[WorkinLonghorn]

"Total advance" referrs to the mechanical advance ( the centrifical weights system) plus what the INITIAL setting is (set by positioning the distrib itself). VACUUM advance is set for CRUISING or part trottle operation only. On a stock engine the vacuum advance goes back to 0 degrees as the throttle opens past the little port in the venturi that opens vacuum to the advance unit.The reason for vacuum advance is that under part throttle the cyinders are just partly filled which results in a low compression ratio and much more advance can be tollerated and is needed for economical operation.Racers don't give a rats rear about economical cruising so they don't have any vac unit,and I've heard that the guys with rad cams like to hook the vac up to full vac for better idle etc.But if you want good mileage you will install an ADJUSTABLE vac advance and experiment until you are just a bit below the knock point at cruising/part throttle operation.-Brian