Low vacuum - should I ditch my vacuum advance distributor?
I've kind of always understood vacuum as an economy thing. And when I hear leave horsepower on the table I'm not thinking of economy nor the cam you described.
Just recently I purchased a summit distributor that was plug and play with the electronic ignition in it for $325. It likely is the same thing as the Edelbrock one for $450. Anyways it gives like the MSD distributor I have in my duster the option to change the mechanical curve whether it be the springs or the amount of timing. Also it has a vacuum advance that's adjustable and can be taken off as well which I did because I don't usually build economy cars.
Also with theforementioned distributor it needs a particular coil that has less than .500 amps... So beware that that may be another $75 investment....
This is just what I did others may have a million different ways but I'm just throwing my hat in and my two cents for another way...
Vacuum advance has a lot more effect than just economy. Throttle response, part throttle cruise, gear changes, idle quality and the way a stick car behaves at low rpm are all affected by vacuum (load based) advance. I use my efi to control the timing on my dart, and it's an entirely different car when it's programmed with no additional advance based on MAP reading. With the vacuum advance programmed in, it runs better and has a ton more part-throttle torque and runs through the revs so much faster that it's silly. After a year of experimenting with the programmable ignition map I wouldn't run without some vacuum advance in the mix. I'm able to crank at 15 degrees, idle at 28, and then keep my initial at 24 with an all-in of 34, and cruise with 50 degrees of advance. I don't care what the MPG is, it just plain drives better and pulls harder any time the pedal is pressed. The driving experience is just better all over.
The hard part about doing it with most vacuum pods is making them work with hopped-up parts. Low manifold vac at idle and reduced vac at cruise make OEM style pods behave strangely. Excessive vacuum advance can also be a pain and requires limiting how much can be pulled in over the mechanical. But once it's dialed in, it's amazing. Keeps the pops and farts to a minimum when granny shifting, keeps the plugs clean and happy, keeps the engine cool at idle without having to run a 12 gallon cooling system (yes that's an exaggeration).
The common wisdom I've run across lately is that the vacuum canister should pull completely in at 2" lower vacuum than the engine produces at idle. This helps prevent the vac can from fluctuating at idle and causing instability issues or a 'hanging' idle situation, which makes sense to me.
Seems like most cans available these days operate at much higher vacuum readings (beginning at 8 and all-in by 16-18"). The MSD ones are the worst offenders. The adjustable canisters on most cheap distributors (which used chevy points-style cans) seem to be able to be adjusted down to an all-in vacuum of about 10-12", which is usually just enough to still be funky unless you like 1,000+ rpm idle speeds. The low vacuum 'B28' cans (operates from ~3" to ~8" of vac) for those cheap distributors are no longer available anywhere, and the Crane adjustables which could match them are also long gone :(
There's an excellent article done by
@Mattax Here.
