I have a 450 holley with a vacuum port that comes straight out on the on the right side of the primary metering block and is manifold vacuum. The ports I have encountered that come bent at a 90 (out of metering block) have almost no vacuum at idle.I have heard it called metered vacum, ported vacuum, and spark timed vacuum but I cant think of any pre-74 distributor that uses it. Not positive about that for sure though. I like to use full manifold advance on my 250 because it gives my cam the timing it needs to idle well, and gives me better mpg at cruise, but it also works as kind of a mechanical spark retard when starting engine. But I have a flat 4 (bug) that runs way better with the vacuum advance disconected altogether, go figure
