Vacuum advance information and myths
I want vacuum advance for,
Mileage
Throttle response
Cooler running
Idle quality
Best vacuum for accessories
Tunability if engine parts are changed
Did I miss anything ?
You're describing general benefits of using vacuum advance.
1. Mileage Yes
2. Throttle response It might. Wouldn't guarentee it.
3. Cooler running Yes. Whether it will always be a noticible difference is another story.
4. Idle quality Depends on the distributor setup. If the advance curve is correct for the application, no difference.
5. Best vacuum for accessories. Not really. It uses vacuum as an indicator of load and density. It doesn't create vacuum.
6. Tunability if engine parts are changed. Depends on perspective. For drag racing there are good reasons not to use it. If the engine has been altered in a way that effects the burn at light to moderate throttle, then yes the vacuum advance should be tuned to match that.
Another way to look at it is functional.
When the burn rate is slower due to lower density of the fuel and air, more lead time is needed for maximum pressure during the maximum crank leverage.