Ported or manifold Vacuum to Dist.
Okay, I've always used ported vacuum to dizzy on all my Mopars. I want to go through my Dizzy on my 340 /4 spd. After boring 20 over , basically stock with the exception of Eddy heads. Then recheck my RB 500 with A/T next year. I was reading this in (How to Article) and there saying to run from manifold to Dizzy ? I know it would take a lot of re-curving , weights,canister,Timing ect. I just want to know the opinions of this. I guess I'm old school but I value my knowledgeable friend here on FAB. My Ragtop 340 in particular needs a better fine tuning for sure and I want to do this myself. They claim idling it will run cooler which could help my RB 500 in traffic. I'd like to use my canisters on both for better mileage when cruising. Here's the article.
Everything you wanted to know about ignition advance
It's wrong. And the AFR chart in the Eddy instructions is wrong too.
That doesn't mean using manifold vacuum is wrong. Use whichever works best with the mechanical curve and for the situation.
IF, the mechanical timing and advance can't be brought up to the where it ought to be at idle speed, using manifold can be used instead. Downside it may not be quite as consistant or stable as doing it mechanically. That's 'cause as load is added (placing it in gear) the manifold vac will drop.
"Where it ought to be"
The timing at idle should be set for highest efficiency. On a non-CAP/CAS vehicle this is all that matters. The engine produces little power at 600 - 700 rpm. So especially with an automatic transmission its important to fire the cylinder so maximum pressures occur when piston has the best leverage on the crank.
Early emissions equiped cars (Chrysler CAP then CAS) had to compromise idle efficiency to reduce HC and CO by purposely running a little leaner and hotter burn. Idle speed was turned up to offset the loss of power. At least for the first years, the reduced initial timing was offset by super quick mechanical advance.
Bottom line is there is no one size fits all.
However, the best practice is to use mechanical advance to compensate for the loss of time with rpm, and to use vacuum advance to compensate for leaner burns conditions. Contrary to what many beleive, the most efficient power at idle is a relatively rich burn. Concern about emissions meant that had to be compromised, and along with it timing. Only in the catalytic converter era did initial timing on some vehicles return somewhat to the pre-smog numbers.
