Ignition Advance Question (General)

[baj25]

I understand well that ignition advance depends on engine rpm and load. But, when you are at revs high enough for 'centrifugal' advance to be all-in, does the 'vacuum' advance advance the timing even further? Say on a pinto with centrif advance all-in at 36deg at 3500rpm, and you are cruising at 3500rpm on part throttle, will the vac advance take timing past 36deg? I've looked at lots of megajolt maps and most top out advance at say 36 degs, though I have seen odd ones beyond 40 deg at low load. TIA, Brian

[Snapperpaul]

No vacuum at WOT so no it does not advance past that given by advance weights

At lower throttle opening and on throttle shut off yes it does as there is now vacuum to turn the vacuum advance plate

It's all about the burn time, throttle shut or part open means less O2 so burn time is longer spark needs to happen earlier so all fuel burnt

There is also a need to achieve maximum pressure on the piston at just after TDC.

As always in physics there are compramises

 

A spark plug on 1 side of a combustion chamber takes longer to ignite all the fuel than 1 in the centre thus the Zetec has less advance than a Pinto

Some engines run twin spark plugs but this only works properly if they are diametrically opposed so the flame front meets in the centre

[IanS]

you are probably coasting down hill to get vac advance past the 36 degree 3500RPM limit.

But the old bob weight / vac system will go to these sort of advances I am not certain how much MPG you actually gain in these extreem points of the curve.

[cb750]

In theory, the vacuum advance would add to the total advance (static + mechanical) of 36 degrees but as IanS says you would have to be in situations where the revs are at 3500 plus and the throttle backed off.