If you run the can on a locked dizzy, it can bring 18 to 20 degrees in on top of the locked amount. So the engine could see something like 54* at about the time that the manifold vacuum peaks or even sooner; and at rpms below that it could be way over advanced; so that would be bad. But if you put the mechanical back in and run idle-timing at something like 16* then,with the can, it might only be adding up to 34, at that same time, and it would be ever-changing depending on what the throttle is doing . If you manipulate those two, you can build a sweet responding lowend into your combo that might otherwise be lazy or have a hard-to-find hesitation. And as mentioned, if you get it right, you could get the cruise timing at something like 48 plus degrees, or what ever the combo likes, and gets the best mileage, which you cannot do with a locked timing.You might be surprised how much timing the engine wants when cruising. Mine likes over 48*. Some situations, she likes over 54*!
Plus the engine will run cooler and smoother as she gets the timing shes been craving.
And the best part is when you hit the skinny pedal, all timing reverts to mechanical.So whatever you set the max to, that will be what you get.
Since a street car spends a huge amount of time under the torque peak, it pays good dividends to get this worked out, even if it means a small sacrifice in power timing (at the lower rpms,) and even idle timing.
For instance; if your combo likes 18* at idle, but does not like any more for another 600 rpm, what can you do? Easy. Dial it back 4 to 6 degrees and bring it in with the can instead. But what if at 2800 and light cruise the engine complains about the 34 mechanical plus 20 vacuum. Easy. Put a staged second spring in there a lot stiffer than the first. This will put a kink in the mechanical curve somewhere; it will be fast to a point and then crawl up to the max power-timing. If you get it right the engine will always be running near optimum timing. Or you could just get a computer......and really make it tight.