I didnt see that he actually verified the 14 at the can. It was my understanding that he was just quoting a spec, so I was guessing that he could have a generic 14 vac advance can, but was never pulling enough to actually achieve it.
That's what was happening to me, and why everyone recommended the can that came all in at 9hg.
If he verified that, then I missed it.
From my reading, rule of thumb is: "Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable."
"GM made an assortment of vacuum cans based on the engine, cam, and engine's vacuum. Grandma's Pontiac would not have the same vacuum can as an L88 BB Chevy, but you can interchange them. These canisters have different vacuum ratings. One might have a vacuum advance that begins to pull at 3"-5" of vacuum and only gives you 8 degrees maximum advance on a big cam engine pulling 10" of manifold vacuum while another might have the vacuum advance begin to pull at 6"-8" of vacuum, gives 14-16 degrees of maximum advance at a high of 24" of manifold vacuum. So it is possible to find a vacuum canister that will work if you are willing to experiment."
Points type distributor vacuum cans are different from the factory HEI distributor vacuum cans - they do not interchange, so make sure you get the can for your distributor type.
These are for points type distributors, but there are many more factory cans:
The NAPA/Echlin VC1810/B28 specs - 0 @ 4Hg", 16 degrees
[email protected] 8Hg" of vacuum.
The NAPA Echlin VC680/B1 specs - 0 @ 8Hg", 16 degrees @ 16Hg".
The NAPA Echlin VC1765/B20 specs - 0 @ 6Hg" and 16 degrees @ 12Hg".
(FYI: Check RockAuto for vacuum cans as I got mine there and it was cheaper)
If your engine exhibits a momentary detonation when transitioning from cruise to WOT this slightly less aggressive cannister (B20) might lessen or eliminate it while still providing full vacuum advance at idle.
Some of these cans can provide too much advance and can be limited by using a stop or an adjustable stop. You can fab your own or purchase one of the aftermarket versions.
From another race website:
"If the engine has too much mechanical advance, then you need to limit the amount of advance inside the distributor and give it more “initial” timing on the crank to achieve the 34-36 degrees of desired total. I usually like to see about 10 – 14 degrees in the distributor and the rest on the crank in most performance engines.
Most stock distributors are set to get full mechanical advance at about 4,000 RPM. You want all of your advance in at about 2,400 RPM. 3,000 is too high, 2,000 “might be” a bit low, so somewhere in there is where you want to be. Not having full advance at or below 3,000 RPM is pretty useless considering an engine needs its advance to run at its best and to get that car moving. Stock, or un-curved, distributors that don’t see full advance until 4,000 – 4,500 RPM are a bit late in most cars and WILL cause your car to not move off the line or accelerate very well below it’s full advance RPM. The trick is to know how much advance you have, and WHEN it comes in. If it comes-in too late, then you need to change the springs and maybe even the weights to make it come-in sooner. If it has too much advance, then you need to limit the amount of advance inside the distributor and give it more “initial” timing on the crank."
This may hold up as a rule of thumb for most engines, but keep in mind that you may also have to adjust your timing and timing curve to prevent detonation. Increasing octane is the way to go to prevent detonation, and retarding the timing to prevent detonation may not be the answer as you will lose power and the engine will run hot. The better route, as often noted on the forum, is to lower the compression ratio and then you can maximize your timing and timing curve for that lowered compression and be able to run on lower octane pump gas with very little loss of HP as compared to the higher compression when run with the correct octane to run high compression. Retard the high compression engine because it "pings" and you lose power and it is no longer maximized for HP - so the drop in HP may be what you will have with a low compression engine with its maximized timing/timing curve running on pump gas.
