Distributor Vacuum - Use or Don't Use?

Guys,

I am still tinkering on my Charger and I am curious if I should once again hook up the distributor to vacuum or not.

The backstory is that when I had my car dyno'ed a few years ago, we found that the engine pinged at WOT (still does on warm days) so the dyno guy said to just leave the distributor disconnected from vacuum to help fight that. It has been that way ever since.

Right now the engine is at 16* initial timing and it advances to 36* total by 3000RPM's or so. This is with the dizzy vacuum plugged.

Would I gain anything by connected the distributor back to manifold vacuum? I know that the ported vacuum is more of an emissions thing as it kills the high vacuum signal at idle. If I use the manifold vacuum, will that improve my idle by advancing my idle timing further or am I just wasting time here?

Thoughts?

First you need to find a new dyno guy. Vacuum at WOT is zero or very near zero so the vacuum advance has absolutely nothing to do with your detonation problem.

Hooking the dist up to manifold vacuum should clean up your idle IF you have enough idle vacuum to operate the advance.

A little more info on the combo will help to dial it in.

Kevin

Stock, there is no vacuum advance at idle, either. One time I accidentally hooked up vacuum advance at idle and I could not get my idle speed to settle down properly. Once I figured out what I had done and fixed it things were fine.

The motor specs are in my signature. Vacuum is about 14" idling in gear. I have noticed that the engine kind of "wants" more idle timing as the RPM's pick up if I turn the distributor to advance the timing. I just have not left it past 16* initial due to the pinging thing at WOT.

One thing I have always noticed is that it seems the idle drops more than it should when I put it in to gear (about 100 RPM drop) and then if I add A/C, that's another 50 RPMs.

Will it hurt me if the idle vacuum advances the distributor well beyond the 16* @ idle it has now?

There used to be a great write-up on the Davis Unified Ignition site about vacuum advance but it appears to be gone now.. The bottom line being there are more advantages than disadvantages.

Here is what it currently says:

Q: What’s the difference between mechanical and vacuum advance?
A: Mechanical advance is the centrifugal weight and spring assembly located underneath the rotor that provides the largest part of the
timing curve. The vacuum advance canister, located on the side of the distributor, provides additional timing only when the engine is pulling
vacuum. The mechanical advance along with the initial advance (base timing set at idle) is what gives you total timing. An example of total
timing would be 12° initial plus 24° of mechanical which equals to 36° total. Vacuum advance is not figured into total timing because it will
only function when the engine is not under a load. Example: If your vehicle is cruising on a flat stretch of road or going downhill, vacuum
advance will come in and add as much as 15° to the total timing for increased fuel mileage and cleaner spark plugs. It is not uncommon to
see as much as 50° – 52° of timing with vacuum advance. But don’t be alarmed by this being too much timing as the vacuum advance will
decrease once you accelerate and put the engine under a load. Under hard acceleration the timing will go back to the original total with no
vacuum advance.

16/36/all in at 3K ain't outrageous. I'd start by plugging the can in to ported & if it only pings at WOT I'd slow the curve with a slightly heavier spring on one side. Best would be a bit more octane. If you're right on the edge a next colder plug MAY help. Experiment with manifold too & you'll for sure need to shorten the slots. With manifold I'd set the initial with the vacuum gauge method then shorten the slots to 35/36 then a heavy enough spring combo to stay just under the pinging point at WOT up thru the gears on your hottest/driest day. Best is more octane but most individuals are set in stone where they are at with that choice. Bottom line NO pinging allowed at any time.

What I am trying to do is clean up my idle/part throttle some so wouldn't I want to skip the ported vacuum as it has no signal at idle? I know this won't fix the WOT pinging by itself as it can only advance the timing.

What I was envisioning is maybe I could dial back the distributor to maybe 10-12* base timing and use the vacuum advance to keep the advance up at idle-part throttle but drop off at WOT which would then top out at 30-32* which might help the pinging?

Does this make sense or am I missing something??

First, I'd get the pinging situation solved. Either limit the total, or reduce the initial. It should never ping, and adding the vac advance won't do anything to change it.

Two schools of thought on ported vs manifold for the distributor:

Ported:
No vacuum at idle. You can run a high(er) initial timing, but too high can cause kickback/starting issues. Easier to tune, since you know the timing won't change between neutral and drive (at least, it shouldn't).

Manifold:
Vacuum at idle adds timing, but only after the engine is running, so starting won't be fighting a high initial. Vacuum drops out at WOT, which is fine. You MUST make sure that your idle and drive vacuum levels are higher than the vacuum the can needs to provide the advance, otherwise your idle will bounce all over the place.

What you're envisioning should work just fine, presuming you have enough vacuum to make sure the can is always advanced at idle/in drive.

what I would do is set the initial with the vac gauge method then shorten the slots to get the std total (which is initial+slots) for your eng type (ie 35-SB) then springs then plugin/adj the can. this would be for ported. then I would hook the can to manifold and same vac gauge method for initial then shorten the slots for the same correct (for your eng) total then springs & see which one works the best for you. there may be a better approach to this as modding the slots ain't easy & I have no exp with manifold but what I've read up on it, it does sound appealling/worth doing. first, more info on whats going on with cleaning up the idle/part throttle response problem. #1 dont allow it to ping! you want close to it for max power and or economy but never over that line

Yeah I think if I have to shorten the slots, I'm just going to buy the FBO plate as I don't have the time/patience/welding skill to do that myself.

FWIW, this is the Mopar Performance distributor that I got from Summit maybe back circa 2006-ish. I don't think they sell it anymore. Any idea what the vacuum advance is on this?

I'm wondering if I role the timing back maybe 2* to 14* base (34 total), then that might cure the pinging issue. However, I'm wondering what the vacuum advance will do to the idle advance...if it will be too far advanced or not.

(1) that's an excellent plan/easy to do/easily reverseable, try 2 or 3 degrees (2) plugging the can in to manifold will definitely require resetting the initial with the dist housing as a can with 7 stamped on the arm will add 14 initial & (as said/VG point) in regards to the idle vac you'd want a can that maxes out at a in hg number lower than your idle in hg reading

If that dist is the Mallory built MP unit, you can adjust the advance. The procedure should be in the Tech Archive.

No welding required.

Kevin

I would say get the pinging fixed quickly and then go from there

I just read the tech archives and see this:

"Although not discussed in MP's instructions, the mechanical advance is a Mallory adjustable YH type (same as used in most Mallory distributors). It is fully adjustable from zero (locked out) to 28 degrees. We set them at 28 degrees when we ship them to MP. All you need to adjust (or lock out) the advance is a Torx driver to loosen two button head screws. "

How do I know if mine is adjustable? Also where do these torx screws reside?

**EDIT**
I think I got this figured out. If my distributor has this, I will need a set of the keys to set the gaps between those tabs.

So do most people with performance STREET motors run vacuum advance or leave it disconnected?

2 second google search

http://prestoliteperformance.com/media/i...e_kit_29014.pdf

If you're running on the street, there's no reason not to run vac advance. Better mileage and burn during cruise for one reason.

If the car is only used on the strip, I wouldn't use vac advance. Why? On the strip, the car only ever sees WOT, at which point the vac advance is inactive. So may as well not have it at all.

I am worried about having too much timing at idle if I use manifold vacuum. Any idea of a ballpark of how much is too much for an engine combo like mine? I can tell it wants more timing at idle as when I advance it the idle picks up and feel stronger but I have been restricted by the total timing.

It seems like you know what you're doing, or at least have an idea.

Your idea to use a limited mechanical advance with the vacuum advance providing extra at idle is interesting and it could work. Years ago I was convinced that manifold vacuum was the only way to go. I have moderated some, but your idea is sound, unless you develop a bog right off idle when you put your foot down, because advance will drop from 20 to 6 degrees. It's worth a try.

Mazda runs very high compression by limiting heat input into the intake charge. The cooler the intake manifold, the cooler the air sucked through the filter, the more the engine will like it. What I didn't expect is they found that the exhaust residual could also add heat, so on their Skyactiv engines, the exhaust manifold is like a header. Scavenging gets the hot air out.

Points to ponder,
R.

Im missing something here, Why is there no ported vacuum an WOT. I would think that is where there would be the highest ported vacuum as you have the most air flow through the Venturi where ported vacuum comes is created so to speak.

Because there is very little vacuum present ANYWHERE when your butterflies are wide open.

The "ported" vacuum exists off ide, and part throttle conditions. that way vacuum advance exists only at part throttle conditions, by design.

Don't need the MP/Mallory keys to set mechanical advance. There is a list of the key width/sizing for each key. I use drill bit shanks to set the limits.

The springs are nice to have, can find those elsewhere for a bunch less.

This is where all the confusion arises. A lot of people think that the vacuum is generated by the venturi effect when you hook to the "Ported Vacuum" port. This is not the case.

The ported vacuum connection taps into the carb just above the throttle blades so when the blades are closed at idle, the port sees NO vacuum. As is quoted above, once you crack the throttle, the port is exposed to the manifold and you get vacuum to the dist.

The manifold vacuum connection is below the blades so it sees vacuum ALL the time.

Kevin

I appreciate the vote of confidence. I think I understand the theory but I am relatively young (31) and didn't grow up in the golden age of non-computer motors.

I can't imagine I'm the first one to ponder this. What do others here do with their street motors?? I thought the "default" was to use vacuum advance and years before the emissions era, manifold vacuum was the only way to go?

Am I the only one running a somewhat stock ignition/dizzy setup?

That is right. The ported vacum they talk about here is actually manifold vacum but its above the throttle plate as said and that way there is no manifold vacum on that port until the throttle is cracked open. Venturi vacum is the vacum that gets stronger with more airflow through the carb as venturi vacum is what the vacum secondary carbs and the Mopar six packs use to opperate. Ron

Ok I spent a few minutes on tinkering with it today. I dialed back the base timing by 3* to 13* and then connected the vacuum advance to manifold vacuum.

Yes the motor does have enough vacuum to operate the can as the idle timing was now around 33* so 20* advance. This netted me a much higher idle out of gear (1100-1200RPMs) and about 2" more vacuum then before.

I went ahead and dropped the curb idle to 950RPM and put it into gear....drops to 700-750 RPM...then turn on the A/C drops to 550-600RPM and acts like it wants to cut off. I tried to fatten up the idle some but it didn't make much difference. Now when I blip the throttle out of gear the motor revs up fine but then stumbles as it slows down. One time it even died on me! The motor also sounded "muffled" to me if that makes any sense....

I know I can always take this back to the setting I had before but I'm curious about this now. What would cause this behavior? Am I dealing with way too much timing now?

Did you verify the timing as the rpm dropped? Bet vacuum is dropping too.

You need to loosen up the spring inside the vacuum can.

Well "fatten up the idle" can be good or bad, but it's not necessarily good. In other words, more gasoline isn't always the answer to a problem.

Go for lean best idle first then mess with the timing.

R.

I tried to lean it up and it definitely made it want to die so can't go any leaner.

How does one "loosen up the springs in the vacuum can"? Is that the allen key adjustment on it? If I do this, will it take away or add more advance?

Is there a vacuum can that will fully advance at an idle vacuum of 7-8"? Seems like I read about a particular Echlin model with a light spring inside, but it's now unobtanium...

a 3/32" allen wrench in the nipple (CW). A particular can will add twice the degrees of timing advance seen on the dampener as the number stamped on the vac adv arm. that is set in stone unless you want to file the notch in the arm on both sides so it can travel further into the can or epoxy strips of feeler gauge on the can to limit the amount of travel of the arm has into the can. This is TOTAL advance & usually you dont change it. Seperately the allen wrench changes how fast the can "comes in" meaning turning the wrench CW lets it start advancing with less vacuum (sooner) so it reaches the end of its amount of advance at a lower vacuum but the total stays the same. I like to set initial/total(slots)/springs then plug in the can (if going with ported) & slow the rate till you are just under the pinging point in everyday driving at varying speeds/in hg levels after you set the other three. If going with manifold plug the can in right from the beginning & set initial/slots (ie 35-sb)/springs(wot) staying just under the pinging point at WOT for springs then adj the can for everyday driving as described

As RR says, the timing from vacuum advance is built in and not easily changed. What is adjustable is when it starts to occur, in inches of vacuum. That's what the adjustment affects.

SO turning it in will make the advance start with more vacuum or less?

Please explain why I want to do this? What concerns me is that something was so far off the engine just up and died. Is this really going to make that much difference?

Before you adjust anything



If the timing is not changing between N and D (it shouldn't) then your problem is elsewhere.

If it is changing, then you can look at adjusting the screw inside the vac advance can.

"in" (CW) will let it start advancing at a lower in hg.

What else could be wrong that would cause this? Timing was the same within +/- 1* from in gear to out of gear.

The thing that leads me to believe this was too much idle timing was the super large drop in RPM's when in gear and with the A/C on.

Hi. I have regular old 383 Road Runner with factory pattern cam. It still has electro-mechanical ignition.

Recently I discovered the vacuum advance was no longer working because the diaphragm started leaking. After looking around for another vacuum advance canister, then buying another distributor to get one, only to find that one bad out of the box, I gave up on it and capped it off. I always kept it on ported.

I have not noticed any difference, but I usually drive in city and am usually either accelerating or slowing down. When the vacuum advance was working, I noticed car would surge at cruise if I dialed in too much. That was the only time I was aware it was doing anything.

I have 4 spd so I don't have to worry about what it does in drive, but when I had bad brakes, I could lug it way down and almost stop without stalling.

I often wonder why Don from FBO strongly recommend Manifold Vacuum.. I suppose it's in regard to the mopar's vacuum can and not having the option of changing it to one that engages at 8" like the chevys can obtain.

Well I think I am going to ditch the manifold vacuum on the distributor. I think I will either go ported (or no vac adv) and see what that gets me.

I double checked the timing today (vac adv plugged) and I got a 21-22* advance from idle. All in at just north of 3000 RPM's according to my timing light.

When the motor cools, I will pull the distributor and see if it has the Mallory YH adjustable advance or not. If so, I may see if I can adjust it to a 18* advance and see if that helps the pinging at WOT.

One thing I noticed is that once the motor gets north of 3000 RPM's the timing seems to bounce a little (2-3*) around. Is this normal? Below 3000, it is rock solid. It's like once is hits that advance "wall" the weights bounce some maybe?

All I know is that motor does not like the low base timing I did this test at. I set it at 10* initial and it tried to diesel when it shut off. I'm thinking if I can run maybe 16-18* base timing with a total of 34-36*, my engine will idle happy and stay out of pinging territory. I will try the ported vacuum with this and see how it behaves as well.

IF the engine starts ok when hot at 22* initial, there is no reason to not run it there.

Take the advance out of the mechanical to hit your total. If you know the total number where the car pings now, it's simple math.

Why compromise your idle quality with lower initial timing when you don't have too.

A brass colored rotor tower in your distributor will designate it as a mallory style advance system. Cast color is the old style MP advance.

Ok guys,

I pulled the dizzy and I do have the adjustable advance! The slots were wide open...right at .250".

I shortened them to .180" and buttoned her back up. I set the base timing to 18* with a total of 34* now. It looks like we have a 16* advance which I can live with.

I will see if I can take her out and play tomorrow and report back!

If the results are good, I may play with the ported vacuum and see if I like that over just mechanical.

Ok I took her out and no pinging! This is on an about 80* day. So it looks like I may cured the pinging issue. I have two small issues now. One of them I expected is that now at 18* base, the starter can't crank the motor fast enough to get it going for a minute it two after it shuts down. I know there isn't much I can do about that besides to roll the timing back.

The other issue is now when the motor gets good and hot, I got a flat spot right of idle when I blip the throttle. Engine stumbles for half a sec, then comes back....even out of gear. I have a 35 nozzle in it now. Do I need to go up or down? How does the extra timing affect the fuel requirements right off idle?

Success! You can wire a N/C momentary contact switch to the blue/yellow wire in the pentastar ECU connector & (1) open it (2) get the starter cranking for 1 second with ign key (3) release N/C switch while still cranking. first try this with a jumper & if it takes care of it then wire in something permanent. make sure the carb A/P tip in is immediate & confirm float levels/healthy stream/no vac leak. since it is doing it in neutral I wouldn't think it is a nozzle size problem (if no vac leak/nozzles ain't plugged). Are we on ported or manifold?

Neither right now. I wanted to check this without the complication of vacuum advance.

I am thinking I might pull the distributor and maybe let the slots out to give me 18* or so of advance. I know I am not pinging at 34* total so maybe I can knock the initial back to 16* to get easier starting and keep the total around 34*.

If I just dialed back the timing to 16* now (32* total), would I be hurting the WOT performance of the motor much?

I like your idea of the switches to kill the ignition until the motor is spinning but I have all reproduction electrical in A+ condition...so I'm not keen on splicing and dicing it at this point...

you're at 18 initial correct? there wouldn't be a problem backing it off to 16 however that'd put the total a bit low & what I'd do is bump the initial up to 20 right now just to get the total up to 36 (or power time it to 36) for a test & if no pinging then I would back the initial to 16 & if it cranks OK when hot then lengthen the slots to get (back to) 36. yes 32 would hurt you. if it did ping at 36 total & 16 initial took care of the cranking then I'd go back to 34 (16 initial+ slot length to get to 34). Understood on the splicing & hopefully 16 will solve it. Holler how it goes

Just dial back the initial so it will start and then open up the slots to get your 34 total back.

Kevin

If anyone's interested, the Echlin VC-1810 is indeed no longer available anywhere. There are, however, several part numbers for the identical can which I found on this forum (caution: non-Mopar) . It seems that most of the alternate p/n's are "dead"... but one, the Airtex 4V1053, is the same can, starts advancing at 4" Hg. and is all-in by 8". They are available from RockAuto and on ebay.

EDIT - got a 4V1053 cheap. Although it does advance at low vacuum, the problem is that it doesn't fit a Mopar distributor, at least not the big-block one... sigh. Back to square one if I want to use manifold vacuum :P

Amen

Guys,

I'm sorry for the week absence here as I have been out of town for work. I pulled the dizzy last night and lengthened the slots to .206" (from .180"). This is a "split the difference" between the original .250" and .180" which proved to be too short.

Timing mechanical advance is now 18-19* advance. I set the base timing at 16* (back from 18*) and total is now 35-36*. I went ahead and turned the screw to tighten the vacuum advance up 2 turns and plugged it up to PORTED vacuum. Lots more advance at part throttle now....

I took it out and it runs pretty well. It is high 70's when I went out and I did a WOT run and still no pinging. It still is hard to start if I don't let it sit for 3 or 4 minutes after I cut it off but I think some of this is heat soak on the starter. I pegged it around 280-300* when I parked it.

I still have one issue though and that is the stumble right off idle if I floor it. Part throttle is great but if I am idling and I blip it, the motor falls on it's face for half a second and then comes back. I can blip it at a stand still and it stumbles hard and then spins the tires! Even out of gear it dips for a second when you blip it.

This only starts to happen after the motor I thoroughly hot...like after 20 minutes of mixed driving. When it's cooler, it's crisp as all heck.

I went down to a 35 shooter (from a 37 which was from a 42) and that seemed to clean this issue up that I had before. Am I still too large on this? Should I try a 32 next or does that seem a bit puny for a BB stroker motor?? I feel like I should go down...doesn't the motor want a leaner shot when it's hot as opposed to cool???

k.i.s.s. get it good & hot & see if you have a healthy AP squirt. right now forget about fine tuning for the heat & you need MORE fuel there as something ain't right/needs fixing. we gotta find out what is changing between it being cold to hot. EDIT you're on ported & not into the can when you crack it wide open correct?

I don't think this is vacuum advance related as it did this before when I had the timing advanced a bit with not vacuum advance hooked up.

Next time I get it good and hot, I will check to see a healthy squirt. Cooled down an hour, it definitely does.

What is a "typical" AP nozzle size for a mild cam, BB stroker? does 35 sound small, large, or "normal"? The only thing I have to go on is my friend's SBC 350 (330HO) motor which took a 28 I think (Holley 4160 600CFM).

UPDATE

I went to a 37 squirter and still the same symptom. When I blip it (after getting hot), the motor stumbles.

I checked the AP shot and it looks pretty healthy...nice big stream. What else can cause this that I am missing???

This has to be carb-related, right?

Not sure why you went back up in squirter size if going down helped?

Too much squirter can be bad too.