"Every engine is different..mine will not tolerate manifold vacuum..But I've had other engines that like manifold vacuum ..there are no rights or wrongs."
My current and last 455 were not fond of running a lot of timing at idle speed. They roar to life instantly after shut-down and idle smooth, produce plenty of vacuum and the tach barely moves when the trans is placed in gear. If I move over to manifold vacuum to the advance they start to "skip" and "miss" slightly and generally just unhappy with all that timing on them.
I can re-adjust the mixture screws slightly and idle speed but then I'm left with more drop in RPM when the trans is placed in gear and slight "skip" in the exhaust note that is not present when running less timing at idle speed.
The engines that have been brought here for running issues that liked, wanted and responded well to a butt-ton of timing at idle speed have all been lower compression and "hefty" sized cams in them. So there may be scenarios where you can improve things some running the timing off the scale at idle speed. The problem one quickly runs into when doing this is that the VA being used will require a very low cut-in point as engines with low compression and hefty cams may not produce enough vacuum at idle speed to effectively operate most VA units. They make VA units that are adjustable for spring tension and cut-in points for this purpose. I don't like or use them here, I much prefer the VA units that control how much timing is added instead, but they are becoming more difficult to find these days......Cliff
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https://cliffshighperformance.com/
73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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