CREATE TABLE table_203_823 (id DECIMAL, "model name" TEXT, "date" TEXT, "type" TEXT, "thrust  ( kg )  / power  ( eshp ) " TEXT, "fitted to" TEXT)

what was the top thrust achieved by a lyul ` ka engine before 1980 ?
SELECT "thrust (kg) / power (eshp)" FROM table_203_823 ORDER BY "thrust (kg) / power (eshp)" DESC LIMIT 1
SELECT "thrust (kg) / power (eshp)" FROM "table_203_823" ORDER BY "thrust (kg) / power (eshp)" DESC NULLS LAST LIMIT 1