Sorry it took me so long to respond to Jerry's comment below. Been out of town.
Originally Posted by Hotrod Lincoln
One degree of movement of the distributor changes the ignition timing at the crankshaft two degrees, since the crank turns twice as fast as the distributor. It doesn't take much adjustment at the octane selector to make a big difference at the flywheel. Also, one degree of ignition timing over-advance once the best torque point under load is reached robs as much power as 5 degrees of retarded timing from the ideal setting. I've proved that repeatedly on dyno runs. Over advance is trying to make the engine run backwards. ALWAYS err on the side of "not enough" timing advance. That's why monitoring vacuum at idle while changing timing makes no sense at all. Jerry
The radius from the center of the distributor shaft to the outer (curved) edge of the octane selector scale is 1.825”. So a circle with this radius would have a circumference of 11.467”. This circle has 360 one degree arcs, each being 0.0318 inches long. So twenty of them ( +10 to 0 to -10) on the octane selector scale should be either 0.318 inches long if displaying “crankshaft degrees” (2 crank rotations per distributor, each crank degree is ½ distributor degree — most spark timing specs are done this way) or 0.636 inches long if displaying camshaft/distributor degrees.
I suspect that the vacuum advance/octane selector shown below is an aftermarket knock off. The distance along the red curve from 10 degrees advanced to 10 degrees retarded measures 0.403 inches. If it were in “crankshaft” degrees the arc would be smaller at 0.318”. If it were in “distributor/camshaft” degrees it should be larger at 0.636”.
So the answer is “it measures closer to (but more than) crankshaft degrees than distributor degrees”.
Last edited by Bill Hanlon; 05/13/20228:37 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
