Timing and vacuum advance 101

I wanted to post a little something about vacuum advance on distributors. A subject earlier got me to thinking about it and I thought I would share some of this with you guys for discussion. Now...The below article was written by an engineer at General Motors. He makes a couple statements that I do not totally agree with, but in general, he hit it right on the head. Take some time, read it and then experiment with your own vehicle if you feel you have a good grasp of it. You will be please with the results if done correctly. This is not one of those things to say your car/truck/Jeep does not run right the way it is...but rather to make it optimum.....................




"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. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

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.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

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.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts."

 

I would argue some of your recommendation. Through the dyno work I did (also for GM) and several race cars, I came up with a different recipe for a street car. I tune the distributor to give the correct static base timing and a mechanical curve to provide the correct final timing at the correct RPM for best performance. I add in the vacuum advance, on ported vacuum, and limit it to provide the correct cruise timing. This has worked on many engines I've worked on.

One point I'd like to see expanded is your position that running severely retarded timing at idle with full manifold vacuum advance is better than correct static timing with no vacuum advance. For emissions and idle quality, the static timing value must be the same with either scheme. Applying manifold vacuum at idle means that the timing will retard even further as the engine initially accelerates. I don't see how that is an advantage under any circumstances, emissions or performance. I know that computer controlled ignitions do not perform this initial timing retard on normally aspirated engines, and I don't thik even boosted engines do this at idle, only under high boost conditions.

Not arguing a point, just interested in your opinions.

 

Well...as you may note, I said that I am not the author and I do not agree with everything the author wrote.
But...your first paragraph...
You know that ported advance and manifold advance has the same cruise timing as each other. There is not difference at all. At cruise, the ported vac is open, just like straight man vac. So no difference there. If comparing a full charge dyno run (I own a dyno) wide open throttle with port vac or with man vac is exactly the same thing, because there is no vac present at full throttle. overall full throttle total advance remain the same. You set up the dizzy the same (with vac unhooked). From there, you just hook the vac to a different spot.
I am sure many of us have done the following...time an engine by ear...get it to where it sounds it's best, crisp throttle response, smooth idle, etc. Then...when you shut it off, it would roll over because it's too far advanced. So, you retard it, you notice it may not be quite as crisp and smooth, but it starts. This is where man vac really helps. It will idle advanced like that...but when you go to start it, it's at a point where it will roll over until it builds vacuum.

This sort of ties to your second question, running it retarded initially makes it start a little easier, but as soon as it fires, the timing goes right up to where it should be (high). Now...when you get on the throttle and lose vacuum...yes, it retards the timing, but this is where your mechanical advance comes into play. The mechanical advance, advances the dizzy to the spot you set it up previously with no vac hooked up. So, if you wanted it to go to 36 degrees at full throttle, that is where it goes. Vacuum advance has zero to do with full throttle operation. You will notice no difference between port or manifold vac on a dyno run. Because vacuum is not present. Overall performance stays the same. It's just all the stuff in between that changes.

 

I just re-read my first post, thinking I must have missed something. I still did not see where this was mentioned. Can you highlight it for me, I would like to address it better. As I said, I don't agree with everything in the article, but I must keep skimming over that part. I can't find it.

 

I remember 70's Fords were notorious for the hard starting thing, heavily retarded timing did make sense on those (boy, that goes back a few years ) I do recall that many of the Windsor engines, even at 6 to 8 degrees static timing, the engine would buck back against the starter.

Just to throw out theory, mechanical advance should not start until around 1500 rpm, but I have seen mechanical advance start as low as 1000 to 1100 rpm on some engines. I have dealt with many severely worn distributors that did the timing shift on shutdown just as you describe. For anyone following along on this, if you have wandering timing at idle between engine starts, it's critical that engine speed is correct (to keep mechanical advance from starting) and that the distributor weights and springs don't allow the mechanical advance mechanism to move around uncontrolled while the engine is running.

 

Last paragraph, my term might have been too strong. I missed the part that you didn't write the article. I got interrupted about 10 times while I was reading it and skipped over that evidently

 

For a street/trail AMC...I like mechanical to come in at around 23 to 2500 rpm. But that is a preference dictated by what type of motor, cam, dynamic compression, etc that you are using.

 

Good article in general,I enjoyed following the conversation. The historical part of how the ported vacuum developed from the direct manifold vacuum is good reading and should help many understand the" lean mixtures need more spark advance " along with development of emission controls and differences in distributors.

Thanks for sharing, I think a link to this thread from the Density Altitude thread may be useful as Air/Fuel ratios do affect timing. Just a thought, In my mind they are related subjects.

 

Yeah, it's pretty right on. The one thing I disagree with in his article is the invention of ported vac in for emission. While it most certainly effect emissions....Ported vac has been around for much much longer. At least back in the early 50's on some cars. But that holds no real importance to the article, so I did not make too big a deal about it.

 

I disagree with the article about the vacuum advance portion. I have worked in my younger days as a journeyman auto mechanic. My stepfather and my stepuncle were journeyman mechanics for about 35 years each. The vacuum advance does not work off of manifold vacuum. The proper connection for the vacuum advance is for it to be hooked to a venturi suction port of the carburator. The vacuum advance receives vacuum as the throttle is opened up, and airflow through the venturies creates suction. There were variations of the vacuum advance hose hook up, due to assorted emission designs, but the basic connection was for venturi suction to operate the vacuum advance.

 

Thank you for the explanation, but I feel that most of us know how ported vacuum works and where it comes from. However, if you would be so kind as to elaborate as to why direct manifold vacuum will not work and will not work better than ported vacuum I am certain we would find it interesting. Please be specif, as this is not an argument, but rather a place for us to learn new views. So lets hear your side and what leads you to believe it.

 

I follow what you are saying but just want to see if we are using the same definition, When you say venturi port I assume you are talking about the port that is generally between .020 and .030 above the leading edge of the throttle plate, sometimes just a round orfice and other times a small slot. When the throttle plate just begins to open this port is "exposed" to manifold vacuum ( as I was taught ) thus activating the advance.

I could be missing something but I was under the impression that there was not any real "suction" between the throttle plate and venturi but rather a pressure drop from the venturi effect that allowed atmospheric pressure to push fuel out of the float bowl to the booster nozzle.

Are we talking about the same thing?

 

Personally I have ran vacuum to the distributor both ways on my 81 Cherokee with a 360 and my CJ with F-Head and found that using the direct manifold source works well and seems to improve off road low speed ( creepy crawly ) throttle response and should in theory be a little more fuel efficient, My F-Head responds well to 10 degrees of initial with idle speed set at 600 rpm, then hook up the manifold vacuum and pull on another 12-14 degrees. idle speed will increase to 850-900 rpm and can be adjusted back down a little with the idle speed screw. this may seem high at first but it does not seem to "overspeed" idle crawling once under a slight load for lack of a better analogy. Occasionally going down hill on technical stuff you may have to brake a little harder but I have not found that hard to get used to.

I am still a little undecided and plan on playing with it more, It does work and may come down to a matter of preference.

 

Brian, you are correct. The suction through the carb is engine vacuum and the suction at the ported side and the manifold is manifold vacuum. They are completely different entities. Engine vacuum does not control timing. As you mentioned, it controls fuel coming in through the boosters and if you have a vacuum secondary carb, engine vacuum opens the secondary throttle plates. These two things are worlds apart.

 

The two ports were often called manifold and spark, or timed, ports, depending on manufacturer. Theoretically, an enigne can pull almost 30" of manifold vacuum. That's a perfect vacuum, as low as you can go. Spark or timed vacuum (terminology sure can get confusing) rarely rises above 20". This literally is the vacuum seen at the bottom of the venturi's, and it does increase up to a certain throttle opening, then falls off as the throttle opens further. IIRC, maximum spark vacuum occurs around 20 to 25% of throttle opening with no load.

 

I "think" what he is asking is....is the flow going through the carb venturies pulling vacuum on the vacuum advance canister. The answer to that is no. Port vacuum and plain old straight manifold vacuum are the same type of vacuum. Meaning, they are created by "blockage" the blockage being the carburetor butterflies. It's like this...Get a shop vac, turn it on and place your hand over the hose, now crack your fingers a bit and let some of the vacuum through. That vacuum is like manifold vacuum. If you put a vacuum gauge over the small spread in your fingers, it would read high. Now....take your hand off of the hose completely and imaging if there was a hole about half way up the pipe. The vacuum flowing past that hole does create SOME suction (sort of like when a truck passes you on the freeway) the suction is a product of flow, the suction with your hand over the end is a product of resistance to flow.
Alright....So, ported vacuum is still a product of resisted flow, the carb butterflies are acting like your hand over the shop vac. Here is the difference with ported. It is just slightly above the carb butterfly. When you slightly crack the butterfly (throttle plate) it unblocks that timed vacuum port and allows it to put vacuum to the distributor vacuum advance can.
Once you open your throttle plates so far, there is no resistance anymore, just flow and that flow does not have enough suction to operate the vacuum advance, so it stops working and we go back to a mechanicle advance distributor. So...straight manifold vacuum acts the same way, when it has no resistance, only flow, it does not pull suction at the vacuum advance. The ported vacuum port has the throttle plates to act as a switch to turn the vacuum on when you crack the plates. Straight manifold vacuum has no switch. You're advanced already when idling, thus the quicker throttle response.

 

I have enjoyed the conversation, Even though we may have slight differences on how exactly the vacuum acting on the port is created we certainly agree that its a viable source for vacuum advance operation, Especially on later model emission control equipped engines

 

I'm trying to sort out some timing issues on my rig right now and came across this discussion of total timing on the MSD troubelshooter forums. The response is concerned with recurving a distributor, so ignore the bits about changing bushings and springs.

"Normally it is best to have the centrifugal advance in at your cruise RPM. Keep in mind you want as small as a curve between initial and total with out hard starting and you want it to come in as quickly as possible with out detonation. This will generally yield the best performance as well as mileage. The vacuum advance should only be added after you have dialed in the mechanical; advance. Then the vacuum is added to ported vacuum so it will help fuel mileage under light load. If you can get your initial up via a bushing change and the motor starts easy the better off you will be. The vacuum advance will add roughly 16 degrees of timing at 16 inches of vacuum and normally puts the timing in the high 40's low 50's. "

LINK TO THREAD

 

This is the way I understand the two types of vacuum.

 

Just would like to re-state, that there is absolutly no difference between ported vacuum and manifold vacuum when under light load.
When under light load, ported vacuum is pulling on the vac advance can, as is manifold. There will be no difference, the timing will be the same.