Vacuum Advance Canisters

[HEI451]

There are a number of stock GM HEI vacuum canisters available, over 15 or so. Selecting the RIGHT one is tough at best, and takes someone that KNOWS curves and settings to make it all work.

That said, you want the vacuum advance to work on FULL MANIFOLD VACUUM SOURCING, vacuum strong and present at IDLE, and NOT ported vacuum. This source should be at the BASE of the carb, NOT above the throttle plate.

It is also correct that all vacuum advances produce too much timing, IF the limiter bushing isn't installed on them. The limiter bushing is a rubber tube, mostly just rubber vacuum tubing, placed over the pull pin, to limit the pin travel, which reduces the over all degrees of vacuum advance crankshaft timing. This said, the "24" referred to on the canister IS the crankshaft amount of vacuum advance degrees that canister will produce, far too much for our needs.

A nice idle timing we would like to see os between 20 and 22 crankshaft degrees, but, if the initial is set there, the engine is usually too hard to start, and this is where CORRECT vacuum advance souring comes in. You state you now have 8 crankshaft degrees of initial timing, and the canister has 24 available, IF the stop busing is in place. IF the canister is on full manifold vacuum, as it should be, the vacuum advance will take the idle timing to 32 degrees, tar too many degrees. If the rubber limiter isn't there, as they degrade and fall off, never to be replaced when a new advance is mounted, then, the sky's the lkimit for idle timing.

Now, IF we set the initial to your 8 degrees, and make a simple sheet steel limiter and attach it to the vacuum advance mounting bar with two 6/32 machine screws, we can set the pin travel to .120 inch travel, for 12 crankshaft degrees, and make sure the canister is full manifold vacuum sourced, we now have an idle timing of 12 plus 8, 20 degrees, exactly what that engine wants.

We also have a situation where every stock canister has a different vacuum pull, along with different curve numbers, and this is why there are so many of them available, who knows what is what. The extremely simple solution to all this is the Crane 99600-1 adjustable vacuum advance kit, has 3 sets of mechanical advance springs, a pin limiter stop and a fully adjustable canister, giving EVERY stock GM setting, And everything in between them. The only modification is, the Crane stop plate needs to be mounted NOT where Crane says to put it, but on the OTHER side of the pin, towords the canister, NOT the end mounting screw. This is very easily done with one threaded hole and an 8/32 machine screw. The Crane method changed the vacuum pull spring strength with every degree adjustment, my modification separates the vacuum rate from the number of degrees selected, making vacuum advance tuning work, instead of compromising with every adjustment change.

If you are interested, I have a series of pictures and an outline on just how to correctly modify both the GM stock, and Crane advances to the correct way the stop plate should be, for both HEI and stock point distributors, along with the simple home made stops as well. There are even pictures on how to mount the Crane limiter plate to a stock GM vacuum advance, points or HEI.

If you want a picture set, there is no obligation, no ad, just simply e/mail me and ask for the one that fits your distributor, in this case an HEI, and I will send them to you.

Use this e/mail address, please. info@davessmallbodyheis.com and I will send same.

Also, I really don't advocate using ported vacuum, if there are those that still do use it, they have NO clue as to how a "load compensator" (vacuum advance) actually works in a non-EGR equipped internal combustion engine. The ONLY engines that require ported vacuum are those with a functioning EGR valve for EMISSIONS use, and there aren't many of them still running around these days.