Timed vacuum or manifold vacuum?

So what was the earliest Mopar to have ported vacuum to the distributor?

I disagree with post #7 and I don't care if Einstein wrote it. For clarification I believe ported vacuum is referred to as timed vacuum in this post. For a street car you want the ported port, which is really a manifold vacuum signal occurring above idle. I have experienced the timing retard using manifold vacuum as pointed out by A383.

Meep-Meep said:

I disagree with post #7 and I don't care if Einstein wrote it. For clarification I believe ported vacuum is referred to as timed vacuum in this post. For a street car you want the ported port, which is really a manifold vacuum signal occurring above idle. I have experienced the timing retard using manifold vacuum as pointed out by A383.

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Good for you. Try teeing in a vacuum guage to your vacuum advance line and drive your car around. Tell me how much vacuum you have. Especially on deceleration, talk about retarded timing. And retarded at idle too. And manifold vacuum doesn't "retard" when opening the throttle, it drops back to where you want it so you are not over advanced. Like I said. Set your car up any way you want. I dont care.

A383Wing said:

I had same issue with setting up my dual quads..same bucking & jerking at cruise and take off...turns out it was too lean

and vacuum advance only needed for acceleration & cruise...not needed at idle on a motor

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thanks for the tip. i did fatten the carbs up a little and it made little to no difference. what did you end up for jetting? the ported/timed vacuum port is cruising at 18", under 2000rpm, and will go to 20" on deceleration. emissions or no emissions i've never had an issue with the timed port, until now.

lewtot184 said:

thanks for the tip. i did fatten the carbs up a little and it made little to no difference. what did you end up for jetting? the ported/timed vacuum port is cruising at 18", under 2000rpm, and will go to 20" on deceleration. emissions or no emissions i've never had an issue with the timed port, until now.

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Lew,
Recurved dual point tach drive ,ported vacuum and runs like a clock.Distributor was done by Paul Petcou in Michigan,this is what he does for a living plus engine work for fast class racing.Attached are specs,cam characteristics also play a big part in timing,no two are the same.

What I don't understand is why you would the the variability of manifold vacuum on timing?

Figure out what you want for a base timing at idle and set the distributor to that. Then curve it to what works best.

Otherwise when the vacuum changes the timing changes and so does the idle which changes the vacuum.

Hemi Rebel said:

Good for you. Try teeing in a vacuum guage to your vacuum advance line and drive your car around. Tell me how much vacuum you have. Especially on deceleration, talk about retarded timing. And retarded at idle too. And manifold vacuum doesn't "retard" when opening the throttle, it drops back to where you want it so you are not over advanced. Like I said. Set your car up any way you want. I dont care.

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If your initial is set to 10 BTDC and you run manifold vac to your vac advance, which has another, say 10 deg in it, your initial is now 20 BTDC. Gently snap the throttle open and the mark will go backwards if the distributor weights can't compensate for the rapid snap back of the vac advance canister. This will likely cause a hesitation and a false symptom of requiring more pump shot. I personally don't want to band aid a stumble by dumping a quart of gas at every pump shot, which will only wash the rings down and waste fuel.

The vac advance can does not "retard" the timing but rather return it to the initial setting as it was timed by moving the distributor. With a ported signal you can only go advanced based on the mechanical curve plus any vacuum signal to the diaphragm. If no vacuum signal to the diaphragm then you default to the mech advance in the distributor, which has the basic timing curve necessary to run the engine under load. The latter is very important. The ported vac advance is brilliant since it cranks in a bunch more timing under light load thus improving efficiency. It's basically free gas mileage. As another twist on timing curves, my 72 Porsche 911 used a vacuum retard in another brilliant way. The timing was set to 5 ATDC with the retard can hooked to manifold vacuum. This was for emission reasons of course. But as soon as you blip the throttle the spring in the vac retard unit gave it a sharp five deg advance then the mech curve took over from there.

62MAX said:

Lew,
Recurved dual point tach drive ,ported vacuum and runs like a clock.Distributor was done by Paul Petcou in Michigan,this is what he does for a living plus engine work for fast class racing.Attached are specs,cam characteristics also play a big part in timing,no two are the same.View attachment 193101View attachment 193102

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i've been trying different centrifugal curves and vacuum canisters without much luck. i just came back from a road test with a full centrifugal distributor and the car drove fine. i'm using a 1406 carb for the primary and noticed from my driving test there was little difference in vacuum between ported and manifold at low speed cruise. i think the problem for me is simply too strong of a vacuum signal at the timed port. i tried a restrictor and it made no difference. i have also noticed that the location of the timed port on the holley 6-pak i was using is in a different location than the 1406. the 1406 port is right at the throttle blade and the holley is further up the venturi. maybe using ported or manifold vacuum may depend more on the carb than other factors. right now i'll have to say that neither ported or manifold is the best in all circumstances. this mess sure makes me love the 6-pak even more. anyhow, thanks for your info and chiming in.

anyone have an opinion on the "heavily cammed engine" and the ECHLIN dist. vac. adv. Mr. Hinckley talks about? Clearly a chebby PN. but does MoPar make one? Adjustable?


Right now I have that situation and am running off the ported vacuum side. I'll play with it this weekend.

Alot depends on the combo. Just about all stock Mopars used ported vacum advance from the factory. What many dont realize is ported or timed vacum is actually manifold vacum but its from a port above the throttle blades so it wont put any vacum to the vacum advance untill you step on the gas. Just like manifold vacum will still be fairly strong even around half throttle. At part throttle whether you use manifold or ported you will still have enough vacum to work the vacum advance. Manifold vacum is strongest on decell and then at idle. At wide open throttle manifold vacum is 0 which is the same for ported or manifold vacum. The only eng vacum signal that increases all the time with more air flow and rpm is venturi vacum but its not a real strong signal as thats why Mopar used a vacum amplifier on some EGR systems. Venturi vacum will work the vacum secondary on the carb but not much more. When emission problems came around in late 60's and on some manufactors used temp actuated advance control valves like Ford did. Its a 3 port valve in a coolant passage that uses ported vacum but if the car gets hot the valve switches the ported to manifold vacum to speed up the eng and turn the fan faster along with advancing the timing to help cool the eng down. When it cools down the valve moves back to ported vacum advance and the idle slows back down. Also many cars in the emission years used decell valves that would let the eng take advantage of high vacum on decell and these valves would advance the vacum advance timing on decell only as normal driving it had normal vacum advance. So the question is what do you use ?? Its really depends on the combo. If you have a dist with very fast mech advance as I do you may idle with 20 to 30 degrees of advance if you have a large enough cam and idle at 1200 so if you throw the vacum advance in at idle you may have up to 45 degrees at idle. Not what I would want but if you have a mild combo with 15 degrees mech advance at idle then throw the vacum advance in and have 30 at idle and it may live ok with that as you have to try it and make sure it dont ping at the hit of the throttle. And thats because with manifold or ported vaucum when you go right to half throttle the vacum drops some when you first hit the gas but will stabilize at half throttle and have a fairly strong steady cruise vacum signal even at half throttle. Once you go past half throttle you start loosing vacum and at full throttle its 0. Many people really dont understand how the vacum and ported vacum work. Ron

Heavily cammed you will need to crank in a lot of initial, maybe even lock it at full advance minus 2 degrees and let that heavy spring give you the rest over the RPM range. This will keep the weights under a load and prevent bouncing. Vac advance on a full lock out advance setup won't buy you anything IMO. If you don't need to lock it at full advance then maybe crank in 20 and limit the advance to give you the proper max advance and all in by maybe 2000 RPM. Ported vac advance might help you for cruising but beware of too much timing under load. The beauty of ported vac advance is the signal drops off as the throttle is opened more and more. Same with manifold vac but it doesn't affect your timing at idle.

I notice I am seeing more and more guys running locked out dist and even some do it on the street. But it does depend on your combo as what works best for you. Ron

383man said:

I notice I am seeing more and more guys running locked out dist and even some do it on the street. But it does depend on your combo as what works best for you. Ron

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If it's a heavily cammed engine, locked out timing will be a blessing. You'll love how much easier it is to tune the carbs by completely eliminating the nuisance of variable timing. These engines usually like a lot of timing at idle and will never need less than max power timing (34 degrees or so). They'll run much cooler and idle cleaner with all that timing added in at idle.

later today I'll pull the cam specs and check my initial timing and post.. see what you guys think...

Meep-Meep said:

Heavily cammed you will need to crank in a lot of initial, maybe even lock it at full advance minus 2 degrees and let that heavy spring give you the rest over the RPM range. This will keep the weights under a load and prevent bouncing. Vac advance on a full lock out advance setup won't buy you anything IMO. If you don't need to lock it at full advance then maybe crank in 20 and limit the advance to give you the proper max advance and all in by maybe 2000 RPM. Ported vac advance might help you for cruising but beware of too much timing under load. The beauty of ported vac advance is the signal drops off as the throttle is opened more and more. Same with manifold vac but it doesn't affect your timing at idle.

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good info, this is basically what i do. i never toss the heavy spring. i think the conflict i'm having is a engine that wants to idle at 24-26 degrees intial and a carb with the timed port at the throttle blades and small venturi; this combo just doesn't mix well. a shorter cam with a wider LSA and slower centrifugal advance would work fine. i also think a holley carb, which isn't possible with the intake, would work.

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2059 said:

anyone have an opinion on the "heavily cammed engine" and the ECHLIN dist. vac. adv. Mr. Hinckley talks about? Clearly a chebby PN. but does MoPar make one? Adjustable?


Right now I have that situation and am running off the ported vacuum side. I'll play with it this weekend.

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mopar vacuum units are adjustable. more info your engine is needed.

just got back from a test drive with a different combo using ported vacuum, (tried manifold vacuum and that didn't make out of the garage, lol!). i rtv'ed .055" spacer into the vacuum pod, (10 degree pod), to limit the travel. ended up with 48 degrees total at 15" of ported vacuum, (engine will cruise 20"). would like to have had 50-52 degrees but the thing just won't run with an aggressive centrifugal and vacuum advance. the new combo seemed work fairly good, will only need minor tweaking. i think the next step is to modifiy one these edelbrocks by moving the timed port up; looks like this is possible. straight manifold vacuum may work ok for low vacuum engines but i don't think it's the best on high vacuum engines. either has it's place but neither is "the" best for all combos.

pic of the new headache.

This has me all quite confused. I have read the blurb in post #7 twice....

Why would "timed" vacuum be considered an "emissions" compromise when my '62 (dual quad 300H motor) used it as well as every other old Mopar I ever owned? For lower emissions? I doubt it...

62 Dart Convertible said:

This has me all quite confused. I have read the blurb in post #7 twice....

Why would "timed" vacuum be considered an "emissions" compromise when my '62 (dual quad 300H motor) used it as well as every other old Mopar I ever owned? For lower emissions? I doubt it...

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i agree with you. i was looking at some pre-emissions cars with carter afb's, non-mopar, and they used the timed port that is in the same location as mopar emision and pre-emission carters, plus the edelbrock stuff. for my 2 cents, work with the timed port

ok, I dug up the cam card and double checked the timing.

Stats:
Mopar perf. 426 Hemi (built by Eddie Haines, if it matters)
dual quad cross ram
Manual trans
comp. cam hyd. roller .566I/.544E .280I/.288E Degreed +2
10* BTDC at 950 rpm (lowest it will go and still idle)
Mopar elect. ignition.
Vacuum adv. on the ported side.

I've only changed the oil since I got the car finished. In other words I haven't changed any components/settings of the ignition system since install. I have about 300 miles on it and it runs great. A little warm, one notch above the middle on the gage. Don't know the temp. but I have a 180* therm in it. I didn't bring in the dyno sheet but I remember my A/F ratios looked good throughout the pulls.

Opinions??

It will run cooler with some more timing at idle. What's the cam duration at .050 lift? Have you mapped the entire ignition curve? How much vacuum is it pulling and how much timing is the vac advance adding?