Installed an hei dizzy with a vacuum advance and was wondering if the vacuum advance should be hooked up on the bottom of the carb or on the intake. Been reading up on this and getting mixed opinions. Also, how do I get the correct settings on the vacuum advance. I know you can adjust it by inserting an allen wrench into the vacuum canister, but how do i fine tune it?

 

My HEI dizzy likes the timed (side) vacuum port . I tried both

 

oh, man. here we go!

'mixed opinions' is probably the most definitive you'll ever get.
try both, and do whatever works best for you.

 

Stock configuration is ported vacuum. Where you go from there is up to you.

 

Ported and Manifold Vacuum

Depends on the timing setup...

You have to factor in initial, mechanical and vacuum timing. Your initial and mechanical advance add together to give the total timing of the engine at a specific RPM, which can also be adjusted but is typical around 3,500 rpm. FE engines usually like around 38 to 40 degrees of total timing, stock. Aftermarket changes to the engine can change the total timing requirements.

From what I have experienced/tested, ported and manifold vacuum are the same except at idle. At idle, ported vacuum goes to 0 inHg, which means the vacuum advance arm is not pulling on the breaker plate, advancing the timing. Manifold vacuum at idle can pull 15 to 20 inHg or more, depending on your cam and other mods. That means at idle, manifold vacuum will pull the arm on the vacuum advance through its full range of travel, adding 15 to 20 or more degrees of advance at idle.

Lets say you have:
6 degrees initial
36 degrees mechanical (18L)
Up to 20 degrees of vacuum advance

Timing degrees, using Ported vacuum:
Idle = 6
Part throttle with 15 inHg of vacuum = 26 up to 62
WOT = up to 42 (6+36, vac. adv. should drop out)

Timing degrees, using Manifold vacuum:
Idle = 26 (6 + 20 vac. adv.)
Part throttle with 15 inHg of vacuum = 26 up to 62 (same)
WOT = up to 42 (6+36, vac. adv. should drop out) (same)

26 degrees of advance at idle is not needed for a normal street engine.


Now lets say you have this setup:
16 degrees initial
26 degrees mechanical (13L)
Up to 20 degrees of vacuum advance

Timing degrees, using Ported vacuum:
Idle = 16
Part throttle with 15 inHg of vacuum = 36 up to 62
WOT = up to 42 (16+26, vac. adv. should drop out)

Timing degrees, using Manifold vacuum:
Idle = 36 (16 + 20)
Part throttle with 15 inHg of vacuum = 36 up to 62
WOT = up to 42 (16+26, vac. adv. should drop out)

In the second setup the total timing is the same as the first but initial timing has been raised from 6 to 16 degrees. This extra initial timing is good at producing more torque and HP through the RPM band. But when you hook this setup to Manifold vacuum, you pull too much timing at idle and may cause the engine to stall out, knock or even kick back.

Based on my experience, to run vacuum advance on manifold vacuum, you would need to either have a vacuum advance that has a shorter arm pull(less timing advance) or lower the initial timing of the engine so that you don't pull too much timing at idle. Less timing available on the vacuum advance probably won't hurt performance but could effect economy.

Low initial timing does lower torque and HP output. Raising initial timing will increase them but only up to a certain point, after that they will fall again. Lowering the total mechanical advance will allow you to add more initial timing without changing the total timing.

The vacuum advance is there to add timing under part throttle and light load conditions. Vacuum is cheap way to approximate load on carburetored engines.

Total timing is the key for max engine output. Too much or too little can lower output. You'll have to test and see how much total timing you engine likes.

On my mostly stock 390 FE I am currently running: 16 initial, 24 mechanical, and up to 22 on the vacuum advance which is hooked up to ported vacuum. It makes more power than it did with the factory setup of 6 initial and 36 mechanical. Also, setup the vacuum advance to not add timing from 0 to 7 inHg of vacuum, this helps insure hard acceleration and WOT runs don't have too much extra timing.

I have run across two different types of vacuum advance canisters. Both have an adjustable set screw in the hole for the hose but they don't react the same to adjustments.

The first type increases the internal spring resistance making the arm of the vacuum advance harder to move, but the total travel never changes. Even with the spring at full resistance. As the spring gets tighter is will take more and more vacuum before the arm will start to move.

The second type, the set screw acts like a stop. The more you screw it in the less the arm can travel. When the set screw is all the way in it will stop the arm from moving at all, which is the same as not having the vacuum advance hooked up. But the spring resistance stays the same only the travel distance changes. So the arm will advance at the same rate regardless of the screw position unless all travel has been locked out.

Disclaimer:
I don't fully understand all the dynamics going on, just stating my experiences.
Nor am I'm not going to say one is better than the other.

 

Good read Old Brown F250 ! My timing is set at 10 degrees with the vacuum unplugged at the dizzy. I have a mild cam with a 600 box holley on an edelbrock open bore intake. I NEVER race the truck, just want it to have good throttle response and good fuel economy. Its only a cruiser.


Mike, did you adjust the vacuum can any when you installed your HEI or did you just keep it where it was when you got it?

 

I didn't adjust dizzy vacuum advance at all . straight out of the box .

 

vacuum advance is only in play when the engine is producing a lot of vacuum- like at cruise (and idle).

to test it- maintain a steady speed, then slightly increase throttle.

to fine-tune it, increase the vacuum advance until it begins to ping at partial throttle- then back it off until the ping disappears.

that's the only method i know of.

 

Is your engine stock or does it have a cam installed, etc? Mine acted lazy so I ended up turning it about 4 times, but I think that increased the timing 8 degrees so I dont think I needed to do that! I may have to gety the truck back out this weekend and just go through this thread and the other hesitation thread I posted and see just what is going on.

 

4 turns sounds to much , you should be tweaking it 1/4 turn or so testing then re adjust if needed . i'd go back to square one .
does your exhaust smell like raw gas ?

 

Yeah. My exhaust is pretty strong. I need to go back to square one this weekend and recheck some things.

 

i'd check your float adjustment first . to high and raw gas will be flooding the engine causing all kind of problems . (idle problems )
you tube has videos on how to adjust floats correctly

 

Ended up using the ported vacuum from the carb. I also read that you can adjust the vacuum advance cannister all the way in until it stops, then back it out 1 full turn and adjust from there........that seemed to work from me. Wide open throttle and she takes off, so I guess I am leaving it there. Now I posted a thread about my modular valve so you guys please chim in and let me know how to tackle this problem!