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Discussion Starter #1
I have to say, vacuum advance and were it gets connected has been one of the most confusing topics I have ever come across. I have always been reasonably sure that it gets connected to the full vacuum port (not ported) for a long time. I see all kinds of posts on forums but, now I see this on Chevy High Performance..

Now, is Chevy High Performance correct or am I confused again?
http://www.chevyhiperformance.com/howto/97438/photo_09.html
 

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Chevy High Performance is not correct when they state "the vacuum canister must be connected to ported vacuum."

Too bad they didn't have us proofread the article first.

Here's the pertinent paragraphs from Vacuum Advance 101:
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. 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.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
 

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Discussion Starter #4
Yeah, I read that and seen much of the same statements on the web. But I came across the one on the Chevy High Performance site which states totally the opposite. :confused:
 

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I'm cut and pasting this in response to your PM question about maximum vacuum advance... (and to enlighten others):

Lars Grimsrud said:
Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees - NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you're talking to is referring to the distributor degrees, or if he's talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet,
will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you "see" with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of
total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.
I don't know for sure what's the highest advance can available. The advance can PN chart lists a few with 15 degrees (which is 30 at the crank)
example: VC1865 AR34 1973-74 350 Vette Special Hi Perf ~15 degrees @ 8.5-11.5" Hg.
If someone knows of anything more than that let us know.
 

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I've read a ton of posts and articles about this subject because I've had questions as well. I was at a pre-49 street rod show yesterday with around 200 or so cars and I looked at every one of them. Almost every one had an Edelbrock Performer carburetor on it and almost every one I saw had the vacuum advance hose hooked to the timed ported side of the carb. There were a hand full that had it run to manifold vacuum port side. Edelbrock instructions that come with these carburetors say to use the manifold vacuum port for non-emission engines so who knows.
 

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Once you touch the gas pedal both ports run the same so it's just a question of the engine liking the extra advance at idle. Most do. With some cars just changing from ported to manifold vacuum speeds up the idle enough to let you close the throttle enough to put the transfer slots back to their proper position. It's an emissions thing. Can also be a crutch for the wrong or misadjusted vacuum advance can.
 

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Discussion Starter #11
Once you touch the gas pedal both ports run the same so it's just a question of the engine liking the extra advance at idle. Most do. With some cars just changing from ported to manifold vacuum speeds up the idle enough to let you close the throttle enough to put the transfer slots back to their proper position. It's an emissions thing. Can also be a crutch for the wrong or misadjusted vacuum advance can.
I have to disagree, Ported vacuum does not act like manifold vacuum under any condition. Both under acceleration do not pull vacuum the same. Ported vacuum is for emission equipped cars only.
 

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If you want to understand more abut what vacuum does in the engine, install a vacuum gauge in your car.
You can also get better mileage if you learn how to use it.



The one rule is most just can't get there head around the difference in ignition timing.

Initial timing, total timing. Advance timing!

Some get freaked out when they set total to XX° and then hook up the vacuum advance and see it go off the chart!

______________________________________________

Initial timing is a setting to allow the engine to crank.


Total timing is when the mechanical advance in distributor met it's max. It is regulated by RPM, the springs and weights control the RPM & amount of XX° when this is achieved.


---------------------------------------------------------------
All my reference to timing xx° is at the crankshaft measured with a timing light. REAL WORLD CONDITIONS. not the dist angle:eek:

---------------------------------------------------------------

Vacuum advance timing is controlled by engine vacuum and the vacuum control itself. Most standard vacuum controls give between 8°-16° more timing.

Vacuum in the engine is very sensitive. And a great tunning tool. You can evaluate engines condition with a vacuum gauge.

Engine vacuum greatly changes when engine load is changed and throttle position. It is very sensitive to these conditions.

The main thing when playing with your timing light, you can't be under the hood while placing a load on the engine. Unless you have a wheel DYNO!!!:eek:

This is why you can't see how it works.

vacuum advance operation basics.

When the engine is under a light load "At cruise" vacuum is high in the engine and the advance is applying more timing.

When the engine is under acceleration or heavy load the vacuum drops and the vacuum advance stops applying timing in hole or part.

-------------------------------------------------------------

Adjustable vacuum advances.

the main purpose is a application of a engine compression being higher than the fuel octane ratio. So the vacuum advance puts Example: XX° timing is applied at engine cruise and you get spark knock, maybe during partial throttle.

One thing you hear, my car is spark knocking going up a hill, I stomp the pedal and it goes away! this is a prime example of a vacuum advance putting to much timing. So you get a adjustable unit to reduce the amount of timing.

This is a very common in a 9.5 higher engine with overdrive or low gears. remember engine load is a important factor. gear ratios play a important part, vehicle weight, you can even have a problem when "FAT AL" rides in your car!!:eek:


______________________________________________

Also understand there is no magical Timing number!

Every vehicle is different. Where you live plays a important part. Elevation, close to the ocean.

Most go by this 36° total!
This is a good ballpark. But even in a stock engine. You can find a optimum setting for your conditions and use.

vacuum advances used before the unleaded days gave a average of 8°-12°

Later models car of the low compression days (80's) the advance could change as much as 20° to help the pigs go down the hwy!

Then came electronic fuel injection and engine controls to end these aggravating days!

I can really give you a head ache, I am going to lock down total timing, removing the mechanical advance and using the "Adjustable crank timing retard":cool: built into my Mallory Box:yes::)
 

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I too have had arguements to this very point and sometimes get confused as well. In my case what I've noticed with engines that have big cams(lots of overlap) that the engine needs alot of initial timing. Well thats fine but in some cases it causes hard starting or "grunting". But if you pull the timing back so it starts then the engine is lazy and pops thru the carb. That would be one arguement for having vac advance hooked up at manifold vacuum. Set it where the engine starts good and when it is running it pulls on the canister to advance the timing.

Now the flip side. I recently tuned a 67 el camino with a 383 stroker and it had a crane 510/[email protected]" cam on a 110LSA and I set up the timing both ways. With the advance hooked up to manifold vacuum I noticed when I stepped on the gas it hesitated and didn't rev up. I had the initial timing at 12* then hooked up the vac to manifold and it was at 25*. Idled good and could lean out the carb but driving the engine was sluggish at best. Then I switched it back to I had 16*initial and hooked the vac adv to ported vacuum. Now when I gave it some gas the engine responded and it pulls nicely. Very easy to drive and cruise nicely. No problems starting either.

I also have been told by my machinist friend who does alot of cylinder head work and she says she has seen lots of exhaust seats and valves taken out by running vac advance while cruising. My take is too much advance and causing a lean condition. I'm sure the gas we use out here doesn't help either.

So bottom line its finding the right balance for a driver. If it were a street/strip or race type deal car not driven alot I'd forgo the vacuum advance all together and get the centrifical advance dialed in so it comes in quick. High stall converters and low gears help it out too. Just my .02.

Oh one thing I left out was on my 76 GMC truck I spent hours trying to find the right balance of power, fuel economy and having the timing set to stock specs for Ca smog inspections. I used an adjustable vacuum advance unit and played with the weights and springs so it would be as responsive as can be without pinging and still get decent mileage, not start hard and pass the timing portion of the smog test. After dickin with it for hours and hours I found the best set up was the stock weights and springs and using the adjustable vac advance so I had 4* initial at idle, but it had 36* at 3,000 with vac advance hooked up. Centrifical advance was 24*. Once I passed smog I would bump the timing up alittle for some more power.
 

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If you want to understand more abut what vacuum does in the engine, install a vacuum gauge in your car.
You can also get better mileage if you learn how to use it.



The one rule is most just can't get there head around the difference in ignition timing.

Initial timing, total timing. Advance timing!

Some get freaked out when they set total to XX° and then hook up the vacuum advance and see it go off the chart!

______________________________________________

Initial timing is a setting to allow the engine to crank.


Total timing is when the mechanical advance in distributor met it's max. It is regulated by RPM, the springs and weights control the RPM & amount of XX° when this is achieved.


---------------------------------------------------------------
All my reference to timing xx° is at the crankshaft measured with a timing light. REAL WORLD CONDITIONS. not the dist angle:eek:

---------------------------------------------------------------

Vacuum advance timing is controlled by engine vacuum and the vacuum control itself. Most standard vacuum controls give between 8°-16° more timing.

Vacuum in the engine is very sensitive. And a great tunning tool. You can evaluate engines condition with a vacuum gauge.

Engine vacuum greatly changes when engine load is changed and throttle position. It is very sensitive to these conditions.

The main thing when playing with your timing light, you can't be under the hood while placing a load on the engine. Unless you have a wheel DYNO!!!:eek:

This is why you can't see how it works.

vacuum advance operation basics.

When the engine is under a light load "At cruise" vacuum is high in the engine and the advance is applying more timing.

When the engine is under acceleration or heavy load the vacuum drops and the vacuum advance stops applying timing in hole or part.

-------------------------------------------------------------

Adjustable vacuum advances.

the main purpose is a application of a engine compression being higher than the fuel octane ratio. So the vacuum advance puts Example: XX° timing is applied at engine cruise and you get spark knock, maybe during partial throttle.

One thing you hear, my car is spark knocking going up a hill, I stomp the pedal and it goes away! this is a prime example of a vacuum advance putting to much timing. So you get a adjustable unit to reduce the amount of timing.

This is a very common in a 9.5 higher engine with overdrive or low gears. remember engine load is a important factor. gear ratios play a important part, vehicle weight, you can even have a problem when "FAT AL" rides in your car!!:eek:


______________________________________________

Also understand there is no magical Timing number!

Every vehicle is different. Where you live plays a important part. Elevation, close to the ocean.

Most go by this 36° total!
This is a good ballpark. But even in a stock engine. You can find a optimum setting for your conditions and use.

vacuum advances used before the unleaded days gave a average of 8°-12°

Later models car of the low compression days (80's) the advance could change as much as 20° to help the pigs go down the hwy!

Then came electronic fuel injection and engine controls to end these aggravating days!

I can really give you a head ache, I am going to lock down total timing, removing the mechanical advance and using the "Adjustable crank timing retard":cool: built into my Mallory Box:yes::)
AMEN AL!!! ain't it the truth.
 

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I have to disagree, Ported vacuum does not act like manifold vacuum under any condition. Both under acceleration do not pull vacuum the same. Ported vacuum is for emission equipped cars only.

Ported vacuum does behave similarly to full manifold vacuum under certain conditions. The difference between the two being primarily at idle.

At WOT, both ported vacuum and full manifold vacuum behave exactly the same. They are zero. At mid throttle they also are similar. It's at idle where the big difference is. Full manifold vacuum has a high vacuum signal at idle while ported is virtually zero.

The reason you hear some guys claim that they see no real difference between the two vacuum sources is usually because they are running radical cams with low vacuum signals to begin with. The vacuum advance mechanisms they are using don't respond to their low vacuum signals so it really doesn't matter. They don't have any vacuum advance anyway no matter what and are essentially running their distributors as-if they were full mechanical.

That's why with a performance cam that affects the vacuum signal, you need to change the vacuum advance mechanism to one that responds to a lower vacuum signal like the performance one Paul listed or go get an adjustable one like Al mentioned.
 

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Discussion Starter #16 (Edited)
Totally agree. From what I have seen in playing around with my ZZ4. It runs better with full manifold vacuum. I run about 15 in of vacuum at an idle. No more hesitation off idle (like it was when I was using ported) and its more responsive. I just need to change out the mechanical stop to allow the mechanical advance to add 5 more degrees of mechanical advance. Its limited at 15 deg right now and I want to see how it does with 5 more deg at the higher RPM range.

I have initial set at 14 deg, vacuum adds another 15 at an idle.. Total right now is 32 deg with the mechanical limiter (MSD dizzy).. I have run with the vacuum locked out, 36 deg total with no problems. So, now that I have the vacuum setup and working, I'm going to allow the mechanical advance to add another 5 deg for a total of 37..

When I first installed the MSD, I screwed up and installed springs that were way to light and it was messing me all up.. The mechanical advance was coming in way to early. It was so bad, the mechanical advance was coming in at around 800rpm.. I did not realize that till Al made a statement about it and I checked deeper and found out.

I'm learning and getting it dialed in closer and closer..
 

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It sounds like you are on the right track. You may want to even try it at 38 degrees total and see what happens. That's where I run mine.

What you are taking the time to do is what separates the ho-hum engines from the all-out ones.
 

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I too have an adjustable vacuum advance canister on my HEI and I'm not sure I've got it right. My plugs are a nice caramel brown color and I don't get any kind of pinging when I mash down on it. I was getting some but I backed it off two turns. One thing I'm kind of wondering about is the advance springs. The distributor came with some medium ones on it and the papers that came with it say that at it advances 6* at 900 rpm, 8 - 9* at 1200 rpm, and 10-11* at 1500 rpm which is all it will do. Is 1500 rpm a little low to have full mechanical advance? Shouldn't it come in a little later on?
 

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Discussion Starter #19 (Edited)
You may get a few referent answers on this. My advice is this, you want about 12 - 14 deg initial timing with about 15 deg of vacuum advance max. Then the total should be about 36 - 38 deg (that's initial plus mechanical advance). With your dizzy adding mechanical of about 10 deg at 1500 is about right but, it should add a little more then that as you get close to about 3,000 - 4,000rpm. Just need to play with the springs and an advance stop if there is one.
 

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You may get a few referent answers on this. My advice is this, you want about 12 - 14 deg initial timing with about 15 deg of vacuum advance max. Then the total should be about 36 - 38 deg (that's initial plus mechanical advance). With your dizzy adding mechanical of about 10 deg at 1500 is about right but, it should add a little more then that as you get close to about 3,000 - 4,000rpm. Just need to play with the springs and an advance stop if there is one.
I've got 12* of initial advance but what's got me puzzled and I can't seem to find anyone else with this problem is that at idle I have no vacuum from the manifold vacuum port on my Carter carb. I do however have 13-14" at the rear intake port.
 
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