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sorry to hijack this thread on a lesson in old engines,
first off, base timming or inital timing is some what just a starting point.
vacuum advance through a untimed port is accepted by many as the way to go, i blame this on the manufacturers of "muscle cars" in a time when emmision laws were in their infancy, in this case, the manufacturers used untimed vacuum advance to clean up the burn at idle, therefore negating the need for any initial advance at all, they would actually move the keyway in the crank snout to make the engine seem like there was initial advance when there was none at all (ford 460). the only time an engine sees initial timing is just off idle at hard throttle application, when the mechanical advance has not started and the vacuum is at it's lowest.
vacuum advance is there to match the spark to the speed of the burn when the charge varies due to sudden changes of the throttle position, just like a map sensor is there to increase the pulse width on acceleration or the vacuum operated fuel pressure regulator on early injection systems like bosch, it responds almost instantaniously.
the time taken to burn a combustion chamber full of air/fuel at a ratio of 14-1 (lamda) is a constant for any given dynamic compression ratio, unfortunatly, the 14-1 is not always present, when it's say 18-1 you need more advance because it burns slower, likewise when its 10-1 it burns quicker so you need less advance.
there are other factors which control the speed of the burn, if you use 87 octane fuel, it burns quicker than a higher octain, if you raise the compression ratio it will burn quicker, if you create turbulance in the combustion chamber it will burn quicker, the last thing is what has been the most prevalent in modern engines.
as a side line, the best effective turbulance in a chamber would be achieve with a squwish hight of 0.040, that is the distance between the head flat area and the matching flat area on the piston.
this is why most engines are not hemi in their design, when they invented the hemi, it was in a different time with less knowledge than now about burn rates and fuel was fuel so their answer to speed up the burn was compression and a spark plug in the center so the flame front could travel outwards evenly, this is not the world we live in now, in the 60's a man called Dave Degins was racing Triumph motor cycles and developed a hemi design with a squish area at either side of the chamber/ piston which would make 500rpm more down the straightaway than a conventional hemi due to the turbulance it created and would go further on a tank of fuel.
interesting though it may be, i am wandering again.
the vortec head design as you can tell by it's name creates a lot of turbulance to speed up the burn on 87 octane fuel and is quite efficient, if you are using a distrbutor in the old days of Chevy, you would set the total advance at 38-42 degrees and get it there as soon as possible, this no longer applies as we now have turbulance, limit the advance to a max of 36 degrees at 3000-3500 rpm. beause fuel burns quicker it needs less advance and more of the fuel is burnt due to turbulance instead of going out the exhaust.
if as suggeted by oldejack, you pinch off the vacuum hose, this will trap the vacuum in the distributor and will stay advanced, if it doesn't, you need a new vac can on the dissy, you have to let air in to the vac canister by disconnecting the pipe, remembering to plug the pipe to prevent air being sucked into the manifold.
to sum up, you need vacuum advance, you need total timing, initial is just a matter of being there and is totally dependant on total advance and may be altered to gain the total advance required for your engine provided that there is vacuum advance present at idle (direct from the manifold or untimed port) and you do not go into a retard condition with the initial, i.e. after top dead center, if this occurs you will have to limit the mechanical advance at the distriburor.
if you have a stumble off idle however, you may cure this by adding initial as this is a symtom of loss of vacuum signal to the carb, you can check this by adding initial advance to see if this is the problem, if it is, then you will have to go inside the dissy and adjust the mechanical advance to maintain the total at 36degrees. if it does not cure your stumble you need more fuel from the accelerator pump.
do not be fooled by adding initial advance willy nilly with the vac pipe disconnected and hearing the engine speed up, this is just a symptom of a stronger signal to the carb causing it to pull more air and fuel in, thats what you need the vacuum advance at idle for, try a max of 12 degrees and if it doesn't cure the stumble go to the carb.
watch for the dreaded detonation, this must be avoided at all costs. if you get detonation, at cruising speed, disconnect the vacuum and try again if it goes away get a slower stiffer vac can, if it does not then you have too much mechanical advance, or a combination of both.
like in a modern computer controlled engine where you would make a 3D fuel/spark map, you are making a mechanical fuel/spark map with nothing to help you other than intuition, plug color, the tail pipe, poerformance and the detonation point.
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), I wouldn't bother with vacuum advance (they were developed to improve driveability and run on ported vacuum so that they only do something when the intake airflow across the port generates enough vacuum to overcome spring tension in the advance mechanism, rather like an ejector nozzle trying to suck the advance diaphragm into the intake instead of a fluid), using a mechanical only advance will function well without the added bits and tuning issues.
