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Discussion Starter · #1 ·
Guys, I've looked through the searches trying to find my answer so I would not have to reveal my ignorance once again. I'm running an HEI distributor and it's creating some problems with the timing. I had made a block for my V/A and was running around 22 degrees initial and the mechanical taking it to 38 total. My problem is that my camshaft is fairly low on vacuum and with my timing light I can see that when I race the engine it retards with the loss of vacuum before the weights take over. My question is: If I can get 13-15 degrees out of the mechanical advance and remove the V/A will 25-26 degrees of initial be okay? It cranks good there but haven't had it out of the shop yet. Didn't know if it might run too warm there. Thanks.

Danny
 

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Discussion Starter · #3 ·
Thanks 930. I just read it fairly fast here at work. Gonna re-read it tonight a bit slower. I'm sure my answer is in there if I'm smart enough to comprehend it.
 

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If you do a lot of cruising then the vacuum advance is a good thing.

You could always go to full manifold vacuum as your source. However, when I did this I had to drop my mechanical initial way lower, which still gave me 28-30 degrees of initial, then ran the different pushing that gave an extra 24* or so of mechanical advance to achieve the desired ~36* total.

I didn't like this, so I went back to ported. In either case you should not be using any of the vacuum advance when racing as @ WOT the vacuum source is gone.
 

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What distributor and which advance springs?
With the information you gave it appears that your running too stiff of an advance springs causing it to advance to late.
You want the mechanical advance to be all in before the vacuum advance drops out (advance curve).
Typical street/strip deal is usually all in mechanically by 2,200/2,500 rpm which would be well before the vacuum advance drops out.
If that is not the case then you might need to re-curve by changing the weights also.
 

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Discussion Starter · #6 ·
Bradley, you may have hit on something. I just put in a new distributor. I swapped out the springs for the lighter ones that I had been running in the worn out piece. But the mechanical does looked delayed with the light. The weights that are in the new distributor look heavier. Is there a preference on the weights? It does look like that the vacuum is dropping off before the mechanical takes over.
 

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Guys, let's clear up some apparent confusion: vacuum advance, unlike mechanical is not dependent on engine speed at all. Vacuum advance depends on throttle opening and engine load. Correct vacuum can for his application should be fully advanced a couple degrees below the actual vacuum reading when engine is idling, and connected to manifold vac port. The OP's vacuum advance probably needs to be limited to about 12 degrees. Mechanical weights shouldn't have any effect until approx. 200 rpm above idle speed, and fully advanced ~2500-3000 rpm. Mechanical should be checked with vacuum disconnected and plugged. Ignition 101 is worth several readings and following its' recommendations.
 
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Danny,
That's a common problem when changing to HEI distributor.
If you can't get the mechanical timing to come in sooner by changing to lighter springs the weights are next to manipulate. Before going there ( the advance weights ) find the lightest springs you can and give that a try. That might create another problem though - irradic idle behavior. If that happens you'll have to go back to slightly stronger advance springs and look into changing the weights.
I believe that Dave Ray on this site has a list of different HEI weights with their corresponding curve.
In the absence of that information you will need to find someone who has a distributor machine and knows how to use it.
Let us know how you make out.
 

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Unless you bought a GMPP distributor (?), a lot of this is guesswork. We do not know what the center plate is, or what weights are in it...

You can mix one heavier spring with one lighter spring to find in between advance curves. The system is a fly-weight governor style and will move as a unit against the total spring tension (both springs) so tune as needed :)
 

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Discussion Starter · #10 ·
No Broc. It's a cheap aftermarket. I knew I would be discarding the module and other parts. I'm just gonna have to play with the springs and weights and the advance to get what I'm looking for. Thanks.
 

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Back in the late 1980's I would weld a bead onto the advance weight to make it heavier and come in sooner.
I still have some of those weights and it works well and is cheap if you do not have a bunch of springs.
 

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I also have very little manifold vacuum and had trouble getting vacuum advance to work consistantly with a Crane distributor vacuum can, vacuum advance limit cams, centrifugal weights and springs etc..

There very well may be a solution to your set up that it relatively easy, so explore that first, but FWIW, I decided to use a programmable MSD 6AL2 with a MAP sensor and locked out distributor.

All the distributor does for me is send the spark. It's completely locked out and installed at 50° BTDC. The MSD retards the timing from 50° based on rpm and manifold vacuum and also pulls timing for a retard start.
What this means is that with with my low and erratic manifold vacuum (262/272°@50 SR & Vic Jnr), I still get a rock-solid ignition curve, and therefore, steady idle and pretty good street manners along with lower temps and fuel economy that VA offers..

Food for thought. :)
 

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I also have very little manifold vacuum and had trouble getting vacuum advance to work consistantly with a Crane distributor vacuum can, vacuum advance limit cams, centrifugal weights and springs etc..

There very well may be a solution to your set up that it relatively easy, so explore that first, but FWIW, I decided to use a programmable MSD 6AL2 with a MAP sensor and locked out distributor.

All the distributor does for me is send the spark. It's completely locked out and installed at 50° BTDC. The MSD retards the timing from 50° based on rpm and manifold vacuum and also pulls timing for a retard start.
What this means is that with with my low and erratic manifold vacuum (262/272°@50 SR & Vic Jnr), I still get a rock-solid ignition curve, and therefore, steady idle and pretty good street manners along with lower temps and fuel economy that VA offers..

Food for thought. :)
Would the programmable 6AL2 work the same using a vacuum advance canister instead of the MAP sensor, with the timing locked at say 36-38 degrees?
 

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Apologies for any high-jacking with this elaborate reply.. :)
Would the programmable 6AL2 work the same using a vacuum advance canister instead of the MAP sensor, with the timing locked at say 36-38 degrees?
No. :)
You need to completely lock-out the distributor and install it at the maximum amount of advance that you want to run, and let the MSD retard the timing based on the engine's requirements.
At WOT, the MAP sensor "sees" the drop in vacuum and tells the 6AL2 to pull out X-amount of timing to arrive at your desired ~36°. When vacuum is returned, indicating low/no engine load, advance is returned to your maximum ~50° for idle and cruise. It's really basic. ;)
The desired curve you want is inputted via a laptop and you can change/tune/adjust that curve at any time you want.

The main point of using the programmable 6AL2 (in both my application and this advice) is to swap the mechanical-vacuum advance can with a MAP sensor. This allows you to replace a fluctuating, inconsistent and erratic vacuum advance control with a rock-solid, electronic one.
I'm not saying it's impossible to set the MSD up the way you're asking (even though it's against what the MSD's instructions tell you), but it (also) defeats the whole purpose of using the 6AL2, only to still end up with an inferior timing control by keeping the mech-VA can.

The problem with mech-VA and low, erratic manifold vacuum is that it's short-comings compounds on themselves.
Meaning; a drop in vacuum results in a drop in advance, which drops idle speed, which drops vacuum, which drops advance, which drop idle speed and so on. The engine rpm plummets until it stalls.
The same is true with an increase in vacuum which then has the engine racing.
Even if you're a whizz at setting up the mec-VA, you're still gonna have stalling and/or racing issues between a cold and warm engine. For me, this was a real pain in the arse and I never found the "happy medium".

With manifold vacuum being converted to an electronic signal, this fluctuating vacuum signal always remains within the boundaries of the set curve no matter how erratic, and the ignition timing remains fixed, steady and stable.

I haven't explored my 6AL2 very much at all (yet) and I'm sure there are still improvements to be had, but anyway, here is what I have programmed into mine so far.
With the dizzy locked at 50°...
The top "RPM" chart shows no timing is pulled except for 25° during slow cranking/starting. (= 50-25=25° for cranking)
The lower "Vacuum" chart shows 18° is progressively pulled between 4 and 2 in/Hg of manifold vacuum. (=50-18=32 for WOT)
From 15in/Hg to 4in/Hg it doesn't matter how erratic or fluctuating the vacuum signal is, the timing is never changed.
(Yes, that "vacuum chart looks all backwards to me too and was hard to get my head around it, but that's how it is. :p)


I have a 5600 converter and 4.56 gears, so I assume this might be why my vacuum can be pulled so close and quickly to WOT.
I also have one of those MSD Knock Alert sensors helping me avoid detonation. My WOT is also set at a conservative 32° ..which still made 710 on the dyno. :D
Hope this helps. :thumbsup:
 

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I have a technical disagreement with the first page of that writeup regarding the purpose of the VA. My position: The vacuum advance is there to modify the spark timing due to changes in cylinder pressure (which influences the combustion burn rate). It is not there to bounce all around guessing at, or assuming what, the instantaneous A/F ratio is, and trying to compensate for that.


I didn't read past the first page.
 

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IMO it's there as load compensation. heavy load = less vacuum = less advance.

A/F ratio doesn't really enter into it.
 

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Apologies for any high-jacking with this elaborate reply.. :)

No. :)
You need to completely lock-out the distributor and install it at the maximum amount of advance that you want to run, and let the MSD retard the timing based on the engine's requirements.
At WOT, the MAP sensor "sees" the drop in vacuum and tells the 6AL2 to pull out X-amount of timing to arrive at your desired ~36°. When vacuum is returned, indicating low/no engine load, advance is returned to your maximum ~50° for idle and cruise. It's really basic. ;)
The desired curve you want is inputted via a laptop and you can change/tune/adjust that curve at any time you want.

The main point of using the programmable 6AL2 (in both my application and this advice) is to swap the mechanical-vacuum advance can with a MAP sensor. This allows you to replace a fluctuating, inconsistent and erratic vacuum advance control with a rock-solid, electronic one.
I'm not saying it's impossible to set the MSD up the way you're asking (even though it's against what the MSD's instructions tell you), but it (also) defeats the whole purpose of using the 6AL2, only to still end up with an inferior timing control by keeping the mech-VA can.

The problem with mech-VA and low, erratic manifold vacuum is that it's short-comings compounds on themselves.
Meaning; a drop in vacuum results in a drop in advance, which drops idle speed, which drops vacuum, which drops advance, which drop idle speed and so on. The engine rpm plummets until it stalls.
The same is true with an increase in vacuum which then has the engine racing.
Even if you're a whizz at setting up the mec-VA, you're still gonna have stalling and/or racing issues between a cold and warm engine. For me, this was a real pain in the arse and I never found the "happy medium".

With manifold vacuum being converted to an electronic signal, this fluctuating vacuum signal always remains within the boundaries of the set curve no matter how erratic, and the ignition timing remains fixed, steady and stable.

I haven't explored my 6AL2 very much at all (yet) and I'm sure there are still improvements to be had, but anyway, here is what I have programmed into mine so far.
With the dizzy locked at 50°...
The top "RPM" chart shows no timing is pulled except for 25° during slow cranking/starting. (= 50-25=25° for cranking)
The lower "Vacuum" chart shows 18° is progressively pulled between 4 and 2 in/Hg of manifold vacuum. (=50-18=32 for WOT)
From 15in/Hg to 4in/Hg it doesn't matter how erratic or fluctuating the vacuum signal is, the timing is never changed.
(Yes, that "vacuum chart looks all backwards to me too and was hard to get my head around it, but that's how it is. :p)


I have a 5600 converter and 4.56 gears, so I assume this might be why my vacuum can be pulled so close and quickly to WOT.
I also have one of those MSD Knock Alert sensors helping me avoid detonation. My WOT is also set at a conservative 32° ..which still made 710 on the dyno. :D
Hope this helps. :thumbsup:

I don't see not working.
I read the installation instructions on MSD's web site and nothing says it won't.
It works without a map sensor.


I can see where it could possibly be better even in some respects with the vacuum canister over the MAP sensor.
( rotor/ cap phasing)


The MAP sensor and vacuum canister receive the signal from the intake so,(wither or not a transmission is a stick shift or automatic, it is still changes gears) it is just one being done one way mechanically the other electronic.


Low vacuum isn't a problem and if it's erratic you have other issues.
 

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I don't see not working.
I read the installation instructions on MSD's web site and nothing says it won't.
It works without a map sensor.


I can see where it could possibly be better even in some respects with the vacuum canister over the MAP sensor.
( rotor/ cap phasing)


The MAP sensor and vacuum canister receive the signal from the intake so,(wither or not a transmission is a stick shift or automatic, it is still changes gears) it is just one being done one way mechanically the other electronic.


Low vacuum isn't a problem and if it's erratic you have other issues.
The 6AL2 I am referring to is the Programmable 6530 unit.
Perhaps you have read MSD's 6421 non-programmable 6AL2 instructions by mistake..?
The 6350's instructions state on both pages, 1 and 10 that if you are programming the unit, the distributor must be locked out. :)

Yes it will work without a MAP sensor, but then why on Earth would you go to the effort and expense of installing an ignition with a programmable ignition curve, only to resort back to the mechanical vacuum advance mechanism that is responsive to a fluctuating vacuum source? What's the point?
Perhaps re-read what I have said and look again at the ignition curve on the chart. It's very clear that the timing won't move at all, even while the normal vacuum signal of any large duration cam is fluctuating at idle.

Apart from improved part throttle, general driveabilty improvements and having lot's of initial without killing your starter, the real advantage to what I am suggesting here is a far superior idle over using a can.

An adjustable rotor for phasing to the cap is generally required with this type of ignition, and it was required that I do that too. Easy. ;)
 

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The 6AL2 I am referring to is the Programmable 6530 unit.
Perhaps you have read MSD's 6421 non-programmable 6AL2 instructions by mistake..?
The 6350's instructions state on both pages, 1 and 10 that if you are programming the unit, the distributor must be locked out. :)

Yes it will work without a MAP sensor, but then why on Earth would you go to the effort and expense of installing an ignition with a programmable ignition curve, only to resort back to the mechanical vacuum advance mechanism that is responsive to a fluctuating vacuum source? What's the point?
Perhaps re-read what I have said and look again at the ignition curve on the chart. It's very clear that the timing won't move at all, even while the normal vacuum signal of any large duration cam is fluctuating at idle.

Apart from improved part throttle, general driveabilty improvements and having lot's of initial without killing your starter, the real advantage to what I am suggesting here is a far superior idle over using a can.

An adjustable rotor for phasing to the cap is generally required with this type of ignition, and it was required that I do that too. Easy. ;)
I know which box we are discussing, and read the correct instructions.
I have a #6241 6AL2 now and looking into upgrading.
I stated in my first post of this thread, lock it at say 36-38 degrees opposed to the 50 degrees you are using with the MAP.
I don't have an erratic vacuum problem, ( don't know where that came from ) it's just 11 inches at idle.


So will it or won't it work?
First you said no and now yes?


Why or earth would I want to use a canister over MAP?


Lets see.


Rotor phasing as stated before.
Don't have to purchase a MAP sensor.
Don't have to purchase a MAP sensor wiring harness.
Don't have to purchase a adjustable rotor. ( if they make one for my distributor )
I have an adjustable vacuum advance canister.
I can have a locked out distributor by knocking out 2 roll pins and 1 back in.
I intend on using the programing for everything else.
I'm abnormal.
I like to do things the hard way.
Need more?
 
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