Quick Tech With Marlan Davis
Initial, as well as total ignition, timing is usually expressed in crankshaft degrees as read at the harmonic balancer. It describes where the spark plug fires in terms of crankshaft position before top dead center (BTDC). Nevertheless, a potentially confusing aspect of ignition tuning on pre-electronically managed cars is that distributor centrifugal advance curves, as well as the amount of any vacuum advance, are often described in terms of distributor degrees. As the distributor rotates at half the speed of the crank when installed in the engine, just multiply by two to get crank degrees. For example, if a tuner put 11 degrees of centrifugal advance in the distributor and set the initial (base) timing to 14 degrees BTDC at idle on the crank, the total advance when the centrifugal curve fully “comes in” would be 36 degrees at the crank [(11 × 2) + 14 = 36].
Norm Brandes [ A camshaft’s timing-chain (upper) sprocket is twice the size of the crank (lower) sprocket, so the cam rotates at half crank speed. The cam drives the distributor at a 1:1 ratio. Therefore the rpm and the timing in degrees as measured...