Rich mix, like when accelerating, take less time to fully igniteįully ignite means full explosive power, when a spark plug sparks it ignites just a bit of the air fuel mix around it, it takes some time for the flame front to spread and you get the full explosive power to push down the piston and add power to crank shaft.Īnd you want this to occur After TDC of course, lol, but you want it at about 8deg ATDCīefore that and you don't get good leverage on the crank to push it down, and after than you don't get full power from the explosion because there is too much room for it to expand into, so less power is added to crankĪt idle with vacuum advance 20-25deg advance is fine its a leaner mix and low RPMs so takes a few extra millisecond for piston to move to 8deg ATDC and for mix to fully ignite.Īs RPMs go up the time it takes for piston to move from say 20deg BTDC to 10deg ATDC is much faster.Ĭentrifugal advance takes care of the RPM advance for the spark, so spark moves to more advanced as RPMs go upīUT.if you step on the gas, make Richer mix, the Vacuum advance should retard the centrifugal advance a bit because the Richer mix ignites faster than the leaner mix, but as the RPMs get higher from "stepping on the gas" it should start to get more advanced. Lean mix, like at idle, takes longer to fully ignite When vacuum advance is reconnected idle will most likely go up a bit, it depends on the vacuum advance YOU CHOOSE to use, reset idle for transmission typeĪll gasoline engines need to be Choke when cold, so make sure your choke setup is working or cold engine won't run wellįord often used a Dual vacuum advance setupĪ vacuum valve that heated up with the engine and switched between Ported Vacuum and Manifold vacuum, this was mostly for lower emissions.Īnd Ported Vacuum or Manifold vacuum for spark advance is a long drawn out debate, lolĪnd there is usually a screw inside the hose fitting on the vacuum advance unit that can be adjusted for best performance Warm Idle for manual trans should be 650, 750 for automatic Then disconnect vacuum advance, PLUG THE HOSE, and set base timing So get it started and let it idle for 10minutes. Its best to set timing after engine is warmed up The manifold Vacuum is "in effect" throttle position, but not as accurate as a TPS(throttle position sensor)ĭuraspark distributors are the popular choice for HEI conversion as they used similar hall effect sensor in the distributor and are vacuum advance.Įither setup/distributor would have centrifugal advance(weights and springs) in the distributor for RPM Spark advance.īase spark timing is set without computer or vacuum advance connectedġ0-12deg is suggested base spark time for these engines.
HEI was used with Vacuum Advance distributors. If you had a TFI ignition system it needed spark advance info from the Computer, the SPOUT wire, because computer has throttle position data.
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