Not Ranked
There are two main reasons I like vacuum advance.
One:
I can get a higher idle rpm WITHOUT having to open to carb butterflies. This of course requires direct manifold vacuum, ported won't work for this. Why I like it: When I turn off the ignition with the carb butterflies closed more than they would normally be to achieve the same idle rpm, the motor has far less tendency to "deisel" or "run on". So this method helps control "deiseling" on engine shut down.
Two:
Vacuum advance helps improve mpg, in some cases, it improves it a LOT. Not worried about mpg? I only have a 17 gallon tank, the more mpg I get the more range I have per tank of gas. Under cruise conditions, light load, only partial open throttle, the engine can use a LOT of advance. Up to 50 degrees, 45 is safer, but it can use a LOT. Say your base timing is 15, add 20 for mechanical advance, add another 15 or so for vacuum advance and you get 50 degrees. I've been running this amount on my FE for the last 8,000 miles. Even with dual carbs I can get 13-14 mpg while cruising at 70-75 WITHOUT overdrive! There are some other "tricks" I've done to the motor to specifically address better efficiency/mpg, all the little tricks work together to provide a nice "package deal". Primary trick is "quench" in the combustion chamber, something the FE's never had back in the day. Custom pistons have to be used.
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