for the guys running vac advance on their street engines...what's your build and what is your total advance at what would be your cruising speed?(2500 rpm?)
mopar says about 50 total for a smallblock but if your build is more efficient would you expect it to be lower?...45?

Touchy subject on here but I'll stick my head out mainly because I'm very proud of my trucks last highway trip.

Bone stock 5.9L Magnum from a injected van. In an 87 reg cab shortbox pick up. With a 5 speed manual cruise rpm is 2200. 115km/hr or 70mph. 670 Street Avenger, dual plane M1, short headers and single ex.
I run mine on manifold vacuum. Straight highway will make no difference on ported since they both pull vacuum at cruising speed.

288km on 32L. That's 11.1L/100km.
For our neighbours who don't know what a Roberston screwdriver is that's 24.3mpg.

My total timing is 30 and it comes in very early. I'm probaby cruising in total. My vacuum advance adds over 20 so it's probably over 50 anytime my foot is not in the carpet. Which by the way is not easy for me.

Cant ans your Q directly but I'd set the initial/total/springs near spot on (IN ORDER) then hookup the vac can & get it (amt/slope) spot on. You want to work w it (1)at a RPM high enough to where the springs are already maxed out (total all in) and (2) at a steady part throttle (high vacuum) to where the cans' adv amt it can add is maxed out (3) shoot for (A) the most adv the eng will take (file the arm at the notch/limit the arms travel whichever is needed) w (B) the slope starting as soon (in hg) as possible (CW with a 3/32" allen wrench) staying just under the pinging point on (4) your hottest/driest day. EDIT this'll tailor it for what YOUR eng needs/likes

..tx guys.
i have 2 near identical 340's,the difference ls only the cam duration.
they are 15/35/53 tot(238/241 duration)
and 12/36/43 tot(224/230 duration)beyond those totals i get detonation on 91 octane.
i was thinking the short duration has better cylinder filling at low rpm and is more efficient.
just wondering what others have found

I "dialed in" my vacuum advance "can" at probably 1/2 the travel. I used a 3/32 allen wrench.

I run 35 degrees advance at 3500 rpm (small block) . The vacuum gauge I have installed shows good numbers at cruise - over 20 degrees mercury.

My idle is kind of choppy at idle (MP 280 .474 lift cam). Tach shows hot idle at 1500, but other tachs show less RPM.
My tach is not accurate (probably shows 500+ rpm more than actual).

I would think 50 degrees at cruise is a good number to aim for. More advance = better mileage. WOT performance won't be hurt. Good luck.