I recently decided to run my vaccum advance on my 300hp crate 360 magnum on ported vaccum instead of manifold vaccum. Because while running the vaccum advance off of manifold vaccum there was way too much advance at an idle. !4 degree initial plus another 10 degrees from the accum advance wich made it idle at 24 degrees and i could hear thudding from the exhaust. So i turned my initial timing up to 15 degrees wich makes my total timing land at 33 degrees and i set my vaccum advance to add 12 degrees of timing at 18 inches of vaccum. Does this sound about right? i dont know how much vaccum my motor produces at a steady cruise but at an idle it produces 18 degrees so i set the vaccum advance to add 12 degrees of timing at 18 inches of vaccum and plugged te vaccum hose on the ported (timed) vaccum source. It seems to be running real good, i just wanna make sure i am not running to much vaccum advance to cause any pinging, i couldnt hear any pinging, but i do have fairly loud exhaust....
I'm guessing since my total timing is at 33 degrees that with 12 degrees from the vaccum advance my engine is probly running at 45 orr 46 degrees total while at a steady cruise, that is if my engine produced 18 inches of vaccum while crusing down the road at 65 mph. It produces more vaccum like 20 inches or more then my vaccum advance may be adding around 14 or 15 degrees of timing wich make it around 48 or 50 degrees while crusing down the road at 65.
I have nevr tried tuning the vaccum advance for ported vaccum, i have always ran it off of manifold vaccum, but i got sick of not being able to get it to idle right on manifold vaccum and switched it to ported vaccum and whith it on ported vaccum and the initial timing being at 15 degrees i do not hear the thudding out the exhaust while at an idle wich is what i heard with the vaccum advance hooked to manifold vaccum. Oh and i run 91 octane premium fuel. If that makes a difference in how much timing i should have with the vaccum advance?
Please help me figur out i i'm in the ball park with my vaccum advance adding 12 degrees of timing at 18 inches of vaccum and with my initial timing being at 15 degrees and total timing being at 33 degrees. Thanks guys.

..compression and camshaft determine your initial timing.
50 degrees at cruise is not uncommon..just watch for detonation

Connecting vac advance to the manifold leads to very unstable idle, esp. w/ automatic.

50-plus degrees, as noted above, isn't a problem, it leads to increased fuel economy, and, more importantly, lots less fuel wash / ring and cyl. wall wear. All the stock OEM hi-po engines were in the 50-55 degree range. Sound crazy? remember that the second you tip into the throttle, the spark gets pulled way back.

Rick

I'd set the initial it wants with the vacuum gauge method then with that figure in hand shorten the slots to get 35 total (initial on ported/total with vac adv plugged). Then I'd mix/match springs staying just under the pinging point at WOT up thru the gears on your hottest/dryest (most likely to ping) day THEN plug in your vac adv & adjust its' curve (clockwise with a 3/32" allen wrench=faster) to come in as fast as possible staying just under the pinging point on your hottest/dryest day at PART throttle (high vacuum) such as a highway steady cruise at an RPM at or above where your mech adv is aqlready maxed out. When all good there take it out at varying speeds/loads & if any ping then reduce the vac adv curve (clockwise) EDIT meant to say counter clockwise

I have a custom curved distributer and I run 22-23* at idle, 36* total. Much stronger idle with more timing. The vacuum pod got thrown in the garbage. Too inconsistant and irratic...Less than 1/2mpg difference with or without it...

I ran my 340/727 GV combo with ported vac advance for years until I read the write up about manifold vacuum which got me interested in trying it. Now my 340 is mostly stock and cam is close to stock which keeps the can advanced at idle in drive. It runs much better with the vacuum can hooked to manifold vacuum with about 30* advance at idle and about 50* or so at cruise set up the way RR explained above.

I had tried manifold vaccum because it manifold vaccum for vaccum makes sense to me, but there is no right or wrong answer, it just depends on what your engine likes. My 360 magnum doesnt like anymore then 16 to 18 degrees of initial timing. right now my initial timing is sitting at 15 degrees and my total timing is at 33 degrees and i have my vaccum advance set to add 12 degrees of timing at 18 inches of vaccum.
So if i'm crusing down the highway at 65 and my vaccum is at least 18 inches or higher i'm runing at about 45 to 46 degrees.
I tried setting my initial timing at 14 degrees wich landed my total at 32 degrees and then tried running my vaccum advance off of manifold vaccum, i had the vaccum advance set to add 10 degrees of timing at 18 inches of vaccum wich is what my engine idles at. So with the initial of 14 and another 10 degrees from vaccum advance it set it at 24 degrees at an idle and my engine ran extremely well, but while at an idle every once awhile i could hear thuddin out the exhaust. And no matter how well i adjusted the idle air mixture screws it never changed. So i figured it was not likeing that much advance at an idle. but when it idles at 15 degrees of initial timing it dont make any noise out the exhaust.

so is it safe to say i am in the right ball park with my vaccum advance adding 12 degrees of timing at 18 inches of vaccum?
I have my initial and my total timing sitting exactly where i want them and i have my timing coming in exactly how i want it. I just wanna make sure i am at least in the right nieghborhood with the vaccum advance adding 12 degrees of timing at 18 inches of vaccum with the vaccum advance connected to ported or (timed) vaccum.

i have givin up on the idea of runnin the vaccum advance off of manifold vaccum, i'm stickin to ported vaccum for the vaccum advance for the time being.... what i need to know now is if i am in the right ballpark with having the vaccum advance set to add 12 degrees of timing at 18 inches of vaccum.
Remember i know where my initial is and where my total is and i know when my timing is all inn and i am happy with all of that. I just want to know if having the vaccum advance adding 12 degrees of timing at 18 inches of vaccum is in the right ball park on ported vaccum?

Are you talking crankshaft or distributor degrees? Remember, the number stamped on the canister arm is dist. degrees, double that for crank!

""If there is enough manifold vacuum at idle in D so that the can stays fully advanced (against its stop), won't that eliminate the instability?""

Yes!!!
Once you know your in D idle vacuum reading, then choose the correct can. I would suggest that you run a can that is 1+" below your existing vacuum reading in D.
I am currently running a distributor that spec's out the need for 12"hg to keep the can fully advanced. I have tested it with 11.5"hg and the idle starts to float. If I stay above the 11-3/4"hg reading then the idle is steady with the manifold vacuum. My setup makes 11-3/4-12"hg at idle in D, and that is too close to suggest to someone else. This is why I suggest that anyone tries a can 1+ less than their actual in D vacuum reading.

i'm talking distributer degrees. here is what i did, i put my vaccum advance on manifold vaccum and i used my dial back timing light to determine how much advance the vaccum advance is giving. At 18 inches of vaccum my vaccum advance is adding 12 degrees of timing because my initial timing is set at 14 degrees and with the vaccum advance plugged into manifold vaccum my timing light read 26 degrees at an idle. Well this is too much timing at idle for my engine, my engine does not like it. But i was doing it that way so i could determine what my vaccum advance is adding at 18 inches of vaccum wich is how much vaccum my engine produces at an idle.
I figured while at a steady highway cruise i may be producing 18 to 20 inches of vaccum, so i figured with the vaccum advance adding 12 degrees of timing at 18 inches of vaccum that i was in the right ball park, but i put my vaccum advance back on ported vaccum, because like i said before my engine does not like all that advance at an idle, she idles crisp and smooth at 14 degrees of initial timing.
so i will ask again, am i in the right ball park with my vaccum advance adding 12 degrees of timing at 18 inches of vaccum? remember my vaccum advance is on ported vaccum, i was just using it on manifold vaccum so i could determine what my vaccum advance was adding and at how many inches of vaccum.
So with my initial timing at 14 degrees and my total timing at 32 degrees and with my vaccum advance adding 12 degrees at 18 inches of vaccum i gotta figure t a steady cruise my engine is seeing at least 44 degrees of timing maybe even 46 to 48 degrees if my engine produces 20 or more inches of vaccum while crusing down the highwaay at 65 mph. Am i in the right ball park? also remember i am running 91 octane premium fuel, if that makes a difference at all.

i think i will leave it the way it is right now.
14 degrees of initial timing, 32 degrees total timing with the vaccum advance set to add 12 degrees of timing at 18 inches of vaccum and on ported vaccum. She seems to be runnin pretty good.
even tho i would rather run my vaccum advance off of manifold vaccum i just can not get it to run right that way, no matter what i do its like there is a miss somewhere when i have it on manifold vaccum, (thuddin out the exhaust) while at an idle with vaccum advance to manifold vaccum.

Sounds like you're pretty good. If you wanted to experiment with manifold you could reduce the initial then lengthen the slots to maintain 32 total (or even a bit more) EDIT I would drill a 3/8 hole in the top of a junk dist cap 2/3 of the way between center & #1 & check rotor phasing as vac adv alters phasing & you would not want to reduce timing IF rotor phasing (which can be corrected) is the culprit

my distributer was curved and phased on a distributer machine. don from fbo systems built my distributer and according to him it is phased and curved correctly. I told him what my engine is wich is the 300hp crate 360 magnum told him what it is in and what my gear ratio is and what size tire i run and he curved it based on all the information and phased it.

I tried to run my jeep engine off manifold vacuum. It over advanced it at idle so much it killed the engine! Had to disconnect it because my carb didn't have a ported source.