Quick Tech With Mar­lan Davis

Hot Rod - - Contents - hHOTROD. COM/ 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...

Mar­lan Davis

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