i want to know if anyone can tell me were the vac advance really needs to be? some people say ported and othes say full vaccum. well i have a 305 with a performer plus cam performer intake with egr and stock hei.
with full vac i can set the initial to about 6 deg. runs ok not much power tho. then i tried to set it at 10 - 12 deg. and used a ported vac and ran much better as far as response alittle more power. the thing that confuses me is i see that some say dont and some say do..... well the next step i am going to try going is to upgrade hei advance cure and i found a kit from crane cams with lockout and vac advance adjuster so before i go any further i just wanted to get some advice from any of you.

i havent installed the cure or advacne adjuster yet thinking of doing so to increase performance . ed

i don't put much stock in setting inital advance other than to get the engine running. i'd shoot for just under 40 degrees total advance above 2500 rpm. crane advises using manifold vacuum for the distributor but i find ported works better for total advance. at high rpm the manifold vacuum drops but not ported. my 66 283 with an HEI runs best at 50 degrees total, doesn't ping and starts very quickly when cold.

I had the same problem, i had mine on full vacuum and didn't get the performance either, i switched back to ported and better response, etc... i guess thats why edelbrock says to use the ported side.....lol

I just finished hooking up my 1806 andI used the timed vacuum side. Haven't started the engine yet as Bingo is off to work. Will post more information if it adds any value.

Use the ported vacuum port on the carb, that is why it is there. Initial advance should be somewhere around 10-14 degrees, the more the better as long as you can start the engine without starter drag. The total advance should be checked with the vacuum disconnected and plugged; shoot for about 38 degrees at 2800rpms (ie. 12 degrees initial and 26 degrees mechanical). Reconnect the vacuum advance and it will give you another 10-14 degrees advance, when the engine can use it, small throttle opening/high vacuum conditions. This is optimum timing for most smallblock Chevy engines although tinkering around could make improvements.

thanks alot guys, good info. i will give it a shot this weekend and see what happens. have a happy thanksgiving. ill let you all know the results thanks

Ok you guys have me wondering. I have an edelbrock carb (1405) Is ported on the left or right? My vacuum advance is hooked up on the right with 12 degrees initial timing. Throttle respose is very good. Should I hook up to the left side to see what happens?

How can I check to see if I have all the timing in at 2800 rpms? I do not have an advance timing light only an inductive.

Brian

you could put a timing tape on your balancer to check the total advance or you can check your advance at idle then rev engine and guestimate how far it is advancing by observing the movement of the mark. you can get an idea of how far by comparing the distance from 0 on your timing tab to 10 degrees advance and how far the mark is moving from idle to higher rpm.

you could put a timing tape on your balancer to check the total advance or you can check your advance at idle then rev engine and guestimate how far it is advancing by observing the movement of the mark. you can get an idea of how far by comparing the distance from 0 on your timing tab to 10 degrees advance and how far the mark is moving from idle to higher rpm.

I know this thread is basically old/dead... but for future readers there are some misconceptions that need to be corrected.

First contrary to a prior posting in this thread, regarding manifold versus ported vacuum, there is NO difference except at idle. Both follow the same load/throttle responses until you completely close the throttle. In fact a fast idle when cold can be enough to make ported vacuum active all the time just like manifold vacuum.

Note that there may be a slight difference in vacuum tapped off of the carburator (ported or manifold) versus tapped off of the manifold, the manifold tap can see a constant 1-2" more vacuum across the board.

Next regarding whether to use ported or manifold to drive vacuum advance, both can work. However the physical properties of engine combustion at idle suggest and support using manifold vacuum. This is especially true with higher duration/lift camshafts. In many combinations the engine will just not run right without vacuum advance at idle. Adding vacuum advance at idle will also reduce the starter load as startup will have zero vacuum and thus less advanced ignition timing.

I recommend doing a search for the "ignition 101" thread for a deeper understanding of how this all works and thus an idea of how you should set it up for your engine combination.

There is a BIG difference between manifold vacumn and ported vacumn. Manifold vacumn is under the throttle plates so with the throttle closed the vacumn will be at it's highest and as the throttle opens the vacumn declines till it is a 0psi or near 0psi at WOT. Ported vacumn is just the opposite because it is ABOVE the throttle plates. It is easy enough to verify this with a vacumn guage.

There is a BIG difference between manifold vacumn and ported vacumn. Manifold vacumn is under the throttle plates so with the throttle closed the vacumn will be at it's highest and as the throttle opens the vacumn declines till it is a 0psi or near 0psi at WOT. Ported vacumn is just the opposite because it is ABOVE the throttle plates. It is easy enough to verify this with a vacumn guage.
BS

Obviously ported vacuum is not the opposite or it would be at it's highest at WOT.

The port for ported vacuum is, as you say, above the throttle blades. *Just* above. As soon as you open the throttle the plates move above the port. You can verify this by looking into your carb at the location of the vacuum ports and throttle plates. Back when my ignition setup was all screwed up I used to tweak the idle speed stop screw until the ported vacuum was just closed. So it is possible to have ported vacuum active with nothing more than a fast idle.

Interestingly enough the idle speed jumped significantly when the ported vacuum was turned on. I just did not know at the time what/why and how significant and indicative that was.

I would suggest you hook up 2 vacuum guages and go drive around to see what happens. Also consider that you need/want vacuum during cruise and the throttle blades are actually not open very much during cruise. So worst case the ported vacuum must be activated somewhere between idle and cruise.