[Formulajones]

Originally Posted by 68 Firebird

Thanks Shiny,
cleaning out old stash of used parts, & I came across this piece.
Curious to see if adding more vacuum advance would have any benefits.
The Vac Can in my HEI currently adds 10 and works well. Did not want to modify it,
just wanted to try another with lower start point.
Always tinkering!!
Gerry

If your distributor is dialed in as it should be with a decent amount of mechanical advance that works with your engine (34 degrees for example) then adding another 20 degrees of vacuum advance on top of that is generally not a good idea, and most engines will not like it.

The additional 10 degrees you have now with your current vacuum advance is about where you would need to be for best results that generally works well on most any engine.

However, if you need a lower starting point, and if this new can offers that, you can simply limit it's ability to add 20 degrees by welding the slot. You would want to limit it's travel to only give an additional 10-12 degrees of advance at the crank, like your current can, and then enjoy it's earlier starting point to see if that offers any benefits for you.

That would be the proper way of doing it so you don't risk engine damage.