Depending on year on 225 piston height and gasket 9:1 to 9:5. My stock 78 225 had pistons .170 in the hole cut my slant head .100 to .45cc heads that motor barely got to 9:18:1 But that had me at a cranking psi of (155-160) from a weak 120psi previously my other slant six 1966 block had the pistons closer up .120 in the hole got me in the 9:4:1 range after everything was calculated, its not rocket science although a 170 engine has the piston height higher almost flush to the deck so i would watch out for that .100 cut will get you close to 10:1 on a 170.
