Setting vacuum advance

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Demonx2

Demonx2

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So I've turned the 408 Demon into being able to drive on the street a little bit. A few short cruises, get ice cream, etc, etc. You don't want to go far in this car as it was more built for racing. Loud, noisy, rough, and probably not the most reliable! It's not what the car was built for but we thought we'd have a bit of additional fun with it this way. You get some strange expressions when you pull up next to someone at a light and they hear this thing idling!!

The engine is timed for the track with about 35 degree total timing advance (all mechanical). But with some light load street driving, and no vacuum advance hooked up, the engine is obviously retarded from where it should be if you were to optimize light load timing on a dyno. I hooked up the vacuum can on the dist and it gets too much advance - we're talking high 50's. The engine did not like that as you can hear evidence it's too much advance. Yes, it could be rotor phasing and such but let's not go there. I just want to limit the max vacuum advance to allow it to max out at about 20 degrees total vacuum advance on top of whatever mech advance it has. Is there a way to do that on these distributors/vacuum cans? I know you can put a small allen key in them and adjust the spring load impacting the level of vacuum when the advance starts but not sure of a way to limit the actual total amount of vac advance.

Looking for those out there that "been there, done that" and how you did it. I figure someone out there has already crossed this bridge. Thx!!
 
Demonx2 said:
So I've turned the 408 Demon into being able to drive on the street a little bit. A few short cruises, get ice cream, etc, etc. You don't want to go far in this car as it was more built for racing. Loud, noisy, rough, and probably not the most reliable! It's not what the car was built for but we thought we'd have a bit of additional fun with it this way. You get some strange expressions when you pull up next to someone at a light and they hear this thing idling!!

The engine is timed for the track with about 35 degree total timing advance (all mechanical). But with some light load street driving, and no vacuum advance hooked up, the engine is obviously retarded from where it should be if you were to optimize light load timing on a dyno. I hooked up the vacuum can on the dist and it gets too much advance - we're talking high 50's. The engine did not like that as you can hear evidence it's too much advance. Yes, it could be rotor phasing and such but let's not go there. I just want to limit the max vacuum advance to allow it to max out at about 20 degrees total vacuum advance on top of whatever mech advance it has. Is there a way to do that on these distributors/vacuum cans? I know you can put a small allen key in them and adjust the spring load impacting the level of vacuum when the advance starts but not sure of a way to limit the actual total amount of vac advance.

Looking for those out there that "been there, done that" and how you did it. I figure someone out there has already crossed this bridge. Thx!!
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That's not too much. It's not uncommon to see 60 and a little more including the vacuum can when at part throttle. Perfectly normal.
 
Some diaphragms are adjustable with a small Allen wrench and some are not. Like many, I run no vacuum advance but compensate with 15* initial.
 
The Allen Key adjusts the spring rate, so that the VA total amount does not fluctuate. Best to screw the AK fully CW [ softest setting ]. If the engine accepts that without light throttle ping, all is good. If it pings, the spring may need tightening [ AK, CCW ]...or....you might have too much total VA. Some experimentation will be reqd.

To limit total VA. Couple of ways to do it. If there is a marking on the VA arm, it is the amount of VA added in dist degrees. I did one last week, marked 10.5, which is 21 crank degrees. I wanted 17*. So I measured the movement of the arm & by proportion calculated the reduction in travel needed to give me 17*. Made a little metal stop to give me the correct gap & I silver soldered it to the arm, just before the bend in the arm to keep the heat away from the diaphragm.
Another way that you could limit VA travel is by limiting the movement of the plate with a screw or similar; haven't tried this but can see no reason why it would not work.
Long duration cams, single plane intakes, low CR, big carbs all want more cruise timing so 60* is not excessive with some combos.
 
Some real misinformation being posted here.
Go to the How to section. Hops and I have covered this and the info is there. In particular look at the table in Direct Connection/Mopar Performance instructions.
 
All these tips are good . Your absolute limit of advance @ cruise is 52*. Thais at CRUISE . If it pings or rattles , you gotta back it off . Enginesw ith long duration cams don't make a lot of vacuum , so , that will in itself limit the advance . Make sure your vacuum line is connected to a source that is ported (above the throttle blades . good luck
 
Mattax said:
Some real misinformation being posted here.
Go to the How to section. Hops and I have covered this and the info is there. In particular look at the table in Direct Connection/Mopar Performance instructions.
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Thx to all for your input. The car is quite loud so detonation/knock is hard to hear! It does have a long duration cam and single-plane intake so I know it can use more advance than I have with the all-mechanical that's in it now (35 total). I know it could safely run something in the low-mid 50's and was hoping someone sells a bolt-on piece to limit vacuum advance. I'll look at the can/arm to see what it's stamped. Sounds like a little old-fashioned welding job or set screw or some type of mechanical stop on the vacuum mechanism is in order.

Mattax - what "How to" section are you referring to? I'd like to check it out. I searched in the Ignition and Small Block engine forums but don't find it.
 
Demonx2 said:
Mattax - what "How to" section are you referring to? I'd like to check it out. I searched in the Ignition and Small Block engine forums but don't find it.
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Sorry. I was prtty tired this am.
How to is in the top bar.
upload_2021-7-19_12-29-32.png


Just because something is in the How To section doesn't make it better, but Hoppy and I put the information there because it does come up a lot. The DC/MP info is at the end of How To Limit and Adjust Chrysler Vacuum Advance Cans
 
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Demonx2 said:
I hooked up the vacuum can on the dist and it gets too much advance - we're talking high 50's.
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How do you know that's too much?

If your D is locked at 35,
and your total from all sources is 50,
then your V-can is only bringing in 50 less 35=15.

If the V-can is plumbed to manifold vacuum,and
if your stall is 2000, then 50* at 2000 under load, YES, that will be too much, and pinging is the evidence.
But if your stall is 3500, then, then the vacuum will rapidly drop as the engine goes towards WOT, and will soon drop too low to activate the V-can; so yur good to go.
In between, will get you varying results.

The goal of all timing mechanisms is to START the burning process at the right time for the engine to achieve maximum cylinder pressure at a specific crank position, to impart to the crank, the maximum amount of push. This position is about 25 to 28 degrees after TDC. The amount of ignition advance required to achieve this, from idle to about 3500rpm, varies with rpm and load. After 3500 or so, at WOT, the advance requirement usually varies very little.

The biggest cam I have tuned is the 292/292/108, and I only tune SBMs; and only for street. I can tell you that for best results, that tune required a "normally working" distributor. That is to say; Not locked, and ported vacuum advance.
And at a cruise rpm of 2600, it liked over 50 degrees.
With a 4-speed, a car will hit 2600 FOUR times at Part Throttle, before hitting hi-way speed.
You can easily figure out what no-load timing your engine wants at any rpm, just by bringing the rpm to that specific point and attempting to keep it there, while advancing the timing. When the rpm no longer picks up, you have found the maximum no-load timing. You can build a map by doing this every say 400 rpm, from stall rpm to cruise rpm, or a little higher. Then smooth it, and I subtract 3 degrees for cruise-load.
If your engine is too loud to hear detonation, I want to say bad things to you and about you, but in your case, I understand that the car is Not a streeter, so then, you will probably want to invest in a stand-alone knock-sensor. Then you can build a street power-curve.
 
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Yes, you can. I have done it so many times, I have lost count. The AK moves a plate that compresses or un-compresses a spring to change the rate.
You are the one providing misinformation.
 
383Scampman said:
All these tips are good . Your absolute limit of advance @ cruise is 52*. Thais at CRUISE . If it pings or rattles , you gotta back it off . Enginesw ith long duration cams don't make a lot of vacuum , so , that will in itself limit the advance . Make sure your vacuum line is connected to a source that is ported (above the throttle blades . good luck
Click to expand...

You should read the MP engine book and actually DO some of the things they say, as your statement is simply untrue. I've seen cruise timing at and over 60*. It's perfectly normal, combination dependent.
 
Bewy said:
Yes, you can. I have done it so many times, I have lost count. The AK moves a plate that compresses or un-compresses a spring to change the rate.
You are the one providing misinformation.
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It doesn't change the rate. It changes the when the vacuum advance is fully applied. The rate is "whatever" the spring allows, just like you cannot change the rate of a valve spring. Same principle.
 
Bewy said:
Yes, you can. I have done it so many times, I have lost count. The AK moves a plate that compresses or un-compresses a spring to change the rate.
You are the one providing misinformation.
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:rofl:

You're hilarious!
Spring rate is not the same as spring force.
You need to review your high school physics.

Considering you've never seen the inside of a Chrysler distributor until looking at the photos people posted here, you have a lot of gall to claim I'm providing misinformation.
Everyone one of those photos I posted are distributors I've taken apart. I've run quite a few on a distributor machine and quite a few measured on an engine.
You argue about how a vacuum advance works with Halifaxhops who has run hundreds of distributors and done concours restorations on many.
I've told you several times that Chrysler vacuum pods are not the same design as GM or Crane, but you're the expert. OK
 
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Kendog 170 said:
View attachment 1715768391You can control it with a wire. This thread may help.

Adj Vacuum Advance Can

Also I believe the VC # on the arm dictates how much advance as well but don't no for sure . Good luck !
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Yea.
And whose picture is that?
wink-gif.gif

Came from my camara.
Posted in my how to. I may not have mentioned it in the thread but the answer to your question is in the How To.
And I'm pretty much always glad to answer questions or discuss different ways of doing things or figuring things out.
If what I wrote there isn't clear (could be my writing, could be the reformatting from seperate posts) be glad to try clarify.
 
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I don't know **** about this subject compared to Halifaxhops and Mattax. If and when I get in a jam I will be listening to you two on Dizzy's. Just like I listen to 67Dart273 when I need electrical advice . I hope and try not to give misinformation but can't help trying to help if and when I can .:)
 
Thats how you learn nothing wrong with that. BTW the number on the arm is the Max distributor degrees it moves.
 
Gosh - I didn't know that asking how to limit the maximum amount of vacuum advance could generate such "animated" responses!! Quite a lot of passion around the topic of timing!! Heck, ask a simple question and we got guys picking at each other and AJ wants to "say bad things to you and about you" (talking about me)!! I'll chalk that up to this internet thing as someday I think it would be great to meet up with a bunch of you. After all, we all share a common love for these old cars!!

The "How to" post really helped - I like the set screw on the side idea but until I get time to pull the dist and take it apart and look at it, I'm not sure which method I'll use to limit the advance.

As for the timing, I'm ok with the mechanical advance part. The 35 total is in line with many published articles about what these engines want (this is a 10.5 CR Eddy head engine). If I recall when I set the timing, that 35 degrees is all in by about 2800-3000 rpm. It's close enough for my little motor anyway. This dist has 22* total mechanical advance so that means the idle gets about 13-ish. I haven't measured the idle vacuum on my car yet but I can tell you it certainly ain't much!! So ported or MV would not make much diff at all.

For part throttle street operation, a bit more advance is all I'm after. Without having it on a dyno to optimize every speed/load, "a little bit more" will be just fine. I'll limit the max vac advance to about 20* so that will add to whatever the mechanical advance is at the rpm the engine is spinning. That will most definitely help with fuel consumption. Is it ideal or perfect? Nope - that's what electronic systems are for and this is just a simple dist. I'm just trying to make it a tad better than it is today without any VA. It's not even so much about burning less fuel; it's as much about keeping the plugs cleaner driving on the street. (Although this thing DOES suck some fuel puttering around. Not sure if it's the engine or my right foot!)

Thx again!! I truly enjoy this FABO forum and all the info shared on it!!
 
Demonx2 said:
Gosh - I didn't know that asking how to limit the maximum amount of vacuum advance could generate such "animated" responses!! Quite a lot of passion around the topic of timing!! Heck, ask a simple question and we got guys picking at each other and AJ wants to "say bad things to you and about you" (talking about me)!! I'll chalk that up to this internet thing as someday I think it would be great to meet up with a bunch of you. After all, we all share a common love for these old cars!!

The "How to" post really helped - I like the set screw on the side idea but until I get time to pull the dist and take it apart and look at it, I'm not sure which method I'll use to limit the advance.

As for the timing, I'm ok with the mechanical advance part. The 35 total is in line with many published articles about what these engines want (this is a 10.5 CR Eddy head engine). If I recall when I set the timing, that 35 degrees is all in by about 2800-3000 rpm. It's close enough for my little motor anyway. This dist has 22* total mechanical advance so that means the idle gets about 13-ish. I haven't measured the idle vacuum on my car yet but I can tell you it certainly ain't much!! So ported or MV would not make much diff at all.

For part throttle street operation, a bit more advance is all I'm after. Without having it on a dyno to optimize every speed/load, "a little bit more" will be just fine. I'll limit the max vac advance to about 20* so that will add to whatever the mechanical advance is at the rpm the engine is spinning. That will most definitely help with fuel consumption. Is it ideal or perfect? Nope - that's what electronic systems are for and this is just a simple dist. I'm just trying to make it a tad better than it is today without any VA. It's not even so much about burning less fuel; it's as much about keeping the plugs cleaner driving on the street. (Although this thing DOES suck some fuel puttering around. Not sure if it's the engine or my right foot!)

Thx again!! I truly enjoy this FABO forum and all the info shared on it!!
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No where did I see where you have told us what vacuum your engine will produce at idle and at cruz'n speed. You can adjust the vacuum advance all you want and it will no do **** if you have no vacuum to operate it.
OP - How much vacuum does your engine have?
Sorry if I missed it
 
Mattax, completely wrong. But I should have explained spring 'rate' more clearly. As for not 'seeing inside a Chrys dist', I was reworking them probably before you were worn....I still have a couple of cast iron dists on the wall, do you know how old they are?
I notice Halifax shops hasn't chimed in......That is probably because he DOES know how the vac adv works.

Mattax, you obviously have never pulled apart an adj VA unit. I have, many times. The GM & Chrys adj VA units internally, are exactly the same design, only the method of attachment to the dist body is different. They may use a different spring because of the different leverage ratio.
For those following this who might still be non-believers & have a dist with an adj VA unit, do this simple test: put the dist in a vice. Turn the AK fully CW. Then, with a screwdriver, push on the VA plate; it will move fairly easily.
Now turn the AK fully CCW & push on the plate again with the screwdriver. It is much harder because you have compressed the spring, & you are pushing against the compressed spring. The AK adjustment changes the rate of apply [ or un-apply ] of the VA unit. With the spring set to the softest setting, the plunger will move with less engine vacuum than with the spring set to a stiffer setting. This was the rate I was referring to in post #13.
 
I'm currently going thru this exact subject on my Mopar's. I started with the 74 440ci Ramcharger as it's only getting 6.6mpg, next I'll work on the Duster 360.





I also started a thread on RamchargerCentral to document my steps and before/after mileage for tuning the timing/vacuum advance
Vacuum Advance Timing for Maximum Efficiency
 
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