Quote:
Originally Posted by Cliff R
Several reasons you can get a greater drop in RPM's with manifold vacuum. With some set-ups the load placed on the engine reduces vacuum just enough to drop out some timing from the VA.
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This quote from Cliff explains why some people see wonky idle. This is especially true when you use a vacuum canister that adds a ton of timing and doesn't even begin to work until 12-15 inches of vacuum, then at 20 inches it's throwing a crap load of timing at you.
With engines that idle with 12 inches in gear, then vacuum readings jump to 15 inches out of gear, this just won't work. When the engine is put in park that extra vacuum starts throwing a ton of timing at it and the idle jumps considerably, and as idle rises so does the vacuum and timing and it compounds the problem.
This is why you want the canister to start at the lower vacuum setting, and have it all done by 12 inches or so, just like the old performance cans used to do, so it's not as sensitive in and out of gear.
The 455 I built in the first video above idles up here at 5,000 feet elevation with 12 inches of vacuum. That's going to be about 15 inches at sea level. So with the vacuum advance setup like I described, the engine isn't even dropping enough vacuum in and out of gear to affect the vacuum advance, so you have a rock solid stable idle, even when I turn on the AC.