Will a 1990 crx hf ecu work on a 1991 crx si?

A 1990 CRX HF ECU may not work properly in a 1991 CRX Si due to differences in engine management and tuning between the two models. The Si features a more powerful engine and may require a different ECU to handle additional parameters such as fuel mapping and ignition timing. While you might be able to plug in the HF ECU, it's likely to lead to suboptimal performance or driveability issues. For best results, it's recommended to use the ECU specifically designed for the 1991 CRX Si.