440 cylinder heads

i replaced a set of 906 with a set of stock valve 915's 1.60 exh valve on a 383 hp, the 915 chamber is the same as the 516, they raised my cr about .8, when you replace the valve to the 1.74 and go with a valve that is not concaved the CC's on them drop a good bit. then you will get 1 to 1.2 increase in CR.