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Ported or Constant Vacuum Source? ... Again

Don Gould

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I found this article on ChevelleStuff.Net I don't know the author so I can't give valid credit.
I have tried to explain this theory to 1000's of callers over the years and most get it. However, there's still a constant stream of posts all over the web that still want to argue about how the vacuum can works, why and how.

I first saw this article article years ago in I believe it was Hotrodders.com or something close

It explains it clearly answers all the questions and should be compulsory reading for everyone here. If everyone reads this it will end the argument.

Quote:
"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.


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 "whiz-bang" 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; low vacuum advance cans are available (from real ignition company's) to provides the same amount of advance as the stock can (15 degrees), but are fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.


For peak engine performance, drivability, 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." end Quote.



A few notes from over 100 years of the combined experience of Jim and myself in tuning, setting up distributors and developing our own computer controlled Mopar ignition systems:

*The adjustable vacuum canisters only adjust the sensitivity of the vacuum can, not the stroke.
*Limiting the mechanical advance and increasing idle timing to be compatible with today's fuel formulations are a necessity that can not be overlooked.
*Beware of the Imported distributors that appear to have the hex shaped vac cans that should designate it as an adjustable vac can, many are not. (Spectra is the worst)
*
Any one who tells you that the vac can is not needed with their ignition system or distributor is playing on your lack of knowledge and probably needs to read this article so THEY understand how UN-knowledgeable their statements make them appear or maybe their just trying to increase profits at your expense? Non adjustable vac cans out of China are about $2 a good quality adjustable can is probably closer to $10-$15 to build.

*Last comment beware of the "Snake Oil", our industry is loaded with false and rediculous performance claims, misinformation, country of origin claims, repackaging to hide source and vendors with very little knowledge of how their products really work and why. When you want reliable information go to the guy who engineered it and makes it.

*Our products are all designed in house, manufactured and assembled in the USA and most components are made by ISO 9000 compliant vendors.

*ISO 9000 is a quality management standard that presents guidelines intended to increase business efficiency and customer satisfaction. The goal of ISO 9000 is to embed a quality management system within an organization, increasing productivity, reducing unnecessary costs, and ensuring quality of processes and products.
 
It's Not a debate, unless you have emissions control and cats it goes to constant and that's not debatable.
 
I have found that using ported vacuum on an engine with a bigger cam and lots of initial the car will idle much better than it does with manifold vacuum.
 
I have found that using ported vacuum on an engine with a bigger cam and lots of initial the car will idle much better than it does with manifold vacuum.
if you lower the initial timing (without the advance) and hook it to manifold vacuum, you can achieve the same timing at idle that you are happy with now, then when you throttle it, the vacuum advance falls off but the mechanical advance kicks in. If it works for you then ok but my question would be what is your total timing all in with it hooked to ported advance?
 
Please Re-Read the article, it was written by the Engineer who was involved in the design and development for GM.
This is NOT an argument for ported or constant it is a clear and exact article on how the different vacuum sources work, their specific applications and how they affect your timing events and how your engine will respond. If after reading it and fully understanding the difference between the two you still want to connect your distributor to the emissions port then do it.

BTW... my avatar is also a BB Chevy killing 318, iron heads with a flat tappet cam and no power adders in a 3080# car running 10.90's at 125 MP. 3 track championships, Oregon State Champion, 5 Wally's and a trip to the winners circle at MATS Las Vegas with 34* of Total timing at 6700 RPM for over 20 years... same block, same Crank, same Cam and Lifters.
 
Thanks for the very informative post, but I’m afraid the opinion asking will never stop!
 
Again,
Subject of much much debate. What it boils down to is that for naturally aspirated pump gas street applications it makes perfect sense to run vacuum advance. Take the time to tune it.
 
I couldn't agree with that article more. In fact, I think I posted it in the last ported vs non-ported debate.
 
I believe your author is a guy that goes by JohnZ. Big in the Corvette and Camaro forums. I remeber seeing that article before.
 
I started as an auto tech in 1974 when I graduated from a vocational school where I took auto shop. Most of the cars I saw were from the mid 60's on up and I can say most used ported vacuum that I remember. But mid 60's is about the oldest I worked on as of course the cars got newer as time went on and I saw all the cars from the emission era also. The only time I work on older cars is my own 63 Plymouth or friends older cars which I like to work on. Even my dads 63 Fury wagon with the 318 Poly and Carter BBD used ported vacuum on it.

But I completely understand why you say to use manifold vacuum. But on some cars it just wont work like a friends 340 Challenger. Someone built the 340 with to much compression for this crap gas we have today and it will ping like crazy with the timing even at the stock setting and me limiting the mechanical advance. I even left the vacuum advance unhooked and set the initial timing at factory setting and it will still ping/spark knock/detonation. I had to set the timing where the car looses power so he could drive it without pinging to bad. The eng needs to be built right to run on pump gas. It has no quench built in it and to much comp. The best I could get it was to put ported vacuum on the vacuum advance and still back the initial timing up some so he can drive it without to much ping. With manifold vacuum hooked up it knocks as soon as he touches the gas. I told him he either has to run racing gas or have me rebuild the eng to run on pump gas. And on top of it the eng does not have much cam in it for the comp so it has the cylinder pressure higher then it needs to be.

I also remember that Fords in the mid to late 70's were set up for emission's that if the EGR valve hose came off they would instantly ping until the EGR was hooked back up since the comb chamber temps went up when the EGR did not work as that's how crappy they had them set up at that time. I also remember they used a 3 prong ported/manifold vacuum switch screwed into the coolant and it had manifold vacum on a port and ported vacuum on a port and the last port went to the vacuum advance. And if the car started to run to hot the switch would raise the internal pod and go from ported to manifold vacuum to speed up the eng for more fan speed and it also advanced the timing to cool it off some. I tell you I was glad when technology came along and they went to fuel injection with 02 sensors and much better eng controls as they were some pain in the butt emission setups on cars in the 70's and 80's.

I do understand the full vacuum at idle but I also will say that with hotrods and muscle cars I do agree that you have to work with each combo individually as some may not need as much advance at idle as others. Thanks for the write up on the post. Ron
 
Someone built the 340 with to much compression for this crap gas we have today and it will ping like crazy with the timing even at the stock setting and me limiting the mechanical advance.
I'm not understanding how you can blame this on todays gas. I'm assuming you are talking about ethanol. To my understanding, ethanol increases the octane, so todays gas would be less prone to detonation than "yesterdays" ethanol free gas. Can you elaborate (or correct me), Ron.
 
How fast does the vacuum canister realease the advance at a 60 MPH cruise when you mash the throttle?
Wondering if the piston speed is faster than the timing retard once the throttle is open and the vacuum drops.
Is this a concern?
Seems as if you may have a momentary high load/ high cylinder pressure situation with 50 degrees of timing.
 
Like any non stock combo you have to use the provided tuning devices and actually tune the car. You can tune the vacuum advance canisters, limit the advance etc.

Tuning a car is a skill that has gone by the wayside nowadays.

If youre worried about piston speed and the cruise or part throttle advance not retarded quick enough for your engine combo then its probably forced induction or some borderline non streetable combo and in this case perhaps with a carburetor on your car, you wont be able to reap the benefits of vacuum advance.

But for those that disconnect the vacuum advance and toss it on their bone stock 383 or 440, they are missing out on some driveability they could have
 
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