[CentipedePerformance]

hooked up to manifold only advances the timing at high levels of vacuum such as cruising. As the throttle blade opens further, manifold vacuum decreases, and pulls the vacuum advance out. Ignition timing is set at 37* with just the mechanical advance, if you could put a timing light on it running down the road, timing at cruise is probably 45*. When cruising, the mixture is very lean, which takes longer to burn, so you need to advance the timing more. Difference between ported and manifold, is that the ported vacuum has nothing at idle. Advancing the timing at idle makes for a smoother idle, and decreases exhaust temperatures, as well as coolant temperature at idle. The reason OEMs went from manifold to ported, was because they needed to meet the new emissions requirements. Higher egts along with the air pump system allowed for the excess hydrocarbons to be completely burned further downstream by the Catalytic converter.