Ported vs manifold vacuum advance

[Huskinhano]

I'm having a discussion on another forum about this topic. My basic view and understanding is this. Ported distributor vacuum advance was a result of early 70's emission equipment. It comes off the venturi above the throttle plates and is effectively isolated by the throttle plates from manifold vacuum, more specifically the throttle shaft as the defining point much like an expansion valve in a refrigeration circuit. As such it is my understanding that the amount of ported vacuum is going to be proportional to the amount of air going through the venturi and not in anyway effected or controled by manifold vacuum below the throttle shaft or amount of the throttle being opended.

I personally prefer manifold vacuum because to can keep the throttle plates closed more at idle making idle mixture more precise or mre effective along with vacuum advance backing off on total timing under higher engine loads while ported can't do this. I think if some one is going to run ported instead, they need to know exactly the advance rate vs engine load and set up accordingly. I found a post by a GM engineer who posted on a Corvette forum on this topic. I think my understanding was pretty close to what he stated except he did not go into more about ported as the throttle is opened, only what it was doing at idle. I tried to paste what he said last night on a post but had some trouble and wasn't able to post. I'll see if I can do it today.

[Huskinhano]

This was the post I found on a MOPAR forum during a google search.

This was written by a former GM engineer as a response to a similar question on a Corvette board:


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.

[milner351]

Bravo!

[CDMBill]

Nice informative piece thanks for sharing.

[JeffMcKC]

He is right on a Stock engine, but the last couple Paragraghs he starts hedging when the cam enters the picture. A proper tuned carb or EFI with the correct timing will not have the issues he is talking about. Proper is the key words. Take mine as a extreme it has depending on the cam timing I am using 2" to 4" of vacuum at a idle.

[racingsnake440]

Jeff, are you saying that with a very radical cam you're better with ported vacuum or that if properly tuned you don't need vacuum advance at all?

[Barry Donovan]

ported works on gentle oem.
flopping doubled up inline fours (v8 ) are never gonna get a curve but it is fun to play with.
advance weights slamming to manic lunacy is your new advance curve. have fun in a wheelie, burning tires and whatever else you do.

the only real effort I have found for advance is four cylinders.

my boxer uses both full vacuum and prted, from one double dighram valve. I think it came from a blow through ea81. As n/a it catches a retard off throttle- because a zero is a four cylinder, no overlap to leap into, it has to catch it or stall.and another diaghram that loves progressive for long throttle gains..and it works.

pairing that up with a monojet, where as vacuum literally changes throttle, no slamming game like a quad pumper...it is a simple genius.

over the 180 degree fires (six cyl and beyond) not many care, they don't have to.

edit: I should say 5 cyl and beyond.

[TC]

Lets see here, I don't think they are thinking this through here on the added timing that will happen at cruise when the manifold vacuum is high...... You set your base timing at 12* BTDC, the vacuum advance adds 10* and you'll have a slight advance with the mechanical at cruise to......So all together your going to be going doing he road with about 30* of advance, can you say Ping City........I know on my '99 Z28 that at about 50mph while under light throttle it will register pinging on the Knock Sensor and it's only pulling about 24 degrees of advance at that point.........

But hey real race cars don't have vacuum advance anyways.............

IMO if you want that much timing at idle, lock the distributor out and set your Total Timing as your Base Timing and forget about it...........Hell you want all your timing in by 3000rpm anyways, so if you have a 4000rpm stall what good is having a timing curve in the first place.........

[JeffMcKC]

Jeff, are you saying that with a very radical cam you're better with ported vacuum or that if properly tuned you don't need vacuum advance at all?

I would say with "Most" heavy cams yes (Valve events mean everything here), vacuum advance is not what I would use. I like High Static timing, it works best with mech totaling out. Remember the guy that gets the most work done with the least amount of timing is better off. I have good luck adding more timing down low like first gear and first part of second then pulling some in second and high.

Once you get away from stock and start fooling with stuff the vac advance drops off quick.

I did play with ported Vac on some with good luck, once again high static and limited Mech and Ported vac. Ford dist you can slid a allen wrench in the port and tighten and lesson tension on them to dial ping out at least the ping you hear.

Guys like that are trapped in thier box and cant see outside it.

[Huskinhano]

OK, so we don't get too far off topic, is the amount of vacuum in ported proportional to the amount of air going through the venturi and effected by such or a manifold vacuum function when the throttle plates start to open and retards was manifold vacuum falls?

[JeffMcKC]

If I read you right Yes.

Ported climbs as more air goes threw the carb, Manifold Vac will drop as the blades open but if the Carb becomes a restriction then it will build again, think part throttle at highway speeds. Then when you let off it will climb even more.

At WOT it will climb only if the carb is to small for Max performance when it becomes a restriction

[Huskinhano]

Found the info I was looking for on a AMC forum. I was slightly wrong on my understanding of ported vacuum. Ported is basically manifold vacuum but not functioning at idle while the throttle is closed. What I thought how ported worked is how venture port vacuum used some times. Anyway, look at the 4th response to the posted question. Good info.

http://theamcforum.com/forum/timed-spark-port-vs-full-vacuum-port_topic11416.html

[JeffMcKC]

Its not Maifold vacuum. It works the same way as a header evac does.

This guy seems to understand it better, remember on the Net, no matter how smart the poster may sound, he can still be wrong.......

Also ported vac can come from different places to do different things depending on where its at, ported vaqcuum is what opens a vac secondary on a vac operated secondary...


The ORIGINAL reason the factory used ported vacuum was to help offset stumble when driving in town.
It's THE reason Chrysler and then Ford went with ported in the late 30's. (I'm a student of ICE history and development)
And clear up into the 70's, it was a reason. However, to deal with emissions and other little details, it got more complicated........... vacuum is an indicator of load. Timing needs to change with load and speed.
There is ANOTHER, besides ported or timed and that's VENTURI vacuum - some companies used that. It is taken from higher up, in the venturi area. What happens with MANIFOLD vacuum is that it DECREASES with high load and speed. Venturi vacuum INCREASES with speed and doesn't drop with load. It's typically used in combination with other controls but not always.

[Scott Liggett]

When I originally built my 383, I had problems getting to idle in gear. It's driveability sucked too. I later realized that I was using manifold vacuum port on the carb instead of the ported vacuum port on the side of the metering block on my Holley. It made all the difference. At the time, the engine had iron fuelie heads, Comp XE274 cam, 224/230 dur and .487/.490 lift. and a 650 dp.

[REARSPROCKET]

"IMO if you want that much timing at idle, lock the distributor out and set your Total Timing as your Base Timing and forget about it...........Hell you want all your timing in by 3000rpm anyways, so if you have a 4000rpm stall what good is having a timing curve in the first place........." I'm going to have to agree with TC. Also I could never run ported on my setup because it would or could induce too much timing in cruise or part throttle at the wrong time. Every setup is different that's why there are so many variables in advance curves. If the advance curve is setup for its application then manifold source advance is more predictable and reliable IMHO.

My setup isn't super radical but radical enough to want as much intial timing as I can feed it, because of compression and cam duration. My setup responded very well to locking out the dizzy. The only other area I have been wanting to test is adding more idle advance with a vacuum bulb limited to 10*. Without using the vacuum adv. the timing is more accurate and precise, so I'm not sure it's worth chasing more idle timing and risking ping and less stable timing?