning. It is electrically erasable and rewriteable, meaning it can be programmed repeatedly in seconds using a standard USB burner.
• 27C256 (EPROM): Erasable Programmable ROM. Windowless versions are one-time programmable (OTP). Windowed versions can be erased using ultraviolet (UV) light, which takes 15–20 minutes in a UV eraser box.
• AT29C256 (EEPROM): Electrically Erasable PROM. Compatible with standard sockets and easily rewriteable, but less common than SST flash chips.

2. Speed Specifications (Access Time)#

ECU microcontrollers require extremely fast data retrieval to coordinate fuel injector pulses and ignition events.

• The Spec: Honda specifications mandate memory chips rated at 170 nanoseconds (ns) or faster.
• Recommendation: While some chips labeled 200ns will function because they exceed their rated speeds in practice, it is highly recommended to source chips rated at 150ns, 120ns, 90ns, or 70ns to guarantee stability under high heat and high RPM.

Access Speed Suffixes:#

• -20 = 200ns (risky; use only if verified)
• -17 = 170ns (minimum factory specification)
• -15 = 150ns
• -12 = 120ns
• -9 or -90 = 90ns
• -7 or -70 = 70ns

Historical Example#

Early production runs of the OBD0 Acura Integra PR4 ECU (dated June 1989) shipped from the factory with a windowed 27C256-17 (170ns) chip in a factory socket. Because mass-production mask ROMs were not ready in time, Honda utilized these reprogrammable chips to meet initial vehicle delivery targets.

Factory-socketed windowed EPROM from a 1989 Acura Integra PR4 ECU.

3. Incompatibility Warnings#

The 28C256 EEPROM#

Do not confuse the recommended 27SF256 or 29C256 with the standard 28C256 EEPROM. Although the 28C256 shares the same 32 KB capacity and 28-pin DIP package, it uses a different pin configuration (specifically on write control pins). If plugged into a standard 27C256 socket, a 28C256 will not communicate with the ECU and will trigger a solid CEL. It requires circuit trace modification to function.