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Discussion Starter · #1 ·
I have read numerous posts and "ignition 101" here on TC and have talked to old and young mechanics. I have a '69 396/375 with flat top pistons and a mild comp cam. The question: Is it better to hook up the vacuum advance to a venturi vacuum or manifold vacuum, and why?
 

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it's not necessarily better either way. try it one way and then the other, see which it likes better. The base timing is the same for both.
 

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Well I'm a proponent of manifold vacuum. But you still have to set up the dizzy either way to maximize your ignition curve. You can't just pick your vacuum source whether ported or manifold vac, and plug into the vac advance unit and call it done, there's more to it than that. (My bad ported not venturi)
 

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Is it better to hook up the vacuum advance to a venturi vacuum or manifold vacuum, and why?
Where are you tapping into venturi vacuum? The only venturi vacuum source I'm aware of is the secondary vacuum supply on the Holley-style carbs--and there's no vacuum nipple to attach a hose to.

Ported (sometimes called "timed") vacuum isn't the same as venturi vacuum. Ported vacuum is just manifold vacuum that is shut off when the throttle closes enough.

Try both ported and manifold vacuum; your engine will tell you which it prefers.
 

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Discussion Starter · #5 ·
Maybe I am using a "ported" vacuum and not a venturi, I only know that it has zero inches of vacuum at idle and increase as the rpms do. The guy who recurved my distributor indicated that I would have 1* of advance at 8" and 14* at 12" from the vac adv. What I am trying to accomplish is get a little more out of the lower end at normal accelleration, not WOT. Right now it is kind of sluggish on the lower end and I am told to get more advance in early and it will help. By saying the engine will tell me if it likes venturi or manifold, do you mean I will feel it upon accelleration or just the way the engine runs. I feel like I am rambling, looking for any help here.
 

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yours should go to the port with no vacuum at idle. That is the way it was designed.
Later model engines use full vacuum at idle, so that when you accelerate, timing retards and you don't get spark knock(detonation)
 

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Discussion Starter · #10 ·
I'm thinking I want more low end advance so if I put it to the port, wouldn't I gain advance upon normal accelleration, or do I just give it more initial advance?
 

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Jon, what is your initial timing (vacuum advance disconnected & plugged) and what is your total timing (initial+mechanical)? A good rule of thumb is 16-18* initial at lowest rpm you can achieve, and 36-38* total, all in at 2800-3000 rpm. Vacuum advance at high speed cruise would add another 12-14* to arrive at 50-52* total (initial+mechanical+vacuum). I know this sounds like a lot of advance, but vacuum advance decreases as you load the engine and/or open the throttle.
 

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Discussion Starter · #12 · (Edited)
Randy, right now I have it set at 17* initial, and get 10*@1500, 15* @2500 and 22* @330 mechanical. It wants more advance. I have to check for a true TDC tomorrow, If this damn snow will quit, I am questioning my TDC as it has an aftermarket timing cover and tab. Since I am only getting 15* mechanical at 2500, should I then have maybe 20-24* initial to give me 36-38 total?
 

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Jon, it would probably help to get all your mechanical in at a lower rpm, more like 2800. Maybe try a little lighter springs for the weights. For the street, I've always liked vacuum advance hooked to manifold vacuum, it idles better and runs cooler in stop/go traffic. After doing this, turn your attention to the carb to make sure you're getting a good pump shot when you're standing on it. Carb linkage fully opening the secondaries at WOT? If you're running rich, it'll be sluggish at low rpm. Just some thoughts.
 

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yours should go to the port with no vacuum at idle. That is the way it was designed.
Later model engines use full vacuum at idle, so that when you accelerate, timing retards and you don't get spark knock(detonation)

John Hinckley's data in "Timing 101" contradicts this and after switching to full manifold vacuum and changing to the appropriate VA can I found excellent results. And so have many others who took his advice.

"Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone."
 

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Discussion Starter · #16 ·
If I want to have all my advance, say 36*, in by say 2500, does it matter if I:
(1)increase the initial to 21* and with 15* mechanical get 36* total or
(2) set initial at 16* and lighten the springs to get 20* mechanical for a total of 36*?
 

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If I want to have all my advance, say 36*, in by say 2500, does it matter if I:
(1)increase the initial to 21* and with 15* mechanical get 36* total or
(2) set initial at 16* and lighten the springs to get 20* mechanical for a total of 36*?
Jon, your total mechanical advance is controlled by a stop pin in the distributor, the rpm at which it reaches the mechanical stop is controlled by the springs. You really should install a timing tape on your balancer and check to see what your total mechanical is while someone opens the throttle until the timing no longer advances. Subtract your initial timing from this number to determine mechanical advance. Again, you're shooting for 36-38* total advance (with vacuum advance disabled) at 2800-3000 rpm.
 

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Scott, was re referring to running a manifold advance vs no vac adv hooked up at all??
No, John is stating the advantages of manifold over ported. And definately the advantages of vacuum advance period. It's really all in his article and there is a long and short version. Lars Grimsrud also has good advice and he and John concur on most points. No vacuum at all is something else John addresses. Completely different approach to street vs. track and it's clear in his article.
 
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