This all makes sense and before I go on I AM NOT doubting your knowledge.

As I understand it ported is from above the throttle blades and gives no adv at idle just when throttle is opened.

Non ported (Manifold) is from below the throttle blades and gives vac at idle and cruise speed. But drops out when throttle is opened

Here is the info I was given on how to do my vac adv it is not the exact source but it explains what I was told from a "ignition specialist"............




Ported Vs. Manifold Source: Vacuum Advance


This was written by a former GM engineer as a response to a similar question on a Camaro 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.

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.





Again Greg I am NOT doubting you just trying to understand it all and explain why I had it set up the way I did. When I used the vac adv hooked up to manifold vac my car responded real nice to it in performance and starting, idle, MPG.......

I DO value your help, just as you say it is hard to deal with these issues ''online'' via posts or email. It is also hard to convey how things are being said weather it is genuine or not, so dont take my questions and or ideas the wrong way (not that you are:) ) as I dont mean anything of what I type to belittle any of your knowledge.:cheers:

I never got any feeling of your not being fully engaged and willing to discuss these issues - nor did I ever get a sense that you did not believe me.

SO -- After reading your post about the vacuum advance --- I get all the way to the bottom... and BINGO -- he says EXACTLY what I said about the vacuum "dithering". He used different words -- but said the exact same thing. A MODIFIED engine works completely different than a stock engine.

Having said that -- this is why I said give the thing some more initial timing and go back to a ported vacuum. Basically this is the same "idea"/"theory" that a manifold vacuum advance is doing at idle (as he correctly explained) -- BUT without the dithering as he calls it (I called it vacuum bounce). But when you advance the initial timing - you MUST pay attention to the TOTAL timing that is set up in the distributor. Going to ported vacuum will give you all of the "advantages" that a vacuum advance has but with a cam - at idle - you're going to have timing all over the map. If you don't think so (or one way to actually see it for yourself) - stick a timing light on it and watch it. Also --- does your vacuum gauge bounce around? How much vacuum do you see. Have you run a long vac hose into the passenger compartment and gone out with a friend - and RECORD the vacuum at various driving conditions?

That information is very usefull.

Also - you can "map" your mechanical advance - while sitting in the driveway. Get your timing light -- DIAL BACK ONLY -- and with no vacuum advance hooked up (plug it) - record your initial timing - then bump the motor 500 rpms - record that timing - go another 500 rpms and record again - UNTIL you get no more change in timing. That should be around 3000 rpms or so. Maybe as late as 3500.... but you shouldn't see any change above that. That will give us a timing CURVE.... from initial to total. That would be good info to work with. Tells us a bunch about the distributor you have in there.

Remember that TOO LEAN can give you the same high CO readings that too Fat can give you. :cheers:

Yes I too noticed that last bit about the "dither".

My vac at idle is a solid 17'' Hg with the extra timing from the vac adv on manifold and was only 10-12''HG on the ported vac. When I get the top end put back together and she is running I will re check it.

I will also use a long hose and check the vac readings while I drive. What driving situations should I be ovserving the vac readings and Should I do this on both vac ports? I will also map out my curve too. All I know is i have 35* all in by 3100. I will map it as you described and see what I have though.

Re ignition box, I am guessing the Digital 6A will suit my needs just fine? With the auto trans I shouldn't need a rev limiter that the 6AL has? I do plan to go to fuel injection in the future, will the 6A be fine with that too?

What is a good way/sign to find out what the engine likes as the initial timing?

Here is where I am at now......

16 inital no vac adv hooked up
35 total no vac adv
27 initial with vac adv hooked to manifold vac at idle
44 total with vac adv all in by 3000rpm in neutral no load

The 35* total was set by my engine shop that dynod the engine but not in the car.

Okay - I'll try to respond -- while watching James May tour the Champagne region of France on BBC... :yes:


The reason for checking your vacuum and writing it down -- is so that when you are in your driveway -- you can duplicate that vacuum (using a hand vac pump) to see what the timing is with that "amount". Obviously there is no way to tell what your timing is while driving... and a "real" vacuum number can really only be created under load (can't really get a good number in neutral just revving the motor).... We'll get to that when you get there.

The above is why people choose to do this stuff on a chassis dyno.... you can stuff a sniffer up the exhaust... use a vacuum gauge.... and check the timing all while just hanging out. You can also use the sniffer to see what effect your changes are making etc. Otherwise -- we're just kind of "hunting" for what works.

A quick story -- I 'made' a buddy with a new '32 Ford go to a dyno -- after I fooled with it and got it as good as I could... it still smelled pig fat to me... and while it ran way better after I set a few things - I was able to convince him that this would be money well spent. So at the dyno - owned by a friend of mine that is a genius with a carburetor.... I was saying to Alex (the owner) that I thought it was pig fat etc. Turns out it was lean and he had to put in larger jets... he found 7 or 8 hp... but there was something else "wrong" with the carb that only someone like him would notice - so he fixed that - and I can tell you that the "drivability" went up about 50%... it was under $300 and the smile on my buddies face was worth $500.

Now -- to even attempt to answer how you'd know what the motor wants/likes in a forum... is impossible. It's something a guy learns after doing a zillion or more tuneups etc. You just know when you're on the right track by listening to the engine - watching how vibrates (or not)... and on and on. Sorry. I just can't answer that one... AND I will say.... that while a guy can fiddle fool and get things much better.... you will only really know for sure if you're on a dyno.

RE: MSD box -- 6A or AL -- either way. If you don't need the limiter then there's no need. They will both work fine with EFI.

Make sure your coil is up to the task of MSD (Multiple Spark Discharge) It should be but it's worth checking to make sure. The MSD only multi sparks up to about 3000 rpms I think.

35* is about right for a total. Again - that all "depends". My new Smeding 383 with EFI and aluminum heads is set at 34* with 10* initial. My 406 with EFI and aluminum heads runs 36* total and 18* initial. Neither of them use vacuum advance...

Your settings are in the ballpark of what I would expect - but that doesn't make them right or wrong. There are so many variables - heads - compression/cylinder pressure - cam - etc.

Thanks again for the info Greg :cheers: As you know from my other post I still don't have it running due to my headers!! I hope to have that resolved soon.

I would love to chassis dyno it but with all these set backs I am not sure I can afford the $225/hr to run it on the local one here. I did get a good deal on a A/F ratio gauge and will also use that as well as all the methods you mentioned to try and dial her in.

I hope to fire it up this week weather depending and get started on the tune at idle and confirm my A/F ratio at cruise and WOT. from some of my dyno sheets being posted online I have had some (might have even been you lol) tell me it may be running a bit on the lean side.

Well --- Lean is different these days than it used to be --- the new motors run quite lean in idle and cruise mode.... but they have knock sensors etc to help and the computer to pull timing out blah blah blah....

My 8 stack runs 13:8 at idle -- 14:0 at cruise -- and 12:8 for WOT

You generally are not going to hurt the motor with a leaned out idle or cruise A/F.... it's that WOT and "under load" that needs to have attention paid to it...