Vacuum Advance - ported or manifold

[SuperHart]

I run a 1969 351W in my roadster. The distributor has a vacuum advance pot on it that advances the timing as higher vacuum is applied. OK, where should the vacuum advance hose be plugged in, a PORTED nipple or direct MANIFOLD nipple? There seems to be so much controversy surrounding this question. From what I read, direct manifold vacuum gives me 22 degrees of vacuum advance plus 10 degrees initial timing at idle for a cooler idle with less NOx emissions but ported vacuum gives me only 10 degrees initial timing at idle, slower idle speed but causes not only vacuum advance but also centrifugal advance to increase as engine speed increases.

There must be some expert out there who really knows his defecation (s**t) who can set me straight.

[Hotfingrs]

Ported vacuum is the one to use. You don't want full advance all the time. Watch out for detonation if you run manifold vacuum.

[jwd]

Connect it to manifold vacuum. Ported vacuum was used by the factory for pollution control purposes. Anyone who says otherwise doesn't know what they're talking about. With it connected to manifold vacuum, you'll get lots of timing at idle and cruise conditions. The further you open the throttle, the more the vacuum advance drops out and at WOT you have no vacuum advance at all. That's exactly how your engine wants and needs it.

Jim

[PeteF]

There are two schools of thought on this, but this is the way I understand it. Pre-emission engines (like these), used manifold vacuum. This gives you full vacuum advance at idle, but no vacuum advance under hard acceleration (load), where the centrifugal advance takes over. This provides more advance at idle and cruise for improved mileage (more complete burning of a lean mixture), while protecting the engine from detonation under acceleration. Typical setting would be 10 deg initial, 24 deg mechanical(in the distributor) and 15 deg vacuum advance. The maximum advance would not exceed 10+24 = 34 deg under load. When emissions became mandated, vacuum systems went to ported. This reduced emissions (NOx I think), but if you run this way, you have to adjust your settings to prevent too much advance under load, because the ported vacuum provides advance with venturi velocity. If you run 10 initial+ 24 mechanical+15 vacuum under load = 49 deg total, which may drive you into detonation. Therefore, you would reset to either less initial timing or recurve the distributor to have less mechanical advance. A third option (what I did) is to remove the vac advance and run 10 initial + 24 mechanical only. Mid 60s high performance Fords ran centrifugal only/no vacuum. You give up 2-3 MPG, but have a simpler, straightforward advance mechanism where you get advance proportional to rpm only. Either way will work with the proper settings. And this will surely bring on more comments and opinions. You can Google the subject and learn more, but the differences of opinion are there too. Good luck

[jwd]

Originally Posted by PeteF

A third option (what I did) is to remove the vac advance and run 10 initial + 24 mechanical only. Mid 60s high performance Fords ran centrifugal only/no vacuum. You give up 2-3 MPG, but have a simpler, straightforward advance mechanism where you get advance proportional to rpm only.

That will work BUT you lose throttle response and the benefits of a cooler running engine. Early Corvettes had mechanical advance only on their high-performance motors. Every one I converted to a vacuum advance dist. ran cooler, had better throttle response and got much better gas mileage all with no loss of performance. GM realized this and changed to vacuum advance on all their engines in 1963.
Jim

[Wbulk]

The end of this thread talks about some advantages of using manifold vacuum, especially with a high duration cam.
Dist. Advance Re-curve

For street engines vacuum advance is a win win situation. Jim is pretty much right on with this.

[bingo2]

Here is a pretty good explanation that I borrowed from another forum:

"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."

[mdross1]

Originally Posted by jwd

Connect it to manifold vacuum. Ported vacuum was used by the factory for pollution control purposes. Anyone who says otherwise doesn't know what they're talking about. With it connected to manifold vacuum, you'll get lots of timing at idle and cruise conditions. The further you open the throttle, the more the vacuum advance drops out and at WOT you have no vacuum advance at all. That's exactly how your engine wants and needs it.

Jim

This is the correct info you need,once the engine settles in after the WOT event and cruising along then the ignition advances to give great fuel mileage or so we pray for.We all know where fuel prices are going and not in the direction we would all like to see.

[wolf k]

I think we all agree that vacuum advance is a good thing for street driven cars. The question is whether the source of vacuum should be manifold source or ported source at the carburetor.
The only difference between the two, is that manifold source would give you vacuum advance at idle, ported source gives you the same manifold vacuum, but only once the throttle is cracked open. Trace out the the ported vacuum port in a carburetor and you will see that it leads to a small hole just above the throttle blade. You will have the same vacuum signal with manifold source or ported source with any throttle position above idle.
So the the only difference between the two is, do you want vacuum advance at idle or not. The choice is yours, depends on what your engine runs better with. Generally, if you run with manifold source, then you will have to back out your idle screw further out to get the same idle speed, this may be a good thing or bad, depends on your setup.
I have heard some drawbacks with manifold source, is that it puts higher load on your starter because you are getting advance at cranking speed. I do not buy this, because we all start our carburetor engines with the throttle cracked, thereby, exposing the ported source.
If you have run-on issues with your engine after shutting off the ignition, manifold source may help you because your throttle blades will be more closed off at idle. Manifold source may also help those lumpy cammed engines idle better.
Again, there is no right or wrong source, it is what ever works better for your setup.

[SuperHart]

Thanks, everyone. Great insight. Looks like it is manifold vacuum advance for me. Car with 351W is street driven, mildly warmed over although it sports a pair of Holley #8007 390 cfm carbs on a Price Motorsports intake. My distributor is set at 10 degrees initial advance with 24 degrees in the vacuum cannister and 26 degrees (#13 slot) centrifugal advance. That's how I have been running but wanted to hear the majority rule position.

[Ozzir]

Wow. I love this forum. You guys are a body of knowledge. I cant wait to get my Cobra and tinker with it and if and when i screw it up i know ill be able to come on here and pretty much have any issue diagnosed and answered..

[olddog]

Pay attention to the part about manifold vacuum at idle. You want the vacuum advance hard against the stop at idle when using manifold vacuum. If you cam gives low vacuum at idle and you do not use the correct vacuum advance unit, you timing will hop around and that will make you idle rpm hop around with it.

[CHANMADD]

I have tried both ways over years, and without a doubt it is easier to get the motor to idle and respond to throttle without any starting or other issues with the ported , above the plates connection.With the manifold vacuum the initial timing has to be retarded too much. I also use a pcv which also taps into manifold vacuum, so a bouncing pcv can cause fluctuations........I think...Just runs great with the ported connection.

[Frank Messina]

Rupert,
Don't forget that you can also use an adjustable vacuum advance if you don't already have one that enables you to tailor the amount of vacuum advance exactly to your engine's needs. All you need is a 3/32 allen wrench which I think you have. In this way you can avoid light throttle detonation by backing out vacuum advance while still maintaning a fairly quick mechanical curve all in by 3000 RPMs. For example I have mine dialed back to 7° vacuum plus 15° initial which allows for easy cranking but a very crisp throttle. Total advance at 3000 RPMs is 35° with no vacuum present. HTH
Frank
PS: Oh and yes, use manifold vacuum only, not/never ported for all the reasons listed above.

[SuperHart]

You know, Frank, you may have given me an idea. At idle I seem to be running awfully rich. I run a pair of Holley #8007 carbs on a Price Motorsports dual quad intake. I wonder if it is because I have the idle screws nearly closed all the way to get the idle down to 800 RPM since I have so much vacuum advance at idle (24 degrees) plus initial advance of 10 degrees. I'm going to dial back the adjustable vacuum advance to 12-14 degrees and then open the idle screws on the carbs to bring the idle speed back up. Let's see if that helps some.

[mreid]

You should be plugging vacuum advance when seting idle speed and base timing.

[Jeff Frigo]

get a new distributor without vacuum advance problem solved it's just not necessary in that light of a car

[CHANMADD]

Extra advance on a light throttle makes a world of difference.

[mreid]

Actually vacuum advance is great on any car. The weight of the vehicle has little to do with the advantages.

[Frank Messina]

The driving difference between no vacuum advance and vacuum advance (manifold source) is quite noticeable in just about every metric. To not have it is to give away performance.

Frank