Switch from port to manifold vacuum advance

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Switch from port to manifold vacuum advance

Post  jones on February 2nd 2017, 2:35 am

Ok I'm confused about using port or manifold vacuum advance.

I am using a Duraspark distributor and port vacuum advance. If I run this distributor on manifold I get heavy pinging during no load highway driving. What changes have to be made to the factory Duraspark distributor to operate with manifold vacuum? Can I do these changes?

(PCV system is no longer being used)

From what I understand engines where designed to run with manifold vacuum up until the emission laws started. Lean mixture at cruising and idle take longer to burn requiring more advance timing. Ford changed to port vacuum when they had to start using EGR systems.

I have a Digital MSD 6AL2 that offers programable timing, should I lock the Duraspark out and use the ignition box? (tow truck)

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Re: Switch from port to manifold vacuum advance

Post  Carl on February 2nd 2017, 1:57 pm

Sounds like you need a slower vac advance curve, or less vac advance. If it's not adjustable on your distributor (not sure if there's an adjustable pot available for those?), port may be the quickest/cheapest/easiest way to go.

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Re: Switch from port to manifold vacuum advance

Post  DeepRoots on February 2nd 2017, 2:23 pm

jones wrote:
From what I understand engines where designed to run with manifold vacuum up until the emission laws started. Lean mixture at cruising and idle take longer to burn requiring more advance timing. Ford changed to port vacuum when they had to start using EGR systems.

I'm pretty certain that is not true.
Manifold vacuum is simply a bandaid for an improper distributor curve.

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Re: Switch from port to manifold vacuum advance

Post  jones on February 2nd 2017, 10:44 pm

DeepRoots wrote:
jones wrote:
From what I understand engines where designed to run with manifold vacuum up until the emission laws started. Lean mixture at cruising and idle take longer to burn requiring more advance timing. Ford changed to port vacuum when they had to start using EGR systems.

I'm pretty certain that is not true.
Manifold vacuum is simply a bandaid for an improper distributor curve.

I have heard countless professionals proclaim exactly opposite from one another. The gentleman's story I care to believe makes sense to me. I was just wanting to know what the timing curve look's like compared to using port / timed vacuum.

My high performance engine's timing is locked out so no worries there. The engine in question is in a tow / hunting truck no longer using a PCV system. I wanted to figure out how to adjust the timing curve mechanicaly rather than using the ignition box. From what I'm understanding I need less total timing for high vacuum situations?

Do you know if all of the Duraspark distributors share the same vacuum pot physical dimensions? (I haven't stuck a wrench in to see if its adjustable yet.)

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Re: Switch from port to manifold vacuum advance

Post  BBFTorino on February 3rd 2017, 3:11 am

Manifold vacuum is generally high at idle, so if you are using it that way, the timing is high, and when you start to accelerate, the high timing could cause the pinging (detonation) you hear.
That means the timing is too much for those conditions.
Ported vacuum will slow down the rate some. I've always thought of ported vacuum to be kind of a Band-Aid solution (remember Ford began using this method in the smog era 70's), but certainly not in every case.

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Re: Switch from port to manifold vacuum advance

Post  DanE on February 3rd 2017, 10:20 am

I have been observing this manifold/port vacuum source debate for the distributor advance canister for a long time, with each side advocating their views.

I have been running on the street, relatively high compression engines with relatively large SFT cams since 1960. And, this spring I will fire up for the first time a 502 BBF for the street and I will guarantee you that it will have manifold vacuum to the distributor advance canister.

The distributor advance curve will be determined by what the engine wants as it is run on the dyno, and the advance canister will be limited to about 8 to 10 degrees advance. Pulling the vacuum from the manifold facilitates a better idle and better part throttle response. At WFO, it is a non issue.

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Re: Switch from port to manifold vacuum advance

Post  tbirdmike on February 3rd 2017, 5:36 pm

vacuum advance has always been intended to run off an advance port on the carburetor.  The advance port in a carb isnt active at idle.   The port starts just above the throttle blades.  As the throttle is opened it causes vacuum which activates the port. As the blades open past a certain point the vacuum is lost and timing drops back down.  

Vacuum advance increases torque which helps off idle accel as well as covering the need for more timing to complete combustion at lean cruise.  Gasoline burns slower as it leans from 13.5 afr so the timing for milage and efficiency is needed.  

Motors that are built and have a high stall converter dont really benefit from this.

Now if the OP is rolling on the highway and getting knock, I would be checking total timing with vacuum advance hooked up.  Really shouldnt be going past 42-44 deg.
Definitely dont run vacuum advance off the intake or anything below the throttle blades.  
If the timing is out of control high then unhook the vacuum and check base and total timing to verify the timing curve in the distributor.  If that checks out then the vacuum advance needs to be limited.
As was said, duraspark has internal adjustment with an allen wrench.

If things on the timing aspect check out, I would question AFR going too lean, or it just simply needs more fuel octane to do what its doing.

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Re: Switch from port to manifold vacuum advance

Post  tbirdmike on February 3rd 2017, 5:41 pm

Just noticed you have a 6al2 box.
It would be simpler with this box. You could make changes easier to see what it likes.

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Re: Switch from port to manifold vacuum advance

Post  DanE on February 3rd 2017, 8:03 pm

One should not make positive statements about a subject that they obviously don't understand.



Posted by tbirdmike  ("Definitely don't run vacuum advance off the intake or anything below the throttle blades")
                               ("Vacuum advance has always been intended to run off an advance port on the carburetor")
These are erroneous statements.


Running vacuum advance from the manifold allows vacuum to advance the timing at an idle for a more complete combustion chamber burn which lowers EGT. The ported vacuum from the carburetor was an attempt to control emissions in the early stages of smog control. And there were a few adverse affects such as burnt exhaust valves. As to the subject of too much timing at high vacuum, there are canisters available that have less advance in them. Also, there are canisters that will activate at less vacuum than standard for those that are running a camshaft that does not pull as much vacuum as a non modified engine.

I am talking about modified engines with a carburetor.

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Re: Switch from port to manifold vacuum advance

Post  BBFTorino on February 3rd 2017, 9:54 pm

I prefer to run manifold vacuum, and limit the advance with an adjustable vacuum pod.

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Re: Switch from port to manifold vacuum advance

Post  DanE on February 4th 2017, 9:53 am

As to the OP's question of what mechanical advance to put in the distributor and how to do it, get in touch with

Scotty J. - AKA "THE MAD PORTER"
ReinCarNationHighPerformance
253 988 6648

Part of his business is setting up DuraSpark distributors. He will know what curve your combination will want. He will do it for you or his FaceBook page will instruct you how to put the proper curve in your distributor and what canister to use. Very Happy

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Re: Switch from port to manifold vacuum advance

Post  DeepRoots on February 4th 2017, 10:33 am

If you are recurving it yourself and getting it set up, you could design the advance curve for ported or manifold vacuum easily.

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Re: Switch from port to manifold vacuum advance

Post  DanE on February 4th 2017, 4:47 pm

DeepRoots wrote:If you are recurving it yourself and getting it set up, you could design the advance curve for ported or manifold vacuum easily.

I'm up to learning something new. I would like to know how you would easily get enough timing at an idle without having the starter kicking back with the ported vacuum source. Wink

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Re: Switch from port to manifold vacuum advance

Post  DeepRoots on February 4th 2017, 6:32 pm

My last build utilizes 24initial and 32 total + ported vac advance.  Full mechanical advance comes in at 2800rpms.
Nothing fancy, powermaster starter and a good battery.  Full power is available at the starter and a trigger wire off the solenoid fires it all.

Starts hot just fine here in the deep south.
Distributor is a stock Duraspark where I welded up the advance slot.
I could have done the same via manifold vacuum but chose not to.
I have a heavy car and advance just off idle with a load was more of an issue, I like the steadiness of only using mechanical advance in this instance as it is somewhat more solid and exact.

Many ways to skin a cat.  I'm sure if you want to run really high advance at idle it might be a problem, especially with a subpar starting system.
If you wanted to do it that way, better/bigger wires, better starter, a 16v system, or an ignition delay are all options.
I did it the way I did as it was the easiest way for me, your experiences or situation may vary..... like anything.

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Re: Switch from port to manifold vacuum advance

Post  The Mad Porter on February 4th 2017, 7:02 pm

When you buy a reman D/S distributor the mechanical curves are ALL over the place. The vacuum advance canisters offer 20+ degrees of advance to cope with EGR and lean part throttle cruise A/F ratios.
The OEM units are dual stage, typically offer far too much mechanical advance coming in too late. 4000+ rpm. I checked one truck distributor this week that offered 28 degrees mechanical @ 5,000 rpm lol.

Properly curving a D/S is fairly easy especially with a sun machine lol. On the vehicle it can be done but is a pain in the ass.

What is your initial timing?
Total timing and at what rpm?


Maybe this will help.

http://www.reincarnation-automotive.com/Duraspark_distributor_recurve_instructions_index.html



Your specific engine build parameters, cylinder head type, vehicle weight, gearing, stall etc etc will determine what advance curve, initial advance amount and vacuum advance curve are needed.
With out more information I can not offer an opinion.

When using a long duration cam with poor idle characteristics manifold vacuum can be used for the vac advance to clean up the idle requiring less throttle blade opening and less initial timing. The problem I run into doing this is when the dash-pot is set to limit advance it also raises the vacuum threshold required to initiate. In these cases I limit the canister with a pin to 10 to 12 degrees and leave the spring tension low to avoid an unstable idle with typically low idle vacuum.

I have never seen a D/S with out an adjustable dash-pot. If it has a 6 sided hex behind the vacuum nipple it IS adjustable. OEM dashpots in some cases had a number stamped on the arm denoting in distributor degrees the amount of advance offered. The remans are unpredictable.



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Re: Switch from port to manifold vacuum advance

Post  DanE on February 4th 2017, 7:18 pm

Thank you for replying DeepRoot. Your way would work, just a lot of initial on the crank.

On my 10.76 to 1 CR 502, I am going to start with 16 degrees on the crank with a 9 degree shoe in the distributor for a total of 34 degrees in at about 2800 RPM. To that I will add a 10 degree vacuum canister sourced from the manifold. That will give the engine about 26 degrees at an idle.
This is what the ignition will be when the engine starts on the dyno. I will adjust to what the engine wants.
I have learned over the years that sufficient advance at an idle lowers the EGT and makes a responsive low RPM engine on the street. This engine is going into a 73 Thunderbird with 3.73 gears.


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Re: Switch from port to manifold vacuum advance

Post  DeepRoots on February 4th 2017, 8:44 pm

The Mad Porter wrote:Properly curving a D/S is fairly easy especially with a sun machine lol. On the vehicle it can be done but is a pain in the ass.

Just requires burning yourself 12 or so times..... what's the big deal Razz

Dan, mine's 10:1 447ci FE with bbm heads, 242/250@ .050 .600 lift, 108/104 with 2x4 carbs. It's a fun engine in my 63.5 Galaxie, which has 4.30 gears and a GV overdrive. Probably my favorite car I've had (took me a couple years and too much money to build tho).
Oddly enough the distributor was a new unit, like $40, but seems pretty decent. I especially like the advance arms as they are held down with a screw instead of that annoying clip.

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Re: Switch from port to manifold vacuum advance

Post  jones on February 5th 2017, 7:46 pm

Thank you for responding.

At WOT vacuum advance doesn't matter as it's not suppose to be working. I always thought the vacuum advance was suppose to be used for high vacuum situations like idle & cruising. But the Vacuum advance is putting in too much timing in those situations.

My plan is to find what the engine likes on mechanical and set it. Then come back and add the vacuum and adjust the amount at why and idle.

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Re: Switch from port to manifold vacuum advance

Post  The Mad Porter on February 6th 2017, 12:27 am

jones wrote:Thank you for responding.

At WOT vacuum advance doesn't matter as it's not suppose to be working. I always thought the vacuum advance was suppose to be used for high vacuum situations like idle & cruising. But the Vacuum advance is putting in too much timing in those situations.

My plan is to find what the engine likes on mechanical and set it. Then come back and add the vacuum and adjust the amount at why and idle.


That is correct. At WOT or when manifold vacuum drops below the start point setting on the advance dash-port vac advance is irrelevant.
Fine tuning part throttle advance is of course always done after the fact...



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Re: Switch from port to manifold vacuum advance

Post  jones on February 6th 2017, 2:24 pm

Is all of the dash pots adjustable that come with the Durasparks? If not what is a good part or part number? Is there anyway to tell if it is adjustable from the outside?

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Re: Switch from port to manifold vacuum advance

Post  The Mad Porter on February 6th 2017, 2:58 pm

jones wrote:Is all of the dash pots adjustable that come with the Durasparks? If not what is a good part or part number? Is there anyway to tell if it is adjustable from the outside?



I touched on that in the last part of my first post. If there is a Hex behind the vacuum nipple it is adjustable. Allan size varies from 3/32" to 1/8".





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