[Jim Brierley]

Good answers by Railcar, Pete and PC/SR! My new head is 7:1 but has a raised area between the intake valve and the cylinder, to make higher ratios more viable. I ran a 7.5:1 Cyclone on the street for many years, on dipper-babbitt, no problems. My current Bonneville engine is still on full pressure babbitt mains, turbo-charged, 7.5:1 Cook OHV, on racing gas or alcohol, 6,000 RPM. Last run in '13, at 167 MPH. I would recommend 7:1 for most street use.