Distributor Advance Expertise Needed

[Road Grabber]

see this post with photos from an earlier thread:
From the A-bodies site:
How To Limit and Adjust Chrysler Vacuum Advance Cans

Here's one of the other many advance threads on this:
Setting Timing and adjusting vacuum advance canister?

By the way, not trying to bust your balls, but it's sometimes easier to find an older thread and read through it and revive the thread with a new post rather than starting a brand new thread on a topic that's been discussed to death in many other threads...I used to be guilty of this, but now read more existing threads first. Actually, this FBBO site has a new feature I love that, when I try to start a new thread, when I type in the title, it self-populates existing similar threads to review before posting a new thread (to avoid too much redundant info).

Yes I did read some other threads. Not as specific to my situation and some used a different canister. Thanks for the replies. I always seem to pick something new up and I’m surprised at the knowledge shared.

 

[topside]

Check out "Bernoulli's principle" on Wikipedia, it applies to velocity through a venturi, and hence carburetors; you can see how a ported vacuum source works. As applied to vacuum advance, a light throttle increases vacuum via velocity. When the throttle is wide open, the vacuum signal drops to essentially nothing as the venturi is now wide open. The detail work is in matching what that signal provides to how much advance the engine wants.
Check the vac port in your vac advance can with a 3/32" Allen wrench; some are adjustable to vary the vac assist. But that won't really crutch a bad mech advance curve, it's a pretty fine adjustment.
I've used the FBO plate on a 512" RB and it was nice - eliminates welding up the advance slot. Actually bought the distributor from him w/plate installed. With that, an MSD 6, and a Ford 5.0 coil, that ignition system could damn near weld, haha.

 

[Road Grabber]

Check out "Bernoulli's principle" on Wikipedia, it applies to velocity through a venturi, and hence carburetors; you can see how a ported vacuum source works. As applied to vacuum advance, a light throttle increases vacuum via velocity. When the throttle is wide open, the vacuum signal drops to essentially nothing as the venturi is now wide open. The detail work is in matching what that signal provides to how much advance the engine wants.
Check the vac port in your vac advance can with a 3/32" Allen wrench; some are adjustable to vary the vac assist. But that won't really crutch a bad mech advance curve, it's a pretty fine adjustment.
I've used the FBO plate on a 512" RB and it was nice - eliminates welding up the advance slot. Actually bought the distributor from him w/plate installed. With that, an MSD 6, and a Ford 5.0 coil, that ignition system could damn near weld, haha.

Thanks for the intel. Will do.

 

[cj's mopar]

Have you try setting to 10 deg initial and plug vacuum 10 plus 28=38 .
36 or 38 on 383 not much more might be just fine .
Then add lifter springs to get advance all in by 2400 rpm . Seams doable.

 

[Road Grabber]

Have you try setting to 10 deg initial and plug vacuum 10 plus 28=38 .
36 or 38 on 383 not much more might be just fine .
Then add lifter springs to get advance all in by 2400 rpm . Seams doable.

The vacuum advance plus the mechanical advance puts me at 48 which I have heard was normal. The vacuum 10 from what I have been told doubles. 28 plus 20 equaling 48. The only way it seems is to use the plate suggested or have the slots customized to limit the mechanical advance.

The car seems to run fine but I would like to know how it runs with 36 total cause my 440 runs great that way after I had it set up years ago.

 

[70rcode]

The additional 17° Vacuum advance was nearly Always utilized in Mopar factory engine tune but seemingly Ignored I'm most magazine articles where just the Initial & Centrifugal mechanical advance curves were illustrated & discussed..... Probably because most high perf speed shops Didn't Connect a vacuum supply line source when on the Distributor machine ...48°--50° Total timing was typical on the Running factory stock Mopars sometimes at idle, just off idle & at light load cruzing speeds.... Of course the Vac advance falls off to near Zero with engine under load or accelerating leaving Just the much mentioned Mechanical centrifugal + Initial advance (about 36°) timing the engine....

 

[RJRENTON]

I had no idea that cruise advance at 50 can be "normal"?

I had my 6 pack distributor set to 14 initial and 36 total. I got that intel from an old Direct Connect Engine Manual years ago and it worked out great. I did bring it to a guy who set up distributors though to do it for me.

So how is mechanical advance restricted? Reshaping/filling in the distributor counter weight slots? Not that I would attempt to do that without a machine....

I tried putting more washers in the vacuum advance but now I realize that doesn't make sense.

Thanks for the reply

WHY....do you think you need / want a vacuum advance type distributor? My 1970 GTX RS23V0A******, has an origional Prestolite dual point distributor setup EXACTLY as you have ..... 15° initial and 35° total advance at 2400 RPM (crankshaft).....set with a SunPro dial back timing light....with the vacuum advance arm connected to the breaker plate, for stabilization....but NOT connected to either manifold or ported vacuum source. The hose is connected to nothing, but looks as if it is, for appearance. The car runs and drives very well. MPG is poor (8-9 mpg) not because of the lack of distributor advance but due to the Dana 4.10:1 gear, the cam (Mopar 292° @ .509 lift advertised at 4° retard), and a relatively high compression ratio (10.5:1). Cruise RPM @ 60 MPH is ~ 3100. But every circumstance is different.....fuel, engine set up, carb jetting, etc. What works for me may or may not work for you......
BTW.....I believe the 1970 383 HP engine used a single point distributor with a sealed vacuum advance diaphragm unit.....the 440 and Hemi used the dual point distributor. Earlier engines (1969 and older 383, 440 & Hemi) used the distributor with vacuum advance diaphragm assembly with the removable end connection to allow shim and spring changes for tuning....but your best source of information is Hallifaxhops at this site. Just my opinion of course......
BOB RENTON

 

[Road Grabber]

The additional 17° Vacuum advance was nearly Always utilized in Mopar factory engine tune but seemingly Ignored I'm most magazine articles where just the Initial & Centrifugal mechanical advance curves were illustrated & discussed..... Probably because most high perf speed shops Didn't Connect a vacuum supply line source when on the Distributor machine ...48°--50° Total timing was typical on the Running factory stock Mopars sometimes at idle, just off idle & at light load cruzing speeds.... Of course the Vac advance falls off to near Zero with engine under load or accelerating leaving Just the much mentioned Mechanical centrifugal + Initial advance (about 36°) timing the engine....

Any thoughts how the two run well at two much different total advance? My 440 is a mechanical advance only distributor and stops at 36 degrees. Yet the 383 using the vacuum and mechanical total advance is ten degrees more. I have heard how a couple degree can have a huge effect on performance. My 440 at 36 degrees total was set that way after I read the Direct Connection Performance Engine manual years ago so this was recommended by Chrysler yet the factory total is 48. The 383 doesn’t have as high compression so it doesn’t seem to have a problem pinging with that advance. I could be missing something but it seems like a broad setting to operate. You would think one is better than the other. I like the plate that controls total advance because it’s not a permanent alteration.

 

[70rcode]

Hey RG,.....I wouldn't get to hung up on Ported or Manifold vac issue as it's Same on the road for advance timing effects as Ported Vac was Only used for EPA smog laws to Makes your engine run Hotter at idle & who needs that....As mentioned,. running 15° Initial, 20° max Mech centrifugal for 35° at 2500 rpm is a good reliable starting point.... Maybe you can adjust the vac can adj screw to minimize the vac advance if you want to use it....Some folks claim the idle & street response torque feels better with the Vac advance connected but it "all depends" kinda deal on total car package..

 

[68 Sport Satellite]

Hey RG,.....I wouldn't get to hung up on Ported or Manifold vac issue as it's Same on the road for advance timing effects as Ported Vac was Only used for EPA smog laws to Makes your engine run Hotter at idle & who needs that....As mentioned,. running 15° Initial, 20° max Mech centrifugal for 35° at 2500 rpm is a good reliable starting point.... Maybe you can adjust the vac can adj screw to minimize the vac advance if you want to use it....Some folks claim the idle & street response torque feels better with the Vac advance connected but it "all depends" kinda deal on total car package..

the adjustment screw doesn't reduce or increase vac advance, it only adjusts the vacuum level at which it starts adding advance.

 

[Geoff 2]

Couple of comments:
- Post #30 is correct about the AK adj screw. Set it fully CW as a starting point. It adjusts the rate of VA adv & ret.
- some sort of stop needs to be made to limit the actuator arm of the VA because in most cases with a modified engine you will not need all the VA that the VA unit can apply.
- low compression engines need more timing, all else being equal.
- most perf engines, & even stock engines, can benefit from VA hooked up to a manifold source [ MVA ].
- much BS has been written about MVA. The link below spells it out correctly. If the link doesn't work, type it in manually. Scroll down to post #6. The last few lines are most important.


www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

 

[Road Grabber]

Couple of comments:
- Post #30 is correct about the AK adj screw. Set it fully CW as a starting point. It adjusts the rate of VA adv & ret.
- some sort of stop needs to be made to limit the actuator arm of the VA because in most cases with a modified engine you will not need all the VA that the VA unit can apply.
- low compression engines need more timing, all else being equal.
- most perf engines, & even stock engines, can benefit from VA hooked up to a manifold source [ MVA ].
- much BS has been written about MVA. The link below spells it out correctly. If the link doesn't work, type it in manually. Scroll down to post #6. The last few lines are most important.


www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

Would you disconnect the hose and plug it going to the VA to set initial timing when using manifold vacuum advance? I will read the link thank you.

 

[Geoff 2]

Yes, when setting the initial timing, always disconnect & plug VA [ ported or man ].

 

[68 Sport Satellite]

Couple of comments:
- Post #30 is correct about the AK adj screw. Set it fully CW as a starting point. It adjusts the rate of VA adv & ret.
- some sort of stop needs to be made to limit the actuator arm of the VA because in most cases with a modified engine you will not need all the VA that the VA unit can apply.
- low compression engines need more timing, all else being equal.
- most perf engines, & even stock engines, can benefit from VA hooked up to a manifold source [ MVA ].
- much BS has been written about MVA. The link below spells it out correctly. If the link doesn't work, type it in manually. Scroll down to post #6. The last few lines are most important.


www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

good points, except for the last part of the first bullet. From everything that I've read and experienced, it technically does not adjust the RATE of Vac Advance and advance retard, it only adjusts the Vacuum level at which Vacuum Advance starts to be added. Good to talk these details out with everyone though. I know for me it can be a bit confusing sometimes

 

[Nxcoupe]

36 without vacuum is about right; with vacuum, cruise advance can be as high as 55.
Every stock or mostly stock BB I've owned really liked 10-12 initial, and the ones with big cams like 18-20; but in either case you have to limit the mech advance.
That's almost always part of re-curving a distributor.

I tune a lot of EFI cars and I get questioned on this all the time. The stock pcms I tune can have up to 60 total at low load cruise. Also close to 14.9 for fuel economy. Always set initial and total with vacuum off and plugged.

 

[Road Grabber]

Engine see to run better since the vacuum advance repair.
I still have a bit of a flat spot and the engine seems to be wanting more fuel. It’s a stock magnum cam with restored original carb for that application.

This hunt has been going on for quite a long time. I tried advancing the timing with no effect.

Maybe plug wires, accelerator pump adjustment/nozzle, points,….

It seems like it’s holding back. I just got it back on the road and will also look at the plugs.

Am I missing something?

 

[Geoff 2]

68 Sport,
Post #34. You are correct about the VA vacuum starting point. What the AK does is compress a spring. So that will alter the rate at which VA applies & also determine how much vacuum will be needed to fully deploy the actuator.

 

[68 Sport Satellite]

68 Sport,
Post #34. You are correct about the VA vacuum starting point. What the AK does is compress a spring. So that will alter the rate at which VA applies & also determine how much vacuum will be needed to fully deploy the actuator.

Hey Geoff, thanks for this tech talk - I enjoy it. With that said, maybe we can straighten out this detail for us and the OP.

I'm not too particular about this detail as far as effect on tuning goes, but some graphs would be good and also something I need to take time to do for my own tuning. I'm just an amateur, but I AM technically minded and curious. Out of my own curiosity, I'd have to see 2 side by side graphs to compare for VA rate. The graph slope would be the rate. Even if the VA spring is compressed, the spring rate remains the same and that is what is controlling allowing the distributor plates to see additional rotation (Advance). Those distributor plates have their own springs which to change that rate requires spring swapping. In my view, the only way to change the rate of VA once it comes in at a given vacuum level would be to change the Vac Advance spring to one with a different spring rate. On the other hand, if the VA spring had a variable coil per length wind then it indeed would have a changing rate of applied VA as the spring is compressed during adjustment.

Do you agree on my thinking or is there something else to explain this?

 

[Geoff 2]

I think the easiest way to understand the VA spring function is to think of a valve spring.
If you set the installed ht at 2", it might read 100 lb/in. Now change the IH to 1.8". It now has 120 lb/in. Compressing the spring the same amount might read 300 lb & 340 lb for each example.
In the case of the VA, it is the vacuum that is opposing the spring tension & the vac has to be high enough to fully deploy the actuator, [ t idle if MVA], against the stop.
Once you move the AK setting from fully CW, you are compressing the spring, so you start with more initial tension & the spring has to be compressed more to a higher tension, needing higher vacuum to fully deploy the actuator.

One of the biggest causes of MVA getting a bad rap is because of bad adjustment &/or non-adj VA units being used. Spring is too strong for the vacuum, so actuator position varies, which causes the idle timing to vary, idler rpm to vary. MVA gets the blame, 'didn't work for me', when it will work when set up correctly. Only with very low vac cams probably under 7-8" of vac, will it not work.

Accel had some graphs of their adj VA units, might be able to find on the net.

 

[68 Sport Satellite]

I think the easiest way to understand the VA spring function is to think of a valve spring.
If you set the installed ht at 2", it might read 100 lb/in. Now change the IH to 1.8". It now has 120 lb/in. Compressing the spring the same amount might read 300 lb & 340 lb for each example.
In the case of the VA, it is the vacuum that is opposing the spring tension & the vac has to be high enough to fully deploy the actuator, [ t idle if MVA], against the stop.
Once you move the AK setting from fully CW, you are compressing the spring, so you start with more initial tension & the spring has to be compressed more to a higher tension, needing higher vacuum to fully deploy the actuator.

One of the biggest causes of MVA getting a bad rap is because of bad adjustment &/or non-adj VA units being used. Spring is too strong for the vacuum, so actuator position varies, which causes the idle timing to vary, idler rpm to vary. MVA gets the blame, 'didn't work for me', when it will work when set up correctly. Only with very low vac cams probably under 7-8" of vac, will it not work.

Accel had some graphs of their adj VA units, might be able to find on the net.

thanks Geoff, what you're saying makes sense and I had forgotten about Hook's law (which is different than spring rate):
Hooke's Law states that the more you deform a spring, the more force it will take to deform it further. Thanks for setting me straight. I need to read up more on spring constant vs Hook's Law. We have now contradicted a few mopar magazine tech articles (unless the changed force on the spring due to position does not equate to applied rate of Vac Adv).

For additional reference:
Here's the inside pieces of the mopar adjustable VA can (sourced from another FBBO thread: Centrifugal Advance vs. Vacuum Advance)

Quick question - if we have a mild cam motor, would it be better to adjust the can so Vac Advance is applied sooner or later, and please specify CCW or CW. CCW means lower vacuum needed to apply the added Vac Advance, but higher manifold vacuum is seen at lower RPM. So would it be true that for a mild cam motor if you set the Vac Adv can to full CCW, the added Vac Adv would not be applied until a higher RPM where the manifold Vacuum signal drops off (or am I backwards on this)? And if so, then for a mild cam motor, would it be better to start at full CW where it takes higher vacuum signal to apply the added Vac Advance? I always get turned around on this one.