timed vs manifold vacuum

[resto75]

Would the be a difference in the fuel consumption between hooking up your vacuum hose to either of these ports?

 

[KenSny]

Which vacuum hose? Distributor advance, PCV, brake booster, vapor cannister, etc?

Makes a big difference if you hook up the distributor. You should use timed for the advance on a street car.

Use manifold vacuum for the others.

Don't think MPG will enter into the equation, even for the advance as it's all in at crusing speed anyhow.

 

[Vettehead Mikey]

what's 'timed vacuum'? Do you mean ported? If so, I think John Z wrote an article very recently which dispell the myths of this subject.

 

[JohnZ]

See the October/November (combined issue) and December issues of "Corvette Enthusiast" magazine for my two articles that de-mystify centrifugal and vacuum advance and deal with the (incredible) amount of mis-information floating around relative to manifold-vs.-ported vacuum for vacuum advance. Ported vacuum was strictly a crude, early emissions strategy so the old A.I.R. systems would work, and seriously compromises idle stability, idle and low-speed cooling, throttle response, and fuel economy.

 

[resto75]

Ok I'm talking dist. vacuum here and just checked the booklet that came with the performer carb. It says the fitting on the passenger side of the carb is the "Timed vacuum port" and the fitting on the drivers side is the "manifold vacuum port". Both are on the front of the carb.

Anyway which would be the best to hook into with the vacuum advance hose from the dist. for a street car.

 

[Robin7TFour]

resto, just hook the line whichever gives you constant vaccum. you will be fine with that set-up


robin

 

[Roy]

Use the "Ported" or "Timed" Vacuum Advance

Using constant manifold vacuum is similar to always having your A/C turned on. You can do it, but why? It's a waste of performance and efficiency. Using constant vacuum can advance timing when you don't want it, such as starting a cold engine because the advanced spark will resist the engine turning over by firing the plugs too soon. Constant vacuum will keep the timing advanced sooner and over a wider range of RPM. Your valves get hotter, your performance at lower speeds suffer, your gas mileage suffers. Plus, you'll go through brake pads quicker because constant vacuum advance holds your engine speed from dropping as fast as it normally would when you let your foot off the gas pedal. And did I mention "pinging" ... yep, constant advanced timing will contribute to pinging or pre-detonation in your engine's combustion chambers. Ouch! Overall, constant advance is O.K. if you are content with mediocre running conditions.

It is important to match the vacuum conditions inside your carb with the timing advance and retard in your distributor, assuming of course that you have a vacuum advance distributor.

Read factory or after-market manuals on the carberator that you are using and follow the directions and settings that are recommended in the manual. Once you have established the standard set up for your vehicle and carberator, then you can experiment with different settings making one change at a time. Do not listen to someone's special secret cure to obtain carberator nivarna. Chances are that their engine is not set up the same as yours ... different cam, compression ratio, or overall condition of the engine and components. Start with standard settings and experiment if you want to.

 

[AKRAY4PLAY]

fuel consumption goes down with more advanced timing, running your timing with straight manifold vacuum will yield the best results. tuning your distributor to the vacuum you are running will improve things ten fold. adjustments are the key, if your set-up is inflexible then you need to compromise with what you have. i would suggest you buy an adjustible advance unit and tune to what you have, Brian

 

[Roy]

Running constant non-ported, non-timed vacuum is like wearing a blanket to work. It may fit and keep you warm, but the tailoring leaves much to be desired.

Constant vacuum ~/= constant advanced timing = not a good thing.

If GM had wanted your timing to be constantly advanced, they would have set the timing advanced at the factory and not bothered to build and install vacuum or centrifical timing advance units.

And how exactley do you tune the distributor to "... the vacuum you are running" when the vacuum is constantly changing under varying operating conditions, i.e. low RPM, high RPM, accelerating, slowing down, low speed and highway cruising?

Read the manuals. Understand the material.

By the way, I have a four page "Edelbrock Performer Carb Tuning" (same/similar to Quadrajet) handout that I can scan and send to those who are interested.

 

[AKRAY4PLAY]

Roy,
several companies make adjustable vac advance units that replace the stock unit. you can adjust the total amount of advance and independently adjust when it operates, ie vac level. for example, if you are running a cammed motor with 10 inches of vac, the stock advance unit won't work very well by not advancing. and if you are running a stroker with higher than normal vac, then the stock unit adds to much advance to soon. for tuning very accurately, you need to be able to adjust when your timing comes in and when it drops out. as for the timing being advanced all the time when running manifold vac, you are correct in saying the vac changes under different running conditions, and under that fact the vac unit will work if set right. ported vac is a way to exaggerate the level "swings" in relation to the operating diaphram of the vac advance unit.

 

[resto75]

My carb was setup with constant vac but I feel it ( or something else) is making the engine sluggish and using more gas then it did before. I am going to try the ported vac and see what happens.

If that doesn't work I might go back to my 750 Holley that I had on before the 600 cfm performer

 

[JohnZ]

Running constant non-ported, non-timed vacuum is like wearing a blanket to work. It may fit and keep you warm, but the tailoring leaves much to be desired.

Constant vacuum ~/= constant advanced timing = not a good thing.

If GM had wanted your timing to be constantly advanced, they would have set the timing advanced at the factory and not bothered to build and install vacuum or centrifical timing advance units.

And how exactley do you tune the distributor to "... the vacuum you are running" when the vacuum is constantly changing under varying operating conditions, i.e. low RPM, high RPM, accelerating, slowing down, low speed and highway cruising?

Read the manuals. Understand the material.

By the way, I have a four page "Edelbrock Performer Carb Tuning" (same/similar to Quadrajet) handout that I can scan and send to those who are interested.

Unfortunately, you have succumbed to all the misinformation about vacuum advance and manifold-vs.-ported vacuum; the ONLY reason for "ported vacuum" was to increase exhaust gas temperature at idle and overrun to ensure a good "secondary burn" when the exhaust stream met the injected air from the A.I.R. pump in the exhaust ports. I was there when it was designed, developed, and released, and I have all the SAE and GM Research papers that were published on the subject - read the articles I mentioned in my earlier post; ported vacuum and the theory and operation of vacuum advance are the most misunderstood subject in the hobby, no thanks to Summit and Jeg's, who don't understand it at all.

 

[Roy]

Any information that you can post would be greatly appreciated.

As far as constant timing advance being your favored approach, you might want to share that information with owners of 1936 Indian motorcycles who had (and have) to manually advance and retard the timing of their machines in order to start, accelerate and cruise at optimal performance. They will be glad to learn that after 67 years that they can discontinue this basic mechanical procedure for enhanced performance and combustion.

The point is ... Timing advance was not born in the 1970's. It has existed since the first gasoline internal combustion engine was created. The proposition that constant advanced timing helped complete combustion during the darkest years of crude poluution control devices does not make it a universal cure-all for subsequent engines with less restrictive exhaust system and more efficient combustion and iginition systems.

If timed/ported vacuum ignition timing is a bad thing, why do reputable manufacturers such as GM, Holley and others continue to recommend it? Why do service manuals desrcibe it's function and benefit? Why do timed vacuum ports still exist on modern GM crate engines that are not restricted by strict 1970's pollution regulations.

I'm curious and look forward to your response that addresses today's engines and intake/ignition systems used in the engines in C3 Corvettes today.

 

[JohnZ]

Any information that you can post would be greatly appreciated.

The point is ... Timing advance was not born in the 1970's. It has existed since the first gasoline internal combustion engine was created. The proposition that constant advanced timing helped complete combustion during the darkest years of crude poluution control devices does not make it a universal cure-all for subsequent engines with less restrictive exhaust system and more efficient combustion and iginition systems.

If timed/ported vacuum ignition timing is a bad thing, why do reputable manufacturers such as GM, Holley and others continue to recommend it? Why do service manuals desrcibe it's function and benefit? Why do timed vacuum ports still exist on modern GM crate engines that are not restricted by strict 1970's pollution regulations.

I'm curious and look forward to your response that addresses today's engines and intake/ignition systems used in the engines in C3 Corvettes today.

You missed the point - full manifold vacuum wasn't the emissions strategy in the 60's-70's - the emissions strategy in that era was "ported" vacuum to the distributor. "Timed" vacuum ports don't exist on modern crate engines - they exist on the carburetors that come on them, which were designed in the 60's, and have 60's emission provisions on them for customers who need them in order to pass emission testing where required.

Service manuals describe it functionally, but don't elaborate on any "benefits" other than emissions; I have a book full of TSB's and Service Letters sent out by GM to the dealers on how to make changes to deal with customer complaints about idle overheating, poor operation and fuel economy with those "ported vacuum" late-timed engines, several of which simply tell the dealers to connect the vacuum advance to full manifold vacuum and re-set the timing. There was no "customer benefit" to ported vacuum; I spent 21 years in Chevrolet Engineering and I know a little about this.

I hate to take up this much space, but below you'll find a paper I wrote a year or so ago that answers your questions; a far more detailed version describing the theory, operation, and application of ignition timing, centrifugal and vacuum advance occupies 6 pages in the combined October/November issue of "Corvette Enthusiast" magazine, with a 5-page followup article on how to tune and optimize centrifugal and vacuum advance at home in the December issue - I think you'd enjoy reading them.


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.

 

[Roy]

Good Article - Where to Attach Distributor Vacuum Advance Hose and More

While I am sure that this debate will go on forever, there is a good articled titled "Setting Ignition Timing Curves" by Scott Crouse, Chevy High Performance located at:

http://www.chevyhiperformance.com/howto/97438/

It is a well written article with detailed photograghs that explain the vacuum advance distributor and the effect of timing advance on engine performance.

One point that is made in the article concerns where the vacuum hose from the distributor should be connected.

"Vacuum canisters advance according to engine vacuum, which is why they must be connected to a PORTED VACUUM SOURCE (emphasis added). Ported vacuum is drawn from just above the throttle blades to make sure the vacuum cannister does not advance at idle."

The Chevy High Performance article goes on to explain the Chevy V-8 HEI distributor and how it operates on both centrifical and vacuum advance devices.

Good article. Hope you enjoy it.

 

[AKRAY4PLAY]

Roy,
nice article, but it has way to many advertisement ques for me. it looks like litterature strait from an ad. i'm sorry but i have to go with John on this one. i have seen dyno pulls both ways, and strait manifold vaccum always tunes easier and better. bottom line, it's your car, tune how you see is the best way. don't get me wrong, i appreciate the posts and they are very helpful to other members. it is always nice to see another point of view. Brian

 

[JohnZ]

Interesting article, and (for the most part) well-written and accurate, for late HEI distributors (which were all designed with "smog" advance calibrations). However, the author hasn't grasped the negatives about what zero vacuum advance does to idle operation, and doesn't understand that distributors used full manifold vacuum for over 30 years before emissions systems were beginning to develop; people who deal with post-emission calibrations and devices as their starting point don't understand how ignition and advance systems were developed and refined for optimum all-around operation and performance in the pre-emission days, with full vacuum advance at idle. People who weren't around in the 60's think "ported" vacuum is normal, since that's all they've ever seen; it's not, but the myth continues to perpetuate, because their history and experience doesn't go back far enough to understand the principles involved.

 

[theo]

John,
Interesting words mate. I'd never even heard of ported vacuum advance before getting a vette and seeing the "great debate" on the forums. As far as i can see, full manifold vacuum is the sensible option, being as how it's such a convenient and (relatively) accurate indication of load. One thing that I wonder about is how the actual magnitude of ported vacuum compares to that of full manifold vac.? From my understanding of this, the ported vac is fed from inside the carb venturi, above the throttle plates. Upstream of the throttle plate the pressure's always gonna be higher (or the vac less!) until WOT, assuming the open throttle gives pretty much zero pressure drop.
So does a vacuum advance canister for a ported car have to be be more sensitive?
This whole AIR system seemed a bit of round-about means of reducing HC's and NOx- surely since full vac at idle gives longer time for combustion, the combustion would be more complete with the non-ported route? Rather than chucking it all out into the exhaust manifold to be burnt. Not much good there! (I have to admit my knowledge of emissions and the combustion side of things gets a little sketchy from here on in, perhaps you can enlighten us!)
I too have access to loads of SAE papers, but not quite as
far back as you're talking about - as you say, there's no SAE activity on the pros and cons of ported vacuum these days, with electronic control being the way.

 

[GerryLP]

I have respectfully disagreed with JohnZ (and JohnZ, I mean very respectfully ) on this issue from time-to-time, and I am not totally discounting or rejecting his manifold vacuum advance theory. I know that he's talking about emission control systems such as Controlled Combustion Systems (CSS) and Transmission Controlled Spark (TCS). These systems, generally speaking, were designed to operate on carburators with a fuel/air mixture relatively in the lean side, and injesting 100 deg F air (CSS) or ambient temperatures above 63 deg F (TCS) together with sensors in the colling system (TCS), and thus the use of the "ported" fitting above the throttle plates in most carburators.

Yes, it also appears true, that after-market products manufacturers added all kinds of stuff to their products in order to obtain goverment approval (the old 50-State blessing). And in order to be able to sell them, they keep those ports in their components.

However, it is also true that part throttle fuel mixture takes longer to burn and therefore requiring added timing advance. However, also recall that in the past, before smog equipment, initial timing was relative low and the advance vacuum canisters calibrated at the factory, so that when a particular manifold pressure setting was achieved and the timing, idle, and carburator were adjusted, then the canister would operate as advertised and provide the engine with the "designed" vacuum advance profile. Fortunetly, we all have also progressed, and have learned all kinds of new tricks to fine tune these engines. New way of thinking many times disprove what was in practice then (remember the old story/argument about horsepower being measured wrong in the old days?).

Also, please recall that the minute one alters the factory engine component combination, a balance in performance is altered, and the factory settings will no longer suffice, so trial and error is the only way to arrive at one's new settings to make the engine perform (together with common sense and a bit of knowledge).

As Brian so well suggested, its best to try and experiemnt, and I plan to try the direct manifold hook-up to the vacuum advance to see how my car behaves over a period of time. Although, my car's problem is that I am not getting a total of 36 -38 degrees BTDC -33 at best, so I will also try to lenghten the advance slots in the distributor.

 

[cuda66]

There is no discussion Mr. John Z, you are absolutely correct.

MSD is WRONG in there instructions and have no clue even how a Vac can works.

The instructions from Mopar Performance, Crane, all those junk Chinese distributors and the worthless crap from Proformance distributors in Australia are wrong.

Everyone tries to copy MSD thinking they actually know something ..wrong. Anyone at MSD who ever knew anything is long gone.

We build and/or recurve 1000+ Performance distributors a year, three Sun distributor machines going steady, I've been curving performance distributors for nearly 40 years and they have always been set up to run on CONSTANT Vac.

I'd like to add a little more insight to your post:
1. MSD distributors use stock FORD pick-up and reluctors
2. The bearing on the shaft is really only required if your running over 7500 RPM constantly
3. A Pro-Billet Distributor has no application unless your engine is making less than 10" of vac at idle.
4. Multi spark only occurs at 3000 RPM or less.
5. No distributor is ready to run in any engine until it has been curved correctly and the Vac can adjusted to the engine vacuum.
6. 50KV systems are mis-leading advertising, the ignition will only produce the voltage required to jump the spark from the electrode to the ground strap and most street cars or bracket cars that run 10 sec. or slower will see no gain from running any ignition system capable of producing any more than about 34,000 Volts.
7. A stock HEI with a good coil and good module can easily produce 48-50KV IF required.

Your standard HEI, Mopar or Ford Electronic Distributors were engineered to run 200-250,000 miles trouble free with minimal maintenance. You won't get 20% of that out of most aftermarket distributors.

Also, you guys be very careful when selecting a new distributor, many of these low end HEI's are Made in China and after about 500-2000 miles the cam gear will wear off and dump all that metal into your oil pan....not good. They also eat the thrust washer out, the pivot points on the mechanical weights are not in the right position and they will not return to the stops. Some even have a detent for the limiter and will skip over the detent at about 5500 RPM and dump another 10-15* of timing into your motor at WOT.

But if you insist on thinking that you can out smart GM with a $200 (or less) distributor go ahead, did I mention we have a sale on GM pistons this month?

In the constant search for that 1/10 or 1/100 of second off the time slip a few of our customers have fallen for the glossy ads in the magazines and replaced one of our HEI’s with a MSD, Crane, Summit or whatever distributor hoping for the quicker ET, most go slower and experience all sorts of Idle problems and performance loss. Even after we curve them correctly they find that the $500 was ill-spent and they gained nothing.

There's also several well known company's out there that claim to be distributor pro's, one of the most infamous has 1 curve for almost everything 12 initial, 36 total at 3000 RPm and I think they run a 10* advance Can on everything...think about it, we offer no set curves. every application is different, I'll wager we have over 100 combinations just for small block Chevy's.

Don’t be fooled, consult with a real Ignition Tech and buy the right components once.