Base timing. Really ?

92b said

I'll try to explain. If this is wrong then someone can delete this.
If we have 2 plates one with short slots and one with long slots. In both cases we want 34 deg at wide open throttle for best power.
Long slot = 10 deg at Idle and 34 deg wide open.
Short slot = 20 deg at idle and 34deg. wide open.
Long slot (10 deg)+ vac adv (?) = just right amount of timing at cruise rpm.
Short slot (20 deg) + vac adv (?) = too much timing at cruise rpm.
? represents the amount of timing your vac. adv. adds.

Click to expand...

You're getting closer.

Lets compare using the advance I measured on a parts store distributor (that I might have changed the spring).
With the inital timing set at 9° the max is 34°
So this distributor effectively has 25° in the slots.
What is the timing at 1500, 2000 and 2500 rpm?


Now lets take this same distributor and set the initial at 18° and limit the maximum advance to 34° using a plate or welding the outer end of the slots.
So this distributor effectively has 16° in the slots.
What is the timing at 1500, 2000 and 2500 rpm?

Now what happens when we add vacuum advance for most efficient/powerful lean fuel operation?
The vacuum advance that came with this distributor adds 22° with 16"Hg and higher.


For drag racing or even street its possible to get away with 30° at 1800 rpm because the converter/clutch is not fully loading the engine, and its moving through that rpm quickly. Even more so with drag racing because the engine isn't fully heat soaked. Probably couldn't get away with it lugging a trailer up a long steep hill unless the gearing was very low.
So without vac advance this curve is OK, and may in fact be an advantage at the strip (ignoring the slew loss at the high rpms).