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Discussion Starter #1 (Edited)
Good afternoon,

Last week I noticed something with my engine timing. 302. Vacuum advance MSD Distributor/6AL spark box. Just decided to validate my initial and total timing. Idle was at 12. Rev'd the motor up to 2500 and saw it hit 38'ish......kept reving to and it kept climbing!!!! VA was plugged in the whole time. Well past an timing marks. Something is way off. Figured it had something possibly to due with stop bushing in the distributor.

So today I put a new bushing in. Old bushing was the big black one w/Blue and Silver (light) spring. I replaced the black with a silver bushing (25) and replaced the old springs with same/same except new.

I went out and disconnected the VA from the MSD. Plugged up the vacuum line with a golf tee and a nipple on the MSD. Intitial coming in at 13 and total looked to be 38. Reved the motor and it stopped. With that combo I would expect 38 total timing reached at 2800RPM. I figured i'm good to go and it was a bad stop bushing or something like that *shrug*

Plugged back in vacuum advance onto MSD and checked timing again. Idle is still 13 but this time where ran to 2500 (by sound) - that total timing kept climbing again. Hit 40 and and kept on going as I gave it throttle.

Got a vacuum gauge hooked up to it and it idles @12 and climbs under to acceleration until WOT and then drops off (as expected)

Oh yeah - car starts like a top and idles great. No pinging or hesitation when I give it throttle.

What gives? This can't be normal behavior.
 

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Discussion Starter #3
Your vacuum advance might be plugged into the wrong port on the carburetor.
But if it’s not pinging... 🙂
Only two on the carb - both manifold vac.

No pinging - just lots of power and torque. I'm confused. I've done a bit of reading and it appears that it is normal behavior to see it go past what you set your total timing to. I'm just confused as to the science and if my source(s) - google - are giving me sound advise.
 

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I think you are describing normal behavior with your vac. advance in play. It will advance you beyond your total mechanical timing (upwards of 50* in many cases), and that is what it is supposed to do. It will behave differently in gear and under load, but that's when it's effects are most beneficial. You are pulling some vacuum until you are truly wide open, which I would be hesitant to do without worrying about overrevving. You can find out exactly how much vacuum advance you are adding by using a timing light with an advance feature, or by simply running the car hooked up to manifold vac. source and noting how much additional timing you are getting at idle vs. your base timing.
 

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Discussion Starter #5 (Edited)
I think you are describing normal behavior with your vac. advance in play. It will advance you beyond your total mechanical timing (upwards of 50* in many cases), and that is what it is supposed to do. It will behave differently in gear and under load, but that's when it's effects are most beneficial. You are pulling some vacuum until you are truly wide open, which I would be hesitant to do without worrying about overrevving. You can find out exactly how much vacuum advance you are adding by using a timing light with an advance feature, or by simply running the car hooked up to manifold vac. source and noting how much additional timing you are getting at idle vs. your base timing.
Yeah - after further reading - its starting to make sense to me.

So - as long as my engine is generating vacuum (acceleration) - the vacuum will continue to advance the timing until WOT. At WOT - engine stops producing vacuum and then the the advance drops off.

Regarding the method to find the vacuum advance...I've got a boring old timing light with no advanced features. I did run the car hooked to a manifold vacuum source and did not notice a difference in idle timing. Was 13 with an without vacuum.

**EDIT**
Sorry - initial was 12 with and without Vacuum
 

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John, that's because you are using ported vacuum off the carb. Your vac advance is '0' at idle, because the port you're using is above the throttle plates. At part throttle, your timing will gain about 12-14 degrees or so
 

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Discussion Starter #7
John, that's because you are using ported vacuum off the carb. Your vac advance is '0' at idle, because the port you're using is above the throttle plates. At part throttle, your timing will gain about 12-14 degrees or so
I think I'm in a manfold vac port (but I could be wrong). See attached pic. Skinny black line goes to the distributor. Fat one goes to my VAC gauge.

Let me know if I'm mistaken.
 

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Discussion Starter #8
@Grimbrand - you maybe right.

I found some documentation that the "outside" port is metered and "inside" port is manifold

I don't have the instructions so I'm on hold with Holley now to confirm.

Thanks for calling that out. I may go swap them anyhow just to see.
 

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Discussion Starter #9
@Grimbrand

You were correct. On the SQ750 - outside port is metered and inside port is vacuum

Will fire it up later and see what the change does. Wonder if this is the cause of my AFR quest.
 

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It seems ok to me. From the description, 12* BTDC at idle both with and w/o vac adv hooked up, you seem to be using ported vacuum.

IMO, the best read explaining vacuum advance (from a GM engineer, no less) is...
http://www.camaros.org/pdf/timing101.pdf

This article may raise in your mind whether you should have manifold or ported vacuum going to the vac advance canister. It really depends on your engine (eg how radical the cam is, etc). But if you ask the question on the thread, prepare for a major "religious debate" ;-) :)
 

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Discussion Starter #11
@P-51 - great article! I’m more “woke” about timing than I was this morning 😁
 

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Discussion Starter #12
So that did it. Intial timing 12* at idle w/o VAC. With "manifold" vac - 32*. 20* too much for vacuum advance?
 

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From that article: "Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with 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."


And (with vac advance + base mechanical timing) you should probably wind up around 20-ish at idle. Your engine's vacuum will tell you where it's happiest, when you tune it. Highest manifold vacuum = where it should be.
 

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Discussion Starter #15
And (with vac advance + base mechanical timing) you should probably wind up around 20-ish at idle. Your engine's vacuum will tell you where it's happiest, when you tune it. Highest manifold vacuum = where it should be.
Same principle hold true for idle tuning. Happier, higher and more stable engine vacuum trumps AFR? I'm at 20 vacuum advance now.

I’d probably adjust the vacuum advance at the distributor to get it to about 36 total and see how it runs there. I’d probably also try initial at 14 and see what that does.
To get there – I think I need to change the stop bushing to the Green one (23). Car seems happiest at 12*…..maybe 13* that would give me 36* total at 13* initial. To get 36* on the Red bushing - I need to retard the timing to 11*
 

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It's all a balancing act. 12 is a nice comfortable place for pump gas off idle at WOT. Your final timing is *more* important though, because if damage from detonation is going to happen, it'll be when you are making lots of torque. If your idle is a little high or low, you might not even notice unless your were lugging the engine.


So once again - set your baseline timing first. Mechanical ONLY. Then ideally, spend time to make your vac advance and idle air bleeds exactly right, and your engine will deliver all the power and economy you could ever hope for.


When you get it close, go drive till you're up to temp, and romp on it! Listen for signs of ping. Drive at part throttle, full throttle, etc. and check your AFR's. Not every engine makes max power just before ping, but to find out you might need a dyno. If it IS pinging, dial it back a degree or two until it doesn't, and then maybe one or two degrees more to be safe.
 

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So that did it. Intial timing 12* at idle w/o VAC. With "manifold" vac - 32*. 20* too much for vacuum advance?
It sorta depends on what your engine "likes". My engine, a stock 302 J-code, is set at 15* max added vac advance. So at idle, using manifold vac and an initial of 12*, I see 27* BTDC. To change the advance on the vac canister (most of them, at least) you use allen wrench down the "snout" and turn it (don't recall which direction removes/increases the advance).
 

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Discussion Starter #19 (Edited)
Interesting. MSD's instructions say to use the ported vacuum above the throttle plate:

Vacuum Advance: The vacuum advance will advance the timing up to 10° during partial throttle
driving (with 15 lbs of vacuum). The vacuum line should be routed to a ported vacuum outlet above
the throttle plates.


More interesting is that the vacuum advance I'm seeing both vacuum manifold port *and* ported vacuum is way higher than 10*

MSD Pro-Billet 8479
 

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If I understand your situation, you have the initial at idle (w/o vacuum advance hooked up) at 12* BTDC. Then when you hook the vac advance canister up to either manifold or ported vac port the initial adv + vac advance jumps to 32*. Correct? If so, that is very odd. Couple of things to check.

(1) When checking initial timing with vac advance disconnected, do you have both the manifold vacuum (very important) and ported vacuum ports on the carb capped off?
(2) If #1 is ok then the only reason that I can think of that both manifold and ported vac cause an increase of 20* is that the idle setting for the butterflies on the throttle is open so wide that the ported vac port is "exposed" to manifold vac at idle. There may be other explanations but I can't think of any. If this is the case you need to set your idle using the idle adjust screws (on an Edelbrock. Holley?) and not using the throttle butterflies.
(3) Either way it seems like the MSD vac canister is pulling in 20* of timing. It is possible that it actually is adjustable (most after market canisters, like Crane, are adjustable up to 20*+). Or it just could be broken (??).

My *opinion* about why MSD instructions say to use ported vacuum.
(1) It became the standard to use ported when emissions became an issue. MSD is going to parrot what the car manufacturers of the day said.
(2) Manifold vacuum has a lot of advantages but it only works if the vacuum at idle is > ~15psi and is steady. If it is lower than ~15psi then the vac advance is not fully engaged and is very sensitive to fluctuations. In this case a disti hooked to manifold vac on an engine with a lumpy cam with lots of overlap (meaning it has both low vacuum and large fluctuations) will see its advance jump around a lot and you can't set the idle in this situation. In this case ported vacuum is the only option. A stock engine with a vacuum leak will also have a problem with manifold vacuum for a similar reason... with ported vac it may run well enough that no knows there is a problem. The takeaway... ported vacuum is a lot more forgiving.
(3) Given #1 and #2 above it is not odd that MSD would recommend ported... it is more likely to work "out of the box" for all engines. But IMO it may not be optimal for *all* engines.

You might be interested in these two videos (both from the same fellow... IMO, the details are fuzzy but he gets the general ideas right):

The argument for manifold vac - here he is *very* sure that manifold vac is the right way to go...

The argument for ported vac - here he realizes (doh!) that for some engines, manifold vac just will not work...
 
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