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Old 10-11-2019, 10:24 AM
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Default Second advance curve?

Anyone heard of using venturi vacuum, located in the smallest part of the venturi, for the ignition vacuum advance to add timing at acceleration?
An "ignition specialty expert" claims this is, per definition, the ported source for the vacuum advance.
As his statement goes against the very purpose of the vacuum advance, as i know it, i have to ask, maybe i missed something?
All i´ve seen regarding the ported ignition vacuum source, in just short of 40 years, is that the vacuum source is located just above the throttle blade, not in the smallest part of the venturi.
But, who knows?

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Old 10-11-2019, 11:11 AM
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Sounds like some one is thinking about big trucks and industial engines that have governors on them. Ones where venturi vacuum was used to pull timing out and control throttle opening. Distributors had two port vacuum advances. One port added and one port took away.

Seems like I can remember a few small foreign cars used a similar two port vacuum advance to control timing. And I do believe the venturi vacuum was labled spark retard.

Not a Pontiac car thing. But might have been used on big GMC trucks back in the 50's that were Pontiac powered.

Clay

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Old 10-11-2019, 11:34 AM
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I read through that Kenth a couple times as I was part of the discussion on modifying the vacuum advance units.

I can't find where he describes specifically about where to hook up to a ported vacuum source and small venturi but I am familiar with the dual port vacuum cans and how they push/pull. My 69 SCJ mustang had a setup like that from the factory. Weird deal but worked.

I do however know exactly what you're talking about. Getting back to a conventional single port vacuum can setup, If one wants to use a ported vacuum source, you have to be careful which one you use if there is more than one offered on a carb. In other words, you have to use the one that is just above the throttle blades if you want vacuum advance to work properly. The one just above the throttle blades only applies vacuum when the blades are opened, so no vacuum at idle, and will completely drop vacuum at full throttle, so no additional timing added at full throttle.

You for sure don't want additional timing added at full throttle. If you have your distributor setup to mechanically supply say....34 degrees total timing where the engine makes best power, and then have the vacuum advance throwing in another 10-12 degrees of timing at WOT, you're going to be picking up your pistons and rings laying in the bottom of the oil pan.

You MUST have the vacuum signal from either a ported source just above the throttle blade if you don't want additional timing at idle, and make sure it's not a source that supplies vacuum at WOT. Or use a manifold vacuum source. Either one of those won't supply vacuum at WOT and shuts down the vacuum advance.

Once the adjustable vacuum cans are modified as I described they work nicely at either port as long as the rest of the distributor is setup correctly. I find most drivability problems are related to distributors when people are chasing their tails around a carburetor.

It amazes me after 60-something years the amount of people that still can't seem to wrap their heads around a vacuum advance unit and want to run without one. Once setup properly there is nothing but benefits from using these things.

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Old 10-11-2019, 11:49 AM
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Well that minimum area does see the highest vacuum level once the throttle blade does open in terms of being the best spot for a ported vacuum source.

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Old 10-11-2019, 12:08 PM
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Quote:
Originally Posted by Formulajones View Post
I read through that Kenth a couple times as I was part of the discussion on modifying the vacuum advance units.

I can't find where he describes specifically about where to hook up to a ported vacuum source and small venturi but I am familiar with the dual port vacuum cans and how they push/pull. My 69 SCJ mustang had a setup like that from the factory. Weird deal but worked.

I do however know exactly what you're talking about. Getting back to a conventional single port vacuum can setup, If one wants to use a ported vacuum source, you have to be careful which one you use if there is more than one offered on a carb. In other words, you have to use the one that is just above the throttle blades if you want vacuum advance to work properly. The one just above the throttle blades only applies vacuum when the blades are opened, so no vacuum at idle, and will completely drop vacuum at full throttle, so no additional timing added at full throttle.

You for sure don't want additional timing added at full throttle. If you have your distributor setup to mechanically supply say....34 degrees total timing where the engine makes best power, and then have the vacuum advance throwing in another 10-12 degrees of timing at WOT, you're going to be picking up your pistons and rings laying in the bottom of the oil pan.

You MUST have the vacuum signal from either a ported source just above the throttle blade if you don't want additional timing at idle, and make sure it's not a source that supplies vacuum at WOT. Or use a manifold vacuum source. Either one of those won't supply vacuum at WOT and shuts down the vacuum advance.

Once the adjustable vacuum cans are modified as I described they work nicely at either port as long as the rest of the distributor is setup correctly. I find most drivability problems are related to distributors when people are chasing their tails around a carburetor.

It amazes me after 60-something years the amount of people that still can't seem to wrap their heads around a vacuum advance unit and want to run without one. Once setup properly there is nothing but benefits from using these things.
Can you post a link to that post?

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Old 10-11-2019, 01:04 PM
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Here you go:

http://forums.maxperformanceinc.com/...d.php?t=834694

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Old 10-11-2019, 01:35 PM
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I tried to understand some of what he was saying, but the guy rambles, jumps all over the place, and uses caps in weird places. It was all just hard to read. He may or may not have some valid points, but I couldn't glean anything out of it. Or maybe more accurately didn't have to patience to read it seventeen times and try to make sense of it all. So I don't want to say he was full of it, he may have very well been super knowledgeable, but I don't believe he got his point across very well.

He offered to send directions via email about something several times.

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Old 10-11-2019, 02:10 PM
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OK, well, if you can't follow a simple explanatio0n of the way things actually work, so be it, but, stand aside and let those that want to fix it right, get to the info, and stop trying to make it seem this does not work.

Now, our distributors have a mechanical advance, regulated by engine rpm's, advancing as rpm's increase, coming back down to a set stop point when the engine returns to its lower rpm operation. We should all agree on that FACT.

And, then, we have, well, most of us with common sense, have a device that will also add ignition timing, in one of two different ways, the vacuum advance.

The way our older engines used vacuum advance is to supplement timing to the engine when the engine was at no, to light loading operation, such as idle timing supplement, or, light cruise operation, to help with no to light load efficiency, idle quality, fuel economy and cooling assistance.

The second method that has been used for a vacuum advance is to operate it from a vacuum source that increases as engine rpm's increase, such as in emissions engines to make higher burn temperatures to burn off more un-burned fuels.

When a vacuum advance is run off a ported vacuum source, the advance degrees are added the same way a mechanical timing curve is added, upon acceleration, NOT at low to no load operation.

As far as what I offer with the vacuum advance modifications info, it is to replace a part that is not added to new replacement vacuum advances by their manufacturers, to bring the new part back into the specification it was supposed to have and doesn't, and to add a part that will not rot, or fall off, as the original ones did. Simple as that, so, if you either refuse to understand it, or, are pig headed enough to not even take a FREE look at the info, keep holding your head under the sand, and don't bother the rest of us, we want to fix the problems, not stop others from getting the information to help get those problems fixed, and, let them learn about it.

Vacuum in a ported source rises as engine rpm's do, and in doing so to operate a vacuum advance, an acceleration advance curve is utilized.

This ported vacuum advance curve added upon acceleration would be, simply, so even a person with limited mechanical ability can understand it, a SECOND ADVANCE TIMING CURVE, ADDED WITH THE RPM REGULATED MECHANICAL ADVANCE...UNDER ACCELERATION, NOT UTILIZED AS A NO TO LIGHT LOAD CRUISE ADDITION.

Simple as that.

Now, our engines do not need a second acceleration advance curve, if they do, fix the first acceleration timing curve, the mechanical curve, it is wrong.

Kenth, you are dead wrong, and you have ben aware of that for a few years now, ao, stop bashing me, and anyone else that tells it EXACTLY like it really is, and not YOUR wrong way. I, for at lest one, am completely and totally fed up sick and tired of the BS some of you push out especially at me, and towords some few others that know what and how it works.

Stop bad mouthing people that actually know how it works right, we few are actually attempting to wade through BS like yours, to help others get their cars running right.

Now, anyone that wishes to see just how vacuum works in a carburetor, google info from the net, there are a bunch of sites with diagrams of how vacuum is created, what type is created in the venturii of carburetors, go look for yourselves.


Last edited by David Ray; 10-11-2019 at 02:16 PM.
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Old 10-11-2019, 02:26 PM
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Quote:
Originally Posted by David Ray View Post
OK, well, if you can't follow a simple explanatio0n
Thank you for making my point

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Old 10-11-2019, 02:29 PM
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Quote:
Originally Posted by David Ray View Post
I, for at lest one, am completely and totally fed up sick and tired of the BS some of you push out especially at me, and towords some few others that know what and how it works.
This is where I get lost. You only have 27 posts. Have you been around here a long time under another name? Or are you talking about past discussions on another website?

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Old 10-11-2019, 02:59 PM
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Now I'm lost.

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Old 10-11-2019, 04:56 PM
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Maybe someone chin safety strap wasn't tightly secured and resulted in a whirling dervish uncontrollably flying around.

I've seen it happen.

Be nice Dave, these people here have been rather well educated. I've kinda grown to like being around em.

They ain't slow by any stretch of the imagination. They learn quickly and won't put up with anyone's crap.

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Old 10-11-2019, 05:23 PM
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Quote:
Originally Posted by Sun Tuned View Post
Maybe someone chin safety strap wasn't tightly secured and resulted in a whirling dervish uncontrollably flying around.

I've seen it happen.

.
Yes could cause some bruising of the brain...wow !

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Old 10-11-2019, 06:14 PM
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Quote:
Originally Posted by Sun Tuned View Post
Maybe someone chin safety strap wasn't tightly secured and resulted in a whirling dervish uncontrollably flying around.

I've seen it happen.

Be nice Dave, these people here have been rather well educated. I've kinda grown to like being around em.

They ain't slow by any stretch of the imagination. They learn quickly and won't put up with anyone's crap.
Yeah.......there was a thread here not too long ago about what each members profession was..............there's a load of VERY smart people here. I didn't respond to the thread, I'm a school bus mechanic...although I have enough hours for a BS degree....

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Old 10-11-2019, 07:24 PM
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I was just wondering David. I know some guys around here are pretty annoying but only 27 posts in and being frustrated must be some kind of new record

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Old 10-11-2019, 08:30 PM
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Dave Ray is known all over the automotive-forum internet, even if he's got minimal post-count on this particular site. He's the Dave Ray of Dave's Small-Body HEIs; taking points distributors and modifying them to work with magnetic triggers and HEI modules. He does damn fine work by reputation--although I've never held one of his babies in my own hands.




The only source of venturi vacuum I am aware of, on domestic automobile carbs made from the '60s onward...is the non-accessible port tied directly to the diaphragm that opens the secondary throttles on a Holley vacuum-secondary four-barrel (or the very similar port in the center carb of a vacuum-secondary Holley 3X2 system, which does have a nipple for a vacuum hose.) If there's truck carbs, or 1930's carbs, or some specialty carb, or European/Japanese/Korean/South African carbs with venturi vacuum ports, I don't know about 'em.

You can't USE venturi vacuum for distributor advance if there's no port to connect a hose to. So that's the end of venturi vacuum as a vacuum source for practically all vehicles, unless you're prepared to modify your carb--drilling holes, tapping-in vacuum nipples, and such. Possible, but inconvenient.

As for using ported vacuum on the distributor advance, at extremely low throttle openings, ported vacuum is different from manifold vacuum. As the throttle opens, ported and manifold vacuum become more alike. Ported vacuum doesn't get "stronger" as the throttle opens except at the very-low end of the throttle range. 'Course, some carbs have more than one source of ported vacuum; and those ports don't have to be the same distance above the throttle plates. I've seen EGR ported vacuum that remained "off" or "Low vacuum" longer than a ported vacuum for distributor advance.

At very low throttle openings, does the ported vacuum vary and thus create a "vacuum advance curve"? Sure. The throttle opening needed to vary the vacuum is going to be very small, perhaps uselessly so. I see ported vacuum as more of a "manifold vacuum that gets shut off at idle" sort of deal.

But what do I know? It's been forever since I've put two vacuum gauges on the same car--one connected to manifold vacuum, and the other hooked to the ported nipple, and compared them.

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Old 10-12-2019, 07:36 AM
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As far as the later Q-jets are concerned the source location for the correct ported vacuum advance is just a tad higher in the main drivers side primary bore than where they sourced manifold vacuum advance. Since they "cut" the slot horizontal it is fully exposed with the slightest movement of the throttle and provides FULL MANIFOLD vacuum to the VA right off idle and it will read the virtually the same as any other manifold source clear across the engines load/speed range.

You are also correct that there are other options for ported vacuum that will NOT show the same characteristics as the source location is either higher or they use a machined vertical slot to reduce the "pull" as throttle angle increased. Most of those ports were for EGR on the later engines.

If you hook up two vacuum gauges and run them to where you can observe the readings, one on the correct ported source and another on a manifold source located in the opposite bore just below the throttle plates (at idle) the readings will mimic each other at every throttle position, engine load or engine RPM except idle and coasting.

At idle and coasting the MVA source will show full manifold vacuum, the ported source at or close to zero.

IF there are or were other source locations for ported vacuum advance on a Q-jet that will show increases with engine speed and load beyond where manifold vacuum advance would I am not aware of them nor have I every seen one.

To this day it simply amazes me at how misunderstood this topic is, even by folks who are pretty well versed with tuning these engines. You'd think by now with all the good information out there folks would understand why your engine needs additional timing at light engine load for best efficiency.

I'll also say this while we're on the subject. I opened up my Saturday mornings in the past couple of years to correct running issues with troubled set-ups. Some of them have came in here from over 1000 miles away, and of course every local resource available to the owner was exhausted including all his beer drinking buddy's and local "gurus" who dabble at this sort of thing.

With most of these vehicles the owner and everyone else was blaming the carburetor, and most of them did have issues with them and they needed some help, but in reality the problems were more related to the distributor and timing issues instead. The biggest culprit were the installation of those cheap POS advance curve "kits" which seldom if every do what they advertise. Those parts, especially the ones for HEI's simply set the geometry up to provide too much leverage and springs not strong enough to where some of the curve is in at or below the engines targeted idle speed. This makes tuning difficult if not near impossible. Just about all of the vehicles brought here not running well also had the vacuum advance hose blocked off and running the mechanical curve only.

The FIRST thing I do when they come in is to remove the carb and fix all the issues there and make sure it has the correct amount of fuel to the idle system for what's it's being used on. I also remove the distributor and re-install the factory parts and use springs that will NOT allow any timing in below 900-1000rpms but a smooth stead advance curve from that point and all-in around 2800-3200rpms or so.

I re-install the correctly tuned carb and distributor and low and behold we can now set the initial timing around 8-12 degrees or so, total advance in the 30-36 degree range, and another 10-15 from the VA. 99.9 percent of the time this immediately cures any and all running issues with the engine provided the build is well thought out and camshaft selection pretty close for the CID, compression ratio and intended use of the vehicle.

The few combo's that I haven't been able to completely cure simply used poorly chosen components such as the cam too large or too small for the rest of the engine set-up. Even with those I can typically "crutch" a decent end result with custom tuning but typically we end up with considerably LESS power than it should be making if more thought were put into the parts selection during the engine build.

Kills me to this day that with all the good info out there, folks still make poor choices for these engines. That simply should NOT be happening these days.......FWIW.......Cliff

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Old 10-12-2019, 07:48 AM
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To add to my post above it's extremely rare to still see any rubber "sleeve" on a vacuum advance or the advance pin on an early type points distributor. The engineers meant well by adding them figuring the rubber would "cushion" things and eliminated any potential wear to the metal parts over the service life of the parts, but in actual use the rubber parts rotten away, fell off, etc and allowed for more advance from either part they were being used on. For the points units I install a custom made stainless steel pin, and may also MIG weld up the high side of the slot to limit the advance curve where we want it;.

For ALL HEI units I also MIG weld the top side of the slot and use a round file to provide a positive stop for the mechanical advance curve precisely where it needs to be. For most of these units I shoot for 10-11 degrees (20 to 22 at the crank and use springs, weights and center cam to provide an advance curve that does not allow any timing in at idle speed, but starts right off idle and all-in no earlier than about 28 degrees and no later than 32-34 degrees. I also modify or select a VA can that starts around 8-9". I seldom use any sort of "adjustable" VA cans as there is still a decent selection of the stock type that are already where we want them to be or I can get there with a quick spot of weld from the MIG and a few strokes with a round file.

Making some sort of "slide-stop" is also very effective if you have the time to do it, and it should be on the stop side of the VA not on the low side like the Crane screw mounted cam does...IMHO.

Anyhow, attached are a few pics of what we do here, and have been doing for coming up on 45 years now I've been contributing to this hobby, the last 16 years on a full time basis........Cliff
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Last edited by Cliff R; 10-12-2019 at 07:59 AM.
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