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Discussion Starter #1
I know there are a ton of timing questions and posts, and I had one a couple of weeks ago, but I'm at a loss. Here's the situation:

289 standard bore
Keith black 9.6:1 hypers
Air gap RPM I stake
AFR 165's
MSD Ready to run dizzy
Long tubes
Holley 670 Street Avenger


Set initial at 18 degrees without vacuum. Advances to 36 degrees.

When I hook vacuum advance into the correct port on the carb , I'm idling at 28 and advancing to 42.9. Seems a little much and I'm concerned with burning a hole in my new Pistons.

The dizzy has the factory stop and I changed the springs to the heavy, silver springs, to get the advance down.

Is this too much total timing with vacuum? Im not happy with the vacuum blocked as the car isn't as "Chrisp" as it is with the vacuum hooked up.

I'm all ears! Really want to get this resolved.

Thanks in advance for your help
 

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I like vacuum advance if setup to run off manifold vacuum since it makes the engine smoother at light throttle. When setup this way it loses vacuum advance at full throttle and your only using your mechanical advance. Some people recommend ported vacuum which is just the opposite of manifold vacuum which in my opinion is just wrong because your total advance would be too high at full throttle.
 

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Discussion Starter #3
Maybe that's my issue. I'm pulling vacuum off of a port on the carb that is supposedly dedicated for vac advance
 

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I like vacuum advance if setup to run off manifold vacuum since it makes the engine smoother at light throttle. When setup this way it loses vacuum advance at full throttle and your only using your mechanical advance. Some people recommend ported vacuum which is just the opposite of manifold vacuum which in my opinion is just wrong because your total advance would be too high at full throttle.

Either I'm losing my mind or you may be confused. Ported vacuum and manifold vacuum should be the exact same at ALL conditions except for 1: when the throttle blades are closed. Any time the throttle blades are open they should be the exact same. At wide open throttle there will be little to no vacuum since the throttle is wide open regardless of ported or manifold. Someone correct me if I'm wrong.
 

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Discussion Starter #5
Either I'm losing my mind or you may be confused. Ported vacuum and manifold vacuum should be the exact same at ALL conditions except for 1: when the throttle blades are closed. Any time the throttle blades are open they should be the exact same. At wide open throttle there will be little to no vacuum since the throttle is wide open regardless of ported or manifold. Someone correct me if I'm wrong.
So dumb question then.....should total advance be set with vacuum hooked up? Set total, with vac at 36 at 3,000 rpm and call it good ?
 

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So dumb question then.....should total advance be set with vacuum hooked up? Set total, with vac at 36 at 3,000 rpm and call it good ?

Here's how I set up my advance: I send my distributor in for a custom curve both mechanical and vacuum based on cam and engine specs. Then set initial timing with vacuum plugged and hook up the vacuum line. I usually choose ported or manifold based off how the car starts (cold/hot) or if I experience a stumble off idle.

In other words I'm probably not the guy to answer your question.
 

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Either I'm losing my mind or you may be confused. Ported vacuum and manifold vacuum should be the exact same at ALL conditions except for 1: when the throttle blades are closed. Any time the throttle blades are open they should be the exact same. At wide open throttle there will be little to no vacuum since the throttle is wide open regardless of ported or manifold. Someone correct me if I'm wrong.
There is no vacuum at all with ported at idle. Ported is taken from the venturi above the plates. When I drive casually I don't go more than a quarter throttle at most. That would still be in the territory of no ported vacuum.

I may be wrong here but I always thought ported vacuum, because it was in the venturi and acted on by the pressure drop in there, was the exact opposite of manifold vacuum. When I was a kid playing with the vacuum gauge at least it always seemed that way.

Sent from my SM-N910P using Tapatalk
 

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There is no vacuum at all with ported at idle. Ported is taken from the venturi above the plates. When I drive casually I don't go more than a quarter throttle at most. That would still be in the territory of no ported vacuum.

I may be wrong here but I always thought ported vacuum, because it was in the venturi and acted on by the pressure drop in there, was the exact opposite of manifold vacuum. When I was a kid playing with the vacuum gauge at least it always seemed that way.

Sent from my SM-N910P using Tapatalk

Hooking a vacuum gauge up to ported vacuum you might be amazed how little throttle it takes to switch it to full manifold. Ported vacuum was designed to give you full vacuum at part throttle.
 

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Well, the 289-4V "A code" without Thermactor was curved to a maximum centrifugal advance of 28* and a maximum vacuum advance of 22* with a base ignition timing of 6*. Hypothetically speaking, at light cruise at highway speed you could probably expect a total timing of close to 50* BTDC. At light cruise, cylinder pressures and temperatures are lower and the additional timing can be tolerated.

What's throwing you off a bit is that you're probably measuring total timing (including vacuum advance) without load, which will reduce vacuum. If you're hearing spark knock at very light throttle it's probably pre-ignition, not detonation, and may be the result of too much vacuum advance at a specific vacuum, which can be tuned with an adjustable vacuum advance diaphragm. Keep an eye on your plugs, as well.
 

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vacuum

66, change your springs back to where they were and quit worrying. 43 under light load is not too much. When you're cruising along at light load, you want more advance to help economy . When you floor it, the vacuum drops and the vacuum can will stop pulling extra advance. Soundds like it was working exactly the way it was supposed to. LSG
 

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Discussion Starter #11
Well, the 289-4V "A code" without Thermactor was curved to a maximum centrifugal advance of 28* and a maximum vacuum advance of 22* with a base ignition timing of 6*. Hypothetically speaking, at light cruise at highway speed you could probably expect a total timing of close to 50* BTDC. At light cruise, cylinder pressures and temperatures are lower and the additional timing can be tolerated.

What's throwing you off a bit is that you're probably measuring total timing (including vacuum advance) without load, which will reduce vacuum. If you're hearing spark knock at very light throttle it's probably pre-ignition, not detonation, and may be the result of too much vacuum advance at a specific vacuum, which can be tuned with an adjustable vacuum advance diaphragm. Keep an eye on your plugs, as well.
I measured total timing, with canister and carb port capped, at 36 degrees. When I hook the vacuum up and run to 3,000 rpm with the heavier springs, I'm at 42.9, 54 with blue springs.

Maybe I shouldn't worry about total timing with vacuum hooked up and roll with the 36 capped setting and let the advanced timing run as it is?
 

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In a nut shell, don't worry. You aren't getting any pinging and / or any other signs of too much advance. The vacuum advance with "most" MSD distributors is going to add another 10 degrees (maximum) to whatever your total mechanical timing is. Even more than that, say 50 degrees at cruise or other low load-low throttle operation is going to be just super. Enjoy the ride, sounds like a fine car.

Z
 

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Discussion Starter #14
In a nut shell, don't worry. You aren't getting any pinging and / or any other signs of too much advance. The vacuum advance with "most" MSD distributors is going to add another 10 degrees (maximum) to whatever your total mechanical timing is. Even more than that, say 50 degrees at cruise or other low load-low throttle operation is going to be just super. Enjoy the ride, sounds like a fine car.

Z
Thank you Z. I'm a worry wart when it comes to my baby. I can over think and over analyze with the best of them. She runs great....no pinging, no signs of detonation. Runs really good.
 

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Well, the 289-4V "A code" without Thermactor was curved to a maximum centrifugal advance of 28* and a maximum vacuum advance of 22* with a base ignition timing of 6*. Hypothetically speaking, at light cruise at highway speed you could probably expect a total timing of close to 50* BTDC. At light cruise, cylinder pressures and temperatures are lower and the additional timing can be tolerated.

What's throwing you off a bit is that you're probably measuring total timing (including vacuum advance) without load, which will reduce vacuum. If you're hearing spark knock at very light throttle it's probably pre-ignition, not detonation, and may be the result of too much vacuum advance at a specific vacuum, which can be tuned with an adjustable vacuum advance diaphragm. Keep an eye on your plugs, as well.

Vacuum advance is NOT added to Initial Timing + Mechanical Timing to calculate Total Timing!

Total Timing in your example would be 34* (6*+28*).

:cheers:
 
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