Best years with the Sox.....
Tiant:
1974, 311.1 IP, 132 ERA+
1976, 279.0 IP, 128 ERA+
1973, 272.0 IP, 120 ERA+
1972, 179.0 IP, 169 ERA+
1978, 212.1 IP, 125 ERA+
Grove:
1935, 273.0 IP, 178 ERA+
1936, 253.1 IP, 190 ERA+
1937, 262.0 IP, 158 ERA+
1939, 191.0 IP, 185 ERA+
1938, 163.2 IP, 160 ERA+
Tiant gave more innings, but Grove's quality was better. Plus, Tiant threw a couple of average years in there (100 and 102 ERA+) during his 1972-1978 run, while Grove was excellent for 5 straight years. It's a close call, but I think Grove wins.