Ported or manifold Vacuum to Dist.

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Kendog 170

Kendog 170

Let the boy go !
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Okay, I've always used ported vacuum to dizzy on all my Mopars. I want to go through my Dizzy on my 340 /4 spd. After boring 20 over , basically stock with the exception of Eddy heads. Then recheck my RB 500 with A/T next year. I was reading this in (How to Article) and there saying to run from manifold to Dizzy ? I know it would take a lot of re-curving , weights,canister,Timing ect. I just want to know the opinions of this. I guess I'm old school but I value my knowledgeable friend here on FAB. My Ragtop 340 in particular needs a better fine tuning for sure and I want to do this myself. They claim idling it will run cooler which could help my RB 500 in traffic. I'd like to use my canisters on both for better mileage when cruising. Here's the article.
Everything you wanted to know about ignition advance
 
Kendog 170 said:
Okay, I've always used ported vacuum to dizzy on all my Mopars. I want to go through my Dizzy on my 340 /4 spd. After boring 20 over , basically stock with the exception of Eddy heads. Then recheck my RB 500 with A/T next year. I was reading this in (How to Article) and there saying to run from manifold to Dizzy ? I know it would take a lot of re-curving , weights,canister,Timing ect. I just want to know the opinions of this. I guess I'm old school but I value my knowledgeable friend here on FAB. My Ragtop 340 in particular needs a better fine tuning for sure and I want to do this myself. They claim idling it will run cooler which could help my RB 500 in traffic. I'd like to use my canisters on both for better mileage when cruising. Here's the article.
Everything you wanted to know about ignition advance
Click to expand...
It's wrong. And the AFR chart in the Eddy instructions is wrong too.
That doesn't mean using manifold vacuum is wrong. Use whichever works best with the mechanical curve and for the situation.
IF, the mechanical timing and advance can't be brought up to the where it ought to be at idle speed, using manifold can be used instead. Downside it may not be quite as consistant or stable as doing it mechanically. That's 'cause as load is added (placing it in gear) the manifold vac will drop.

"Where it ought to be"
The timing at idle should be set for highest efficiency. On a non-CAP/CAS vehicle this is all that matters. The engine produces little power at 600 - 700 rpm. So especially with an automatic transmission its important to fire the cylinder so maximum pressures occur when piston has the best leverage on the crank.

Early emissions equiped cars (Chrysler CAP then CAS) had to compromise idle efficiency to reduce HC and CO by purposely running a little leaner and hotter burn. Idle speed was turned up to offset the loss of power. At least for the first years, the reduced initial timing was offset by super quick mechanical advance.

Bottom line is there is no one size fits all.
However, the best practice is to use mechanical advance to compensate for the loss of time with rpm, and to use vacuum advance to compensate for leaner burns conditions. Contrary to what many beleive, the most efficient power at idle is a relatively rich burn. Concern about emissions meant that had to be compromised, and along with it timing. Only in the catalytic converter era did initial timing on some vehicles return somewhat to the pre-smog numbers.
 
Mattax said:
"Where it ought to be"
Click to expand...
upload_2019-10-11_9-48-24.png

from: The New Distributor (Session 136), the Master Technician's Service Conference

upload_2019-10-11_9-51-57.png

from: Ignition System Analysis, Master Technician's Service Conference 1969

Both Chrysler pamphlets are worth looking at.

For AFR at idle, note in the FSM's as well at the Master Tech booklets that they went from mid 13:1 to 14.2:1 for the '67 up CAP/CAS engines.

Also take a look at the AFR load relationships from Larew which I've posted before. For example here
Need Opinions. Have nothing to compare to.
or
How To Limit and Adjust Chrysler Vacuum Advance Cans
 
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In a relative nut shell...
Manifold vacuum will be high at closed throttle.
Ported will be high at open throttle.

Some older Ford's I have had had both going to the advance canister. And it kind of averaged the two.

The CAP valve does some thing with both to slow the timming return to no vacuum, IIRC. There is a CAP factory manual on imperial club web site again IIRC
 
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IIRC ported was for emissions. But whatever the distributor is set up for would work best.
 
Dana67Dart said:
Some older Ford's I have had had both going to the advance canister. And it kind of averaged the two.

The CAP valve does some thing with both to slow the rimming return to no vacuum, IIRC. There is a CAP factory manual on imperial club web site again IIRC
Click to expand...

Some Volvos had that same push/pull distributor pot, great for turbo builds.
 
Kendog 170 said:
Okay, I've always used ported vacuum to dizzy on all my Mopars. I want to go through my Dizzy on my 340 /4 spd. After boring 20 over , basically stock with the exception of Eddy heads. Then recheck my RB 500 with A/T next year. I was reading this in (How to Article) and there saying to run from manifold to Dizzy ? I know it would take a lot of re-curving , weights,canister,Timing ect. I just want to know the opinions of this. I guess I'm old school but I value my knowledgeable friend here on FAB. My Ragtop 340 in particular needs a better fine tuning for sure and I want to do this myself. They claim idling it will run cooler which could help my RB 500 in traffic. I'd like to use my canisters on both for better mileage when cruising. Here's the article.
Everything you wanted to know about ignition advance
Click to expand...

That is a question like what kind of oil is best.
Most racers use manifold vacuum. (if advance is not locked out)
The BIG difference mechanically is that with manifold vacuum the advance is "All In" at idle and drops back the advance as more throttle is given.
With ported vacuum full advance comes in as the throttle is opened and drops back as more throttle is given.
 
Tanks guys. I think I'll stay ported with the 340 so my dizzy adjustments will be minimal. As for the 500 I have two dist. to play with. Maybe set one up for full and one for ported and make my decision from there .
 
pishta said:
IIRC ported was for emissions.
Click to expand...
A fallacy I believed for a long time. Then I started going through old FSM and looking at pre-smog engines.
TrailBeast said:
Most racers use manifold vacuum. (if advance is not locked out)
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LOL. Most racers I knew used no vacuum advance at all. For drag racing, its one more thing to deal with - and its not needed.
TrailBeast said:
With ported vacuum full advance comes in as the throttle is opened and drops back as more throttle is given.
Click to expand...
Good description!
Diagram showing how that's done from the same '69 MTSC page as linked above.
upload_2019-10-11_10-13-43.png


Dana67Dart said:
Some older Ford's I have had had both going to the advance canister. And it kind of averaged the two.
Click to expand...
The mid 80's AMC jeep engines also used an non-linear valve (NLVR) that mixed ported and manifold vacuum while the engine was warming up. At normal operating termperature, it used ported. If coolant temps got too high at idle, it would switch to manifold. These were cat equiped engines and initial timing on the 360s were 10 to 12* at 600 to 650 depending on which year you look at.
 
Kendog 170 said:
Tanks guys. I think I'll stay ported with the 340 so my dizzy adjustments will be minimal. As for the 500 I have two dist. to play with. Maybe set one up for full and one for ported and make my decision from there .
Click to expand...
When you have time, measure out what you've got. Get a light, a tach and timing tape. Check from as slow as the engine will turn 'til its stops advancing. Then decide what steps to take next.
 
For example, if the timing measured indicates the distributor has long primary advance, intended for an initial timing of 0 or -5* then using manifold vac would worth trying. OTH if the timing shows a curve where initial can be set at 10 to 15*BTC around 600 - 700 rpm, and timing end up in the 30- 35* range around 3000 - 4000 rpm, then ported is fine.

All testing done with the vacuum advance hose plugged of course.
 
Mattax said:
A fallacy I believed for a long time. Then I started going through old FSM and looking at pre-smog engines.
Click to expand...
I guess a GM engineer believed differently:

"..After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it..."

Now, what Mopar did with this prior to air injection (they were late to that party) is another fork of reasoning.
 
The one thing I've found (and I haven't fooled with this in years...but if I ever get my distributor machine up and running I'll test it again) was when you are running ported vacuum advance with anything much over 230 at .050, or you are running close to the limit on CR for your combination or both, using ported vacuum you can't get the rattle out at tip in.

IOW's, when you are at a cruise and you just touch the throttle you get a rattle. I cleaned it up a bit with manifold vacuum, but wrote it off at the time that the mechanicals of the VA unit can't possible react quick enough to pull timing before you run into rattle.

At this point, I don't give up CR just to run a VA, but again, I may change my mind if I have a distributor machine here to test with, and to test full ignition systems.
 
pishta said:
I guess a GM engineer believed differently:

"..After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it..."

Now, what Mopar did with this prior to air injection (they were late to that party) is another fork of reasoning.
Click to expand...
And whoever he is, he's absolutely wrong.
That became pretty obvious when looking at what was actually done. Go ahead - I'm sure you have some 60s service manuals and so forth. We can believe some guy who may or may not have have been involved in ignition system development or we can believe what we read in original documents and see on original vehicles. You already know my conclusion on that.
There were other methods used. I think was Ford who tried a venturi vacuum based advance mechanism (40s?) and that's not the only one.
 
yellow rose said:
The one thing I've found (and I haven't fooled with this in years...but if I ever get my distributor machine up and running I'll test it again) was when you are running ported vacuum advance with anything much over 230 at .050, or you are running close to the limit on CR for your combination or both, using ported vacuum you can't get the rattle out at tip in.

IOW's, when you are at a cruise and you just touch the throttle you get a rattle. I cleaned it up a bit with manifold vacuum, but wrote it off at the time that the mechanicals of the VA unit can't possible react quick enough to pull timing before you run into rattle.

At this point, I don't give up CR just to run a VA, but again, I may change my mind if I have a distributor machine here to test with, and to test full ignition systems.
Click to expand...
Well you know who does have Sun machine up and running. Should give him call. You know he's not a guy to give up on a challenging situation!
 
Mattax said:
And whoever he is, he's absolutely wrong.
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I should clarify what I mean that he's absolutely wrong.
There probably were pre-smog era engines where manifold advance was used. But he is wrong that it was universally used, and he's wrong it changed with the introduction of emissions requirements. And it's certainly not a 'band-aid'.

For example, from the same 1959 published Chrysler Master tech book linked above.
upload_2019-10-11_21-3-2.png


I don't have any early GM books here, but I'll see what I can dig up.
 
Mattax said:
Well you know who does have Sun machine up and running. Should give him call. You know he's not a guy to give up on a challenging situation!
Click to expand...


Yeah, I know. I'm thinking I ought to call him, drag this thing I have over to him and let him look at it. Before I bought it, I called him and talked to him about it. He thought it was a great machine. I'm sure it is, but the electronics are way WAY above my pay grade.
 
yellow rose said:
Yeah, I know. I'm thinking I ought to call him, drag this thing I have over to him and let him look at it. Before I bought it, I called him and talked to him about it. He thought it was a great machine. I'm sure it is, but the electronics are way WAY above my pay grade.
Click to expand...
That guy at Paramount is pretty helpful with Sun machines- at least was when I called for help on my friend's.
I was thinking you'd just take the distributor over and run it on his machine and get his thoughts on setting it up.
You're right about things changing with longer duration cams. At idle, vacuum is lower than stock. Then coming off idle at light throttle vacuum rises, but hard throttle it of course drops more. So maybe that might have something to do with the issues you've observed?
 
Mattax said:
I should clarify what I mean that he's absolutely wrong.
There probably were pre-smog era engines where manifold advance was used. But he is wrong that it was universally used, and he's wrong it changed with the introduction of emissions requirements. And it's certainly not a 'band-aid'.

For example, from the same 1959 published Chrysler Master tech book linked above.
View attachment 1715407094

I don't have any early GM books here, but I'll see what I can dig up.
Click to expand...
Crazy you mention that but my original HP273 AFB carb only has one vacuum port for the distributor. Manifold vacuum. I should snap a picture tomorrow.
 
Mattax said:
That guy at Paramount is pretty helpful with Sun machines- at least was when I called for help on my friend's.
I was thinking you'd just take the distributor over and run it on his machine and get his thoughts on setting it up.
You're right about things changing with longer duration cams. At idle, vacuum is lower than stock. Then coming off idle at light throttle vacuum rises, but hard throttle it of course drops more. So maybe that might have something to do with the issues you've observed?
Click to expand...


I called and talked to the guy at Paramount. I forget his name. I'm horrible with names. He hooked me up with the company he uses to get his meters rebuilt. Damned expensive stuff.

At one point, I had the machine working with points, but I've yet to get I to work with anything else.

It's an Allen 22-250. I'm sure Tuner could get me fixed. I need to call him.
 
toolmanmike said:
Crazy you mention that but my original HP273 AFB carb only has one vacuum port for the distributor. Manifold vacuum. I should snap a picture tomorrow.
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Yes, do. What year? I'll check my '67 FSM for that carb.

I have found the '64 Tornado 230 used manifold vac. At least that's my conclusion from the distributor specs in the FSM.
However it did have PCV. ;)
 
yellow rose said:
I called and talked to the guy at Paramount. I forget his name. I'm horrible with names. He hooked me up with the company he uses to get his meters rebuilt. Damned expensive stuff.

At one point, I had the machine working with points, but I've yet to get I to work with anything else.

It's an Allen 22-250. I'm sure Tuner could get me fixed. I need to call him.
Click to expand...
Maybe make your own magnetic pickup amplifier. I think Tuner posted or PM'd me that can be done with a good genuine GM HEI module. It would need a power supply but probbaluy not too hard to figure out.
 
Mattax said:
Yes, do. What year? I'll check my '67 FSM for that carb.

I have found the '64 Tornado 230 used manifold vac. At least that's my conclusion from the distributor specs in the FSM.
However it did have PCV. ;)
Click to expand...
1966 #4119S AFB
 
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