Think it's in one of my DV books seen it other places just googled it and that's 1st thing came up. 3-4% hp gain per point of cr increase and also said that an increase say 7:1 to 8:1 will give slightly better % than 14:1 to 15:1. So low end cr increases probably the 4 ish and higher ones are 3 ish percent.
