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Message: <blockquote data-quote="AApple" data-source="post: 95887" data-attributes="member: 56">As long as the throttle blades are OPEN, there will be no, or very little vacuum in an engine. Take a sneezle engine, fer instance....they don't create ANY vacuum, because they don't(normally) have a throttle blade, or any control over air intake. That's why they need a vacuum pump.There has to be some sort of restriction in the intake tract to "create" vacuum....that restriction is the throttle blade(s). A carbed engine needs some sort of vacuum to "pull/suck" the fuel out of the fuel bowl. Fuel injected gas engines use a vacuum signal to determine engine load, among other things.Vac 101...class dismissed.... :smilie_happy: Now back to your regularly scheduled timing thread....Looking at the charts posted, I see very little difference in the 4 plotted curves, and mostly the difference is above 3500 rpm...right about where the vac and mechanical advance would start pulling the timing back up(advancing). At least one of the charts shows a larger difference in the low rpms, but not much in the higher rpms.So....I'm starting to unnerstand(it's a MIRACLE!) the programing on the Power Arc/C5.... I may be slow, but...I'll get there! :hihihi:</blockquote>