The biggest fear with vacuum advance is TO MUCH total advance (BASE, plus MECHANICAL plus VACUUM added together). But at what inches of vacuum does it start to advance the timing and by how much? Distributor 'curve' is what were talking about here, and it's tough to nail down.
Normally replica Cobras need more BASE timing advance than other engines because of the aggressive cam profile (compared to street cars). If you don't get enough BASE advance the carb will stumble and you'll have a lack of power until a certain rpm is reached (1800-2000 or so). Around 1800-2000 the mechanical advance starts working, so the motor starts running better.
The new MSD vacuum advance distributors don't even start working until 15" of vacuum. What YOUR distributor works at is a mystery, could be as low as 7", typically around 11" or 12". With an agressive cam profile you might not make 15" even at cruise rpm and light throttle. 11" or 12" is common with Cobra motors.
MSD, in consultation with their 'lawyers', are really conservative with their distributors 'as shipped'. Vacuum advance (non adjustable, assumes you have such a model) is WAY high at 15". Mechanical advance doesn't even start working before 4000 rpm (all units). Total advance is fairly limited (all units). To work properly they HAVE to be 'set up' and that shifts the responsibilty to the consumer! Wise people these MSD folks...
Rule of thumb is: Run as much advance as you can without detonation for maximum performance AND fuel economy. The FIRST thing manufacturers did to control emissions was retard the timing, the FIRST thing that did was reduce horse power!
You CAN run pretty advanced timing (beyond) 40 degrees utilizing the vacuum advance. This assumes crusing speed, light load, high intake manifold vacuum. This might be a GOOD thing, more advance, more efficiency, better mpg.
15 base, even more, may be required depending, again, on your cam\carb profile. There is no 'magic number', all custom engines vary in what they like.
I run about 18 base, 30 some total AND have a vacuum advance. But I got a fairly radical cam and twin carbs, I NEED a lot of base advance timing. Had to buy a special 'geared starter' to crank that engine over with the big base advance.
