I need to learn about vacuum advance cans.
I thought I knew that the ones that have the hex shape coming off the disc are adjustable with a 3/32 allen and the adjustment is what level of vacuum begins to trigger the advance.

When I look for specifications on a particular adjustable can, all they list is a vacuum number - like 6 Hg.
For instance, a Standard VC208 is 6 Hg.

What are we adjusting when we use the 3/32 wrench?

Aftermarket Adjustable Cans from ACCEL or CRANE:
What is adjustable on these?
Are there two adjustments, one for its sensitivity to vacuum signal and one for the amount of advance?

Can someone teach me about vacuum advance cans?

When you turn the adjustment, you determine how soon a rising vacuum starts to advance the timing. You can see it happen if you put a vacuum pump on the distributor canister while you watch the timing with a light. As you pump out the air and create the vacuum you can watch the needle on the pump, and see precisely when the timing starts to move around on the harmonic balancer. When you turn the hex wrench, you change the start point for this. Someone else will have to fill you in on which direction changes it how. I always have to go look that up. Doctor Dodge, or Duster Idiot will know.

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69 Dart Swinger, 230 ci slant, t-5 five speed, intercooled turbo, 4 wheel disk brakes, tubular upper control arms.

Right. That's what I said in the underlined phrase.

But why is the only specification available in the books the parts guys have is some vacuum number?

I've dug pretty deep to find a can in the range between 9.5 and 11, and the books and online resources only give XX Hg, not XX degrees.

Since "how soon a rising vacuum starts to advance the timing" is adjustable with the allen wrench, why is this the number given in the books instead of the maximum vacuum advance number?

btw, turning counterclockwise makes it require more vacuum to move the diaphram.

when you harden or soften the spring you're actually changing the HG rate for that diaphragm. I.E. I have a very particular curve that requires my spring to be hard, cuz due to hig advance #'s at low RPM and advanced timing it tends to ping if I rev it soft to certain point. Meaning: If I stump on it hard, it will rev high no ping, but if I gently drive from 0 to 65, most likely it wil ping a li'l if I try to go from 65 on to 80 accelerating light. My vac reading at 70 is 12... so if the spring ain't hard enough the vac can comes in.

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Juan Ignacio Caino

Please use e-mail button istead of PM'ing. I do log in sometimes but I'll be answering quicker thru e-mail.

There are two cans with 8.5 stamped on their arms.
The book rates one at 6 Hg and the other a 10.5 Hg.
Is there a rule to how to figure the vacuum range from this number?
Hypothetically, if it were plus or minus 50% of that number...
The range for the 6 Hg can would be - 3 to 9 Hg.
The 10.5 can's range would be 5.25 to 15.75 Hg.

The can rated at 6Hg probably would cause pinging even if it were tightened all the way ccw.

The vacuum adjustment can go back and forth as many times as you want. The number stamped on the actuating arm is the number of degrees advance in the distributor. Double this to get the advance at the crank. You can make this more, but not less, and you can not go back once you have made it more. You can increase the length of the narrow section on the actuating arm by removing metal from the arm to lengthen the slot. I used a dremel tool with a cut-off disk in it. It is a rather imprecise job, and if you go too far, you can;t undo it.

Higher advance at cruise can increase your gas mileage. This is advance that goes away as you increase the load on the engine, and the vacuum drops. If you want more advance in the timing at cruise, you can get it this way without changing initial, or centrifigal. Getting the cruise advance with the vacuum canister, allows you to dial it out at light throttle with the hex wrench if you end up with preignition at light throttle. If you don;t feel up to modifying a canister, then look for one with higher numbers on it.

As far as detemining which is best for you, trial and error is the only way I know to find out, since so many variables wil effect when your engine will see preignition. In general, bigger cams can stand more advance as the bigger cams do not build cylindar pressure as much a mild cam. Also, engine temperature, and air/fuel mixtures effect when pre-ignition will happen. A leaner mixture will burn hotter, and tend to have preignition sooner. With a rich mixture and a big cam you can let the vacuum advance hang in there under more throttle with out pre-ignition. This would mean the number for inches of Hg could be lower for that combination than for a lean curb on a mild cam.

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69 Dart Swinger, 230 ci slant, t-5 five speed, intercooled turbo, 4 wheel disk brakes, tubular upper control arms.

I have taken timing out of the vacuum pod by wrapping a thin gauge wire into the slot, the wire takes-up some of the slot length and limits the travel.
DD

I've posted the specs of the various vacuum cans available from Standard-BlueStreak for the \6 distributor. As noted in that thread, I haven't tried using a smallblock V8 advance can on a \6 distributor. Seems to me it may fit and work (wasn't the Crane adjustable advance unit applicable for small block and \6?), and if so that expands the options further.

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Too many people who were born on third base actually believe they've hit a triple.

Question: I am using a points type advance can, stamped 8.5 in my electronic distributer now (a '73 unit, came off the engine I'm running now). I found a proper electronic advance can found in a wrecking yard, also stamped 8.5 on the arm. Will the different mounting holes in the advance plate make a difference in how far the distributer will advance, or is it the same, or is it the same no matter what can is used?