Re: Late 50’s Johnson 18 hp compression? Home › Forum › Ask A Member › Late 50’s Johnson 18 hp compression? › Re: Late 50’s Johnson 18 hp compression? hdtedReplies: 37Topics: 10 May 19, 2018 at 8:51 pm #76341 I had a 1974 25hp NOS motor still in the box. It had 140 psi cranking pressure. I don’t know if there was a difference between the 18 and 25, but they were both 22 Cubic inch motors.