nook
+1y
[quote="holcombe347"][quote="truck action"]
sorry for the book, but if someone could explain how the vacuum advance works it would be greatly apreiciated...
It can get pretty confusing on all the stuff regarding ignition timing, advance rates ect. you could fill books with all the info.
Basically you have 2 advance units in your distributor, mechanical advance and vacuum advance, they work together and work work independently.
Later units don't have the mechanical advance, its computer controlled but we won't go there.
Mechanical advance is the pair of weights and springs inside the distributor, as the dist, spins the weights are thrown outward by centrifical force, and controlled as to the amount and speed they can move by the spring pressure thats holding them.
In therory when the engine is running the timing would be perfect at "0" degrees or TDC, as the piston is moving up the cylinder compressing the air fuel mix, just at the point the piston hits TDC or "0" degrees and starts to move back down thats when the fuel mix should explode to drive the piston back down, but take into account the speed things are moving and the time it takes to fire the plug, ignite the fuel mix, have it explode and expand thats why the initial timing is set a few degrees before TDC so hopefully they can all happen at the right time.
The faster the engine spins the more advanced the timing has to be so it can fire the fuel mix at the correct time just when the piston is breaking over and starting back down, too early (advanced) and detonation, the explosion is trying to stop the piston, too late (retarded)and the piston is already traveling down and the blast doesn't have as much force. Thats where the mechanical and vac advance is doing its job, the faster the engine spins the more the weights move the more the timing is advanced.
The vacuum advance varies a bit during light throttle as the manifold vacuum goes up and down, but under full throttle the manifold vacuum is nothing so the vacuum advance on the dist does nothing.
Now at hiway speed cruising along at 60mph, the engine is say turning 2500 rpm, the mechanical advance inside the distributor is not fully advanced because of the springs holding back the weights, so the vac advance steps in because you have a good strong manifold vacuum so it advances the timing to give better power and fuel economy.
Bottom line, if you have a vac advance use it, old rule of thumb, advance until it pings then back it down a few degrees for best performance, but take into acount that rule is for best performance and hard to deal with for a daily driver that see's varied driving conditions, grades of fuel, ect.
Sorry its not a great explanation but I tried.