ENGINE TIMING – DISTRIBUTOR (NON-COMPUTER)
• MANIFOLD – Below the carb – highest at idle and decreases as the throttle is opened – zero at WOT
• PORTED – Above the carb throttle blades – zero vacuum at idle, increases off-idle and thru part throttle – drops to zero at WOT
Usually the best vacuum performance is obtained by hooking the advance diaphragm can to the ported vacuum source, however many auto trans vehicles will idle better using manifold vacuum.
If there is a throttle tip-in ping problem, the manifold vacuum source may work better. If not, an adjustable vacuum diaphragm may be needed to reduce the total vacuum advance. Stock GM diaphragm units generally have 10° total advance. Check using the manifold vacuum source or a hand vacuum pump, such as a “Mighty Vac”.
TIMING SETTINGS – READ AT CRANK, VACUUM DISCONNECTED
• INITIAL, at IDLE = 10° - 14°, making sure the centrifugal advance hasn’t started to come in yet.
• TOTAL, at approx. 3200 rpm = 30° - 34°, with 10 degrees of centrifugal advance in the distributor (20° read at crank). Do not have too quick of a centrifugal curve, preferably no more than 8° (at crank) at 2200 rpm and total advance not arriving until 3200-3400 rpm.
Street blown motors will do better at 28-30 degrees total at 3400-3600 rpm, so use an initial setting of 8°-10° and a slower curve.
• A sample setting for a mild to moderate performance small block Chevy or Ford would be 12° initial and 32-34° total (3200 rpm at crank, vacuum disconnected), and using a 10° vacuum can connected to ported vacuum (stnd trans) or manifold vacuum (auto trans).
Performance applications using quicker-than-stock advance curves and more initial timing than stock, generally benefit from a port vacuum source rather than manifold vacuum. Engine mods and trans types will affect results. Highly modified motors with auto transmissions require higher stall converters and usually idle better using a manifold vacuum source. Less than 8 inches of vacuum at part throttle/light load should use an external vacuum source. Also, better results will be had using an adjustable vacuum diaphragm on the distributor set at 5°-7° advance (use manifold port or hand vacuum pump to set, then connect to port source for running).
And finally, if you have an opportunity to put your vehicle on a chassis dyno, do so. That one tool is unbeatable for maximizing all your tuning settings.