Join Date: Aug 2010
Location: Ulster County, New York
Bronco Info: 1978 Cummins Bronco
With a small cam and iron heads, 9.3:1 would do best with 93 octane, unless you retard the timing slightly, which will negate the efficiency benefit of the higher compression.
Ox, with iron heads and a truck cam, 10.5:1 will ping unless you retard the timing. My Mustang motor has 10.95:1 with aluminum canted valve hi-swirl heads and a big solid lifter cam. ( 250/260 duration @ .050" ) and it barely gets away with premium gas. Of course, the more aggressive the cam, the lower the cylinder pressure at low rpm and the less octane required. Vice versa for smaller cams.
1978 Bronco Custom, 6" skyjacker & 3" body lift, 37x12.50R16.5s, posi front diff, 3.50 gears, Dynamic Racing C6 w/billet 1200 rpm converter, upgraded brakes, '91 Cummins 12V, Denny T fuel pin, injection pump maxed out & timing advanced, 3800 gov. spring, 60# valve springs, ARP head studs, 4" exhaust.