CREATE TABLE table_16551 ("Engine" TEXT, "Power ,  continuous" TEXT, "Critical altitude This is the highest altitude at which the engine can achieve its full continuous power rating. Above this altitude ,  power falls off with height as with a naturally aspirated engine . See Supercharger#Altitude effects for details." TEXT, "Power ,  takeoff" TEXT, "Compression ratio" TEXT, "Supercharger gear ratio" TEXT, "Octane rating" TEXT, "Dry weight" TEXT)

When critical altitude is sea level, what is the compression ration for a supercharger gear ratio of 7:1?
SELECT "Compression ratio" FROM table_16551 WHERE "Critical altitude This is the highest altitude at which the engine can achieve its full continuous power rating. Above this altitude, power falls off with height as with a naturally aspirated engine . See Supercharger#Altitude effects for details." = 'sea level' AND "Supercharger gear ratio" = '7:1'
SELECT "Compression ratio" FROM "table_16551" WHERE "Critical altitude This is the highest altitude at which the engine can achieve its full continuous power rating. Above this altitude, power falls off with height as with a naturally aspirated engine . See Supercharger#Altitude effects for details." = 'sea level' AND "Supercharger gear ratio" = '7:1'