Adjusting Vacuum Advance Cannisters

[grouseman]

As we all know, most of our Mopar vac advance cannisters are adjustable for vacuum sensitivity, using an allen wrench through the vac. nipple.  If it has a hexagonal shape to the body, it's adjustable.  We can adjust the spring tension for more or less response to engine vacuum.  Turning it clockwise ('tighten') will REDUCE spring tension so advance is very responsive to even low vacuum levels.  CCW ('loosen') compresses the spring, meaning you need high vacuum conditions to get full advance.  This spring adjusts only responsiveness to vacuum, nothing to do with the total amount of degrees available. 

Before monkeying with anything, find out what your setting is currently.  Record how many CW turns it takes to 'tighten' it all the way.  This will pull the spring bearing plate all the way forward towards the head of the screw, and will positively stop.  (If you turn it CCW, you will never feel a 'stop' because the bearing plate is essentially running off of the end of the adjusting screw.)  Turn it back CCW to the original setting.  Now chart the vac. advance curve at your original factory setting.  Tools: timing light, degreed damper, vacuum gauge, tubing, and a T fitting.  All you are doing is plotting how much advance is coming in at various vacuum levels.  Hook up the vacuum gauge in a T with the cannister, and a long section of tubing so that you can apply vacuum to it (either orally or with a vac pump).  Apply 4" vacuum, record how the timing has changed from Static.  Do this every 4" of vacuum until it stops changing.  Sample below:

Vac reading   degrees advance
0                         0
4                         0
8                         4
12                      10
16                      15 
18                      18 
20                      18   

Now do this all over again, once for 2 full turns CCW, and once for 2 full turns CW.  Or more/less if you like.  The idea here is to quantify the relationship between spring turns and the resulting change in the curve. 

Vac              2 turns      2 turns
Reading          CW         CCW
0                     0              0
4                     1              0
8                     8              2
12                   15            6
16                   18            12
20                   18            16 (this one is so tight that 22" was needed to get full advance)

You may need to use smaller increments of vac. to get a better picture of the curve.  Graph it all up, and you can see exactly how things respond. 

So now you have the ability to adjust the sensitivity of timing to your engine's needs.  Turn the adjusting screw as needed to optimize your timing but NOT get any rattle during any throttle position or driving conditions.  You really need to have a vacuum gauge hooked up to read it while you're driving to analyze things. 

The amount of advance is controlled by (if I'm not mistaken) metal tabs on the arm.  If you want MORE advance, you can file them down.  If you want LESS advance, you need to shim them somehow.  And there should be a number stamped on the arm indicating the number of degrees of advance.  (I gotta check this over; I may be confusing car makers here).  And don't forget that different applications would have different amounts of total advance.  A 1976 318 will have very different specs from a 1968 340.  I know these specs are in the old Chilton manuals, so I'll bet they're on the 'net as well. 

Now it's up to you to play with the spring preload, and alter the amount of timing to see what your engine responds to.  A nice way to spend a Sunday. 

grouseman

[Chryco Psycho]

not entirely true , if you turn the screw all the way counter clock wise & apply vacuum to the canister the advance plate will not move so yes it does limit the total amount of advance available

[grouseman]

Good point.  You could put a large preload on the spring, so that you only get a few degrees advance at your highest vacuum levels. 

[Chryco Psycho]

actually the screw as it is backed out [counterclockwise] will hit the housing & prevent the vacuum from doing anything 

[Ghoste]

If you have the vacuum advance dialed in like this, is it still beneficial to run without it at the dragstrip? 

[grouseman]

There is the vacuum diaphragm, the spring, the hexagonal spring bearing plate, the adjusting screw, and then the vac. nipple.  The adjusting screw is threaded normally, and is threaded into the hexagonal plate like a nut.  As you turn the screw clockwise (tighten) the head of the adjusting screw can't move, but the hexagonal plate moves towards it, reducing any compressive preload on the spring.  Fully CW will put the least preload on the spring, and make it more sensitive to vacuum changes. 

If you turn it all the way CCW (loosen), the hexagonal plate goes further away from the head of the adjusting screw and compresses the spring as much as it can.  Now it needs a ton of vacuum to get that spring to move, so very little vacuum advance will come in. 

Most street applications will be in the middle somewhere. 

Next time you come across a junked vac. cannister, take it apart and see what's inside. 

If you're at the drag strip, unplug it, it's too simple to do anything else.  It's one less variable to worry about. 

[Chryco Psycho]

If you have the vacuum advance dialed in like this, is it still beneficial to run without it at the dragstrip? 

Makes no difference , it will not move the advance plate plugged in or disconnected

[Ghoste]

If you can tolerate, one more dumb question then, what is the advantage of NOT using one in a performance car?  I know the cruise condition-fuel economy thing for a street car but if it isn't affecting the street/strip performance...?

[grouseman]

A drag strip-only car basically sees wide open throttle only, which is a very low vacuum condition.  The vacuum advance cannister would never be activated.  All it is, is one extra part to not have to worry about, so they remove them.  They are not GAINING performance by removing or disconnecting the vacuum advance line, they are eliminating something that's not needed.  Like power steering or power brakes, or cruise control, or electric windows.  People buy a new 'race' distributor without the vac, thinking they are GAINING performance because it's 'just like on the drag cars'.  Nope. 

But if you drive on the street, vac advance does wonderful things for you.  Keeps plugs cleaner, better fuel economy, engine runs better and cleaner.  If your engine is developing any level of vacuum while cruising, you should be using it.   Tailor it to your engine, of course. 

[dodge freak]

Well I run about 38 degrees of advance and no vacuum. The dist has very light springs and the advance is in by 2500 and stays in till it drops below 2000 rpms. I can time it by ear, rev it to 3500 or so and keep turning the advance up until the motor gets a little rough and starts to slow down. I can see the motor start  shaking then the  rpms start to drop, I back it off till the motor is smooth running and stop, check it with the timing light and its right about 38 degrees. I tryed the vacuum afterwards and could feel the motor get rough a bit. Maybe it could take 3-4 more degrees and it be ok but it would not make much of a difference I can't believe. As a bonus I can use the pick up coil plates for the lean burn motors that are one piece so my timing is rock steady. If your spark plugs are not staying clean you need a stronger spark box, I have the MSD 7 al now with the macthing coil , motor idles fine even when cold with no choke- holley hp- and motor runs the same all the time , everyday. MSD claim the 7 would not be any better than the 6 but I notice a difference, my 6 box was 8 years old but if you have no smog police to worry about I would just go with the 7 al , it is a better box. If your motor pings with the light springs in the dist. when you floor it and can not use both of the light springs, so the advance is not all the way in till 3500 rpm or so, well then the vacuum advance would be a good thing to use, that way you would have more timing at light throttle and not get any pinging when you step on it.

[grouseman]

I assume you mean 38 degrees TOTAL, rather than initial.  What are your initial and mechanical amounts?  12 & 26? 

The problem I see with what you're doing is that your 3,000 rpm is a no-load, relatively lean condition.  That is very different from a rich, wide-open throttle situation, and the air/fuel mixtures will be very different.  Rich mixtures need less advance than lean ones, and you're adjusting timing to a lean mixture.  38 degrees Init & Mech is probably bang-on for your engine, but I just don't understand why you're doing it that way.  Can you explain? 

[dodge freak]

Well thats a good point, the power valve is close when I set it, but I don't have a way of put a load on it. Yes thats total 38, what it is at idle I don't know oh care, with my 3.91 gears its at 1500 rpms going 25 mph though my sub.  It cranks fine to start it, I do know MSD makes a computer that can control the rate of advance and can even retard the timing at high rpms, but it seems fine and MSD has enough of my money already. Maybe I should back it off a tad and recheck it by driving it, but its so hard to tell what way is better. Maybe next year I look in to the timing controls MSD has. The motor might like 35 or so at idle but it would kill my starter, but of course MSD has a start retard that backs off the timing 20 degrees or what ever you set it at.   This is how Smokey Yunick in his book Power Secrets said he sets it, but his is in all the way at 1500 rpms and they run the engine to 7500 rpms and set the total advance and thats all there is to it.  Been setting it this way for 10 years and did play around with it at first, by running more and less but since I had my motor rebuilt in 2001 I have just left it there. Not even sure if its 38 degrees but that-at least when its lean likes it. I do still have the stock balancer so the timing marks might have move a bit. That book Power Secrets is for racing motors but there is some stuff you can use for everyday motors. I have the first ed. that came out in 1983 not sure if its been updated, I know he past away a few years ago.

[dodge freak]

I just look in my Summit cat. a MSD adjustable timing control is $232, so yeah not this year- I am going though $40 bucks a week on gas, it stinks. Was looking at that Power Secrets book again, Smokey likes the water temp. to be at 200 degrees, claims the Chevy motor will make more power all the way up to 220 degrees but thats pushing it. Theres a few other things that others do different. He likes MSD stuff but runs .035-.040 gap for the plugs, says there is no power gain after .040 and all the bigger gaps do is make missfiring and crossfiring more likely. Like I said its for racing and its all Chevy but still its worth looking at.

[Chryco Psycho]

actually at wide open throttle you have a very high ported vacuum which is where the dist should be connected if using vacuum advance but overadancing at WOT is not good & will cause misfiring which I s why I leave ti disconnected & time engine exactly the way Freak described

[firefighter3931]

actually at wide open throttle you have a very high ported vacuum which is where the dist should be connected if using vacuum advance but overadancing at WOT is not good & will cause misfiring which I s why I leave ti disconnected & time engine exactly the way Freak described

Exactly ! Too much advance at higher engine speeds will bring on detonation....an engine killer if there ever was one. The reality is that at higher engine speeds....the spark lead requirement actually diminishes. 

Lots of racers use a high rpm retard to save their engines from detonation. 

Big cams with long duration and short overlap respond well to lots of base timing and shortened mechanical advance curves from my experience. I allways disable the vacuum advance.

Ron

[grouseman]

I dug around my files and found this from another fellow on another forum for a looooong time ago.  I've had to trim some extranneous parts to get it to fit here. 




As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

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.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don't work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will "dither" in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8" of vacuum, so there is no variation in idle timing even with a stout cam.

[firefighter3931]

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren't fully-deployed until they see about 15" Hg. Manifold vacuum, so those cans don't work very well on a modified engine; with less than 15" Hg. at a rough idle, the stock can will "dither" in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15" Hg. of vacuum at idle need a vacuum advance can that's fully-deployed at least 1", preferably 2" of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8" of vacuum, so there is no variation in idle timing even with a stout cam.

That pretty much sums up the problem with vacuum advance and bigger than stock cams. Not too many guys have in excess of 15in at idle after a new bumpstick/manifold/headers etc... has been installed.  Low vacuum creates surging/sputtering with the can hooked up as described above. That is why i allways disable the stupid thing and just tune the curve after setting the base timing. 

Perhaps on a "stock" engine the vacuum advance can be made to work but i've never had anything that is bone stock....what fun would that be 


Ron

[dodge freak]

If you want stock , you are better off buying a new car.  Buy a new car-Gosh what did I just say??

[mikepmcs]