The EJ251SOHC was the first version of a long line of single overhead cam  engines by Subaru for the US market.
The EJ251 was soon replaced by the EJ253 in many models due to its improved cooling to aid in head gasket life and improved engine management and sensors.
 The EJ251 commonly experienced head gasket failures resulting in interior channel breaches or exterior fluid leaks due to the continued use of a single layer coated gasket first introduced on the EJ25D. Intake volume is calculated by the use of a MAP sensor, unlike the EJ253 which uses a MAF sensor.
Compression ratio is 10:1.

Power ISO: 123 kW (165 hp; 167 PS) at 5600 rpm and 226 N⋅m (167 lb⋅ft) of torque at 4400 rpm
Usage:
Impreza 2.5RS, 2.5TS, 2.5OBS 98-03 (US) (excludes 99)
Forester 99-04 (US)
Legacy 99-01 (US, 4EAT)
Legacy 99-04 (US)
Outback 00-01 (US, 4EAT)
Outback 02-04 (US)
Baja 03-05 (US)
The EJ251SOHC was the first version of a long line of single overhead cam  engines by Subaru for the US market. The EJ251 was soon replaced by the EJ253 in many models due to its improved cooling to aid in head gasket life and improved engine management and sensors.  The EJ251 commonly experienced head gasket failures resulting in interior channel breaches or exterior fluid leaks due to the continued use of a single layer coated gasket first introduced on the EJ25D. Intake volume is calculated by the use of a MAP sensor, unlike the EJ253 which uses a MAF sensor. Compression ratio is 10:1.
Power ISO: 123 kW (165 hp; 167 PS) at 5600 rpm and 226 N⋅m (167 lb⋅ft) of torque at 4400 rpm
Usage:
Impreza 2.5RS, 2.5TS, 2.5OBS 98-03 (US) (excludes 99)
Forester 99-04 (US)
Legacy 99-01 (US, 4EAT)
Legacy 99-04 (US)
Outback 00-01 (US, 4EAT)
Outback 02-04 (US)
Baja 03-05 (US)
Subaru EJ engine