I'll keep this short and sweet. Vacuum advance... Does anyone have any personal preferences to a set-up? Vacuum source, point of initial travel and total vac advance. This is for a mild SBC 305, w/ cam, air gap manifold, and headers. I've read conflicting stuff so give me the straight skinny. Thanks!
Here is a site search on this Forum that has about 600 of the same questions, one should answer it the way you want plus a few good sites. There are sites/info in the Knowledge Base.
I know the same ole question gets old. Thanks for the effort. :thumbup:
I know the same ole question gets old. Thanks for the effort. :thumbup:
Taken from one of the postings, everything you should know about HEI ignition's, the timing is still the same set-up on advance.
manifold vac hook up, below carb
Why the manifold? This will potentially introduce advance too soon, wont it? or is their a trick to keep that from happening?
You want advance at idle.
This is quoted from one of the threads that comes up with a search.
So you disconnect your vacuum advance and set the initial timing, then with springs / weights set the total timing ?
When you reconnect the vacuum advance to a full manifold vacuum you immediately induce advance at idle, so there goes your initial advance up 10º or whatever the vacuum cannister is set at.
As you state "if your static timing is at 10º, at idle it´s actually around 25º with vacuum advance connected" as I see it with the vacuum advance connected it is not static and 25º at idle is way too much.This is why the vacuum advance is disconnected at idle to get a correct static timing reading.
I disagree with the statement of seeing 50º advance if you were going down the highway, (10º initial, 20º - 25º mechanical and 15º from the vacuum) you stated elsewhere when the mechanical advance kicks in the vacuum is not adding anything.
It is also important to get all the advance in before your cruising rpms.
Also if the total timing was set at 34º - 36º how can it get to 50º?
Remember total timing has been restricted to 34º - 36º and even more rpms won´t advance it anymore.
The vacuum advance is an aid to get from idle to mechanical advance and also operates during low engine load when the vacuum is high.
Setting vacuum advance is not measurable it has to be done on the road until pinging is eliminated.
Get yourself a book dedicated to ignitions, I´m going to throw out my computer controlled distributor and go with PORTED vacuum advance, try Ignition Systems by Todd Ryden.
BTW there are several sites in the knowledge base on this.
I have a small block in a 63 Nova with aftermarket intake ,carb,cam, headers etc. I use manifold Vac. and a stop plate to limit the amount of vacuum advance to 8*. I set my initial timing with advance unhooked at 12*. With the cam we have it idles a little rough and when you add the extra 8* degrees advance at idle it smooths out and is much more drivable. I have the mechanical advance set up to work right at wide open throttle such as when at the track. You just need to make sure to use an adjustable vacuum advance and have it set to provide the advance at idle and low rpm's but also not so low that it adds at your cruise rpm. This setup seems to work great for us. ccnova
So you are in fact running 20º initial timing.
Initial timing is set at idle, no advance, to then add vacuum advance ? seems odd.
There is no transition from no advance into the mechanical advance.
How does it react when you pull away from a standstill slowly?
How does it react when you stomp on it ?
GM uses ported vacuum for the advance system. So you need to plug the vacuum line from the distributor to the port above the throttle blades (no vacuum at idle). When the throttle is openned (off idle) the distributor will advance. Pretty simple.
Connecting the vacuum advance to the manifold makes no sense. If that is done then the timing will retard when the throttle is openned which is the opposite of what is needed. Plus the mechanical advance will be advancing at the same time the vacuum is retarding. What is the point of that?
Fords use the manifold pressure for their vacuum advance system. It this case, the high manifold pressure, holds the vacuum advance in retard. When the throttle blades open and manifold pressure drops then the distributor will advance. Different method but the same effect.
from my install instructions with adjustable advance
set the base timing to 8-10
as delivered mech is 24 @ 2800
as delivered vacuum is 14* @ 12 Hg (4 turns)(8 turns possible)
"If surging or pinging is noticed at CRUISE rpm's turn the vacuum adjustment conterclockwise"
If more advance is needed turn clockwise
from the chart table for 1,2,3, etc turns with manifold vacuum
1 turn=3 deg all in by 11Hg, starting at 8Hg
4* turn = 14 deg all in by 12Hg starting at 6Hg, (45deg slope)
8 turns = 28 deg all in at 17Hg but a long slope starting at 4Hg
The vacuum advance is off when you floor it, the Hg is below 6, timing advances only from the mech to the 34 total.
When you back off the gas, as the Hg builds, the vacuum advance works against the mechanical for a smooth transition back to base timing
Your balancing motor load (Hg) to mech advance at any rpms for best burn possible.
Reason I bought it (and tuned both mechanical and vacuum): I can burn 87 octane with 10deg base/36 total (in at 2400) with no ping because of the vacuum advance. It replaced a Mallory dual point mech only that wanted 93 octane at 12 and 32 total
So jealous, you Chevy guys can buy just the adjustable vacuum, I had to buy a distributor.....
I'm impressed with the responses, but the question remains.... Ported or Manifold.. I'm leaning with the manifold source because it leans toward a more dynamic timing curve that does reflect load and throttle position. I'm so confused:confused:
won't hurt anything to try both,
you will see a change, ported vacuum is "0" until ? 1500 rpm+, you have no vacuum advance at idle
so when drive away normal at a green light, it takes a little more peddle or excellerator pump (richer) to compensate for no advance
Why is it there? I don't know this to be THE reason but there are "dual" vacuum advances on some Ford dist, it's a push/pull deal, never used one but makes sense.
My car sucks on ported, runs great on manifold, 90% sure yours will also.
Gona buy that Demon carb, ask them also, direct connection, vacuum advance and carb rich/lean
You be the judge.
Try it both ways and see which works best.
I'm betting you will find ported is much better.
I still think it is simple, when you give it the gas of idle, you need advance. And the only way to get that with a chevy is to use a ported source.
A ported source will also advance some when going down the highway because the throttle is openned. That combined with some part of the mechanical advanvce makes a nice amount of timing for good MPG.
Let us know what happens.
I guess you have a ford by your web name.
Yes Fords need to use manifold vacuum source. On a ford, the vacuum advance retards during high vacuum (idle). The diaphagm pulls on a spring and retards the timing, but once the vacuum drops, the spring in the vacuum advance system is what actually advances the timing. So when you give it throttle and the manifold vacuum drops, the timing advances.
Chevys work opposite, the spring retards and the vacuum advances. That is why a ported source is used on a chevy. But in both ford and chevy, the timing advances when the throttle is depressed.
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