Ported vacuum advance or manifold. Let's debate!

[Geoff 2]

Hunt,
Post #114. Not surprised at your great result. Some are still not 'getting' it, never will....

The dieseling stopped because the idle rpm was lowered......the idle rpm was able to be lowered because the engine was making more HP at the higher rpm & thus idle rpm could be lowered because there was still enough HP to keep the engine running.....the extra timing at idle allowed the engine to make more HP at idle....& above is the result.

 

[hunt2elk]

You guys keep missing where I stated that my idle rpm was 850 when hooked to ported vacuum previously. When I switched to manifold and the initial timing went up to 30, idle rpm went up as well of course. I then turned my idle back down to the 850 I had before. So idle speed had nothing to with engine run on. What I believe did impact run on though is the fact that I was able to turn all 4 idle mixture screws in 1/4 turn to get back to the idle A/F that I previously had. The only other difference is that I now have a couple more inches of vac at idle.

 

[DeltaV]

How about this?
_________________________________________
Ported Vacuum vs. Manifold Vacuum

I found this article while searching for good tech information on the comparison of ported vacuum versus manifold vacuum. I don't know the author so I can't give valid credit. ChevelleStuff.Net doesn't indorse either method and there is a lot of discussion as to which is better. So, I'll present the article and you, the reader, decide.

---

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; 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, 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.
Courtesy John Hinckley
Retired GM/Chrysler Engineer

---

Found it here:
Ported vs. Manifold Vacuum

 

[33 IMP]

So much debate over a simple question.... with a simple answer, already provided by someone here.
Try both, see which works best for you. Done.

 

[4406bbl]

You guys keep missing where I stated that my idle rpm was 850 when hooked to ported vacuum previously. When I switched to ported and the initial timing went up to 30, idle rpm went up as well of course. I then turned my idle back down to the 850 I had before. So idle speed had nothing to with engine run on. What I believe did impact run on though is the fact that I was able to turn all 4 idle mixture screws in 1/4 turn to get back to the idle A/F that I previously had. The only other difference is that I now have a couple more inches of vac at idle.

Throttle blade position nearly closed is what stopped run on, more timing allowed you to close the primaries kind of like an idle stop solenoid on a 6bbl.

 

[hunt2elk]

Throttle blade position nearly closed is what stopped run on, more timing allowed you to close the primaries kind of like an idle stop solenoid on a 6bbl.

That makes sense, thanks.

 

[RemCharger]

You guys keep missing where I stated that my idle rpm was 850 when hooked to ported vacuum previously. When I switched to manifold and the initial timing went up to 30, idle rpm went up as well of course. I then turned my idle back down to the 850 I had before. So idle speed had nothing to with engine run on. What I believe did impact run on though is the fact that I was able to turn all 4 idle mixture screws in 1/4 turn to get back to the idle A/F that I previously had. The only other difference is that I now have a couple more inches of vac at idle.

I think I corrected an error.

So have you driven it? Any pinging?

 

[hunt2elk]

I think I corrected an error.

So have you driven it? Any pinging?

Yes, thank you, I edited it in my post now. I have driven it about 50 miles so far with no detonation that I can hear. It hasn't got extremely hot here yet though, that sometimes makes a difference as well.

 

[biomedtechguy]

How about this?
_________________________________________
Ported Vacuum vs. Manifold Vacuum

I found this article while searching for good tech information on the comparison of ported vacuum versus manifold vacuum. I don't know the author so I can't give valid credit. ChevelleStuff.Net doesn't indorse either method and there is a lot of discussion as to which is better. So, I'll present the article and you, the reader, decide.

---

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; 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, 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.
Courtesy John Hinckley
Retired GM/Chrysler Engineer

---

Found it here:
Ported vs. Manifold Vacuum

My position on the topic?
What the Chrysler engineer said that I quoted.
Don at foursecondsflat set me up with an electronic ignition distributor and his control module and that includes the max advance limiting ring. By increasing the timing at idle, because of a vacuum advance can that responds (deploys) at lower vacuum, that allowed me to adjust my SIX idle mixture screws to a place that dramatically improved my idle and helped me get a better AF Ratio at idle too. ALL GOOD.
Now I'm moving UP to what I consider THE best ignition system for a carbureated engine. The Progression Ignition...and it hooks up to MANIFOLD vacuum too!

 

[4406bbl]

What are the specs on dons vacuum can? Do you have a chart? Most cans for big block mopars start advance at 7-8" vacuum, and with a 9-1 compression engine, a long duration cam and an auto trans it will not help as they will not idle in gear with high enough vacuum to matter, in fact they die when dropped into gear, manual trans no problem.

 

[Rusty Wrench]

[1] Cannot believe the ignorance, post #15.
[2] GM used MVA up to the late 60s, when tighter emissions forced the use of the USELESS PVA. MVA produced more HC than PVA, hence it's demise.
[3] Rarely would a street driven car/engine not benefit from MVA. One case would be locked timing, but even some of these would benefit from a small amount of timing added by MVA.
[4] If the engine was a candidate for MVA [ & most are ], then there is no such thing as it didn't work for me'. What happened was it wasn't dialled in properly. Unfortunately, I have never seen a method quoted, so I developed my own 25 yrs ago. Some of it is in the thread 'Carburetor Tuning' in this section. I suggest you read it & also see what D. Vizard said.
[5] GM cars. As an example. My GTO was driven off the showroom floor, idling at 26*. 6* initial + 20* added from MVA. 10.75:1 comp ratio, mild cam 200* @ 050.
[6] Chry missed the boat on MVA, that simple.

Geez Geoff!

 

[Geoff 2]

Yeah, & you better believe it. This is what you learn when you do it for 40+ yrs.

 

[Kern Dog]

Uhhh.....
#5. I hear far too many guys tell the unbelievable story that they drove/bought their car new off the showroom floor.
I sold new cars in 1985. NO customer ever drove a car out of the building. They always drove them off of the car lot but never out of the showroom. Ever.
This idiom is a tired and old one that has about 0.1 degree of probability.
I'll add that 40 years of doing something sounds great but also proves nothing. I've worked with guys that had double my experience but half my work ethic and skill. Time in the game doesn't always mean competence.
Having manifold vacuum artificially crank up the timing, then a press on the accelerator pulls out 90% of it doesn't seem to make sense.

 

[66 Sat]

I've been trying to get my head around it too.
This is what I want to know: Say for example MV is adding 20 degrees of timing at idle, then when you move off that drops, you're now on half throttle at 1,500 rpm, is MV adding the same amount of timing as PV at this point (even though it's less than it was  at idle)?
If it does, then I get why you might want MV, but on the other hand, if it's only different at idle, and my car runs fine and idles fine, doesn't run hot etc then what's the big deal?
There's a guy on YouTube, The Junkyard Necromancer who has been having some respectful discussions about it and explains it quite well (although I'm still clearly a bit confused) and is currently running some back to back tests with MV and PV to get the definitive real world answer. He's going to measure a bunch of things including fuel consumption, performance etc. He should be posting the results soon so it will be interesting to see which comes out on top.

 

[diesel_lv]

Uhhh.....
#5. I hear far too many guys tell the unbelievable story that they drove/bought their car new off the showroom floor.
I sold new cars in 1985. NO customer ever drove a car out of the building. They always drove them off of the car lot but never out of the showroom. Ever.
This idiom is a tired and old one that has about 0.1 degree of probability.
I'll add that 40 years of doing something sounds great but also proves nothing. I've worked with guys that had double my experience but half my work ethic and skill. Time in the game doesn't always mean competence.
Having manifold vacuum artificially crank up the timing, then a press on the accelerator pulls out 90% of it doesn't seem to make sense.

Engines idle very hot at the exhaust when under 15°-ish. Manifold vacuum at idle, depending on cam and many other variables can be very advantageous. As for dropping timing when accelerating, when you 1st apply throttle during a normal start from stop light etc, your vacuum as throttle blades will not drop, as it will see the manifold vacuum and continue just as a car w ported vacuum. If you stab the accelerator pedal, vacuum is going to drop as normal and centrifugal advance is going to take over. The high exhaust temps of low timing at idle are desirable to burn off pollutants, but not the best for the engine. Many things were done in the name of emissions.

 

[diesel_lv]

I've been trying to get my head around it too.
This is what I want to know: Say for example MV is adding 20 degrees of timing at idle, then when you move off that drops, you're now on half throttle at 1,500 rpm, is MV adding the same amount of timing as PV at this point (even though it's less than it was  at idle)?
If it does, then I get why you might want MV, but on the other hand, if it's only different at idle, and my car runs fine and idles fine, doesn't run hot etc then what's the big deal?
There's a guy on YouTube, The Junkyard Necromancer who has been having some respectful discussions about it and explains it quite well (although I'm still clearly a bit confused) and is currently running some back to back tests with MV and PV to get the definitive real world answer. He's going to measure a bunch of things including fuel consumption, performance etc. He should be posting the results soon so it will be interesting to see which comes out on top.

Yes, as soon as you crack the throttle blades, MV n PV become the same. The MV will only be of benefit at idle depending on engine mods etc and cooler idling.

 

[66 Sat]

This is making more sense now. So the curves are identical but have different starting points? As per my below diagram?

So if I wanted to try MV the only changes I'd need to make would be to the carburetor (lowering the idle speed and what else?)?

How would MV give better throttle response off idle then? I thought I was starting to grasp this but then I went back through the thread and on page 4, a member has lowered the initial timing because he is running MV, but wouldn't that then reduce the overall timing and reduce the amount of advance being provided at part throttle and therefore make the car more sluggish when cruising?

It probably doesn't help either when someone keeps saying "ported vacuum advance is completely useless" - how is it useless if it's doing what it's meant to do when you are driving? Maybe it's "completely useless at idle" but it's not completely useless.

I hope I'm not the only one confused by this.

 

[Black_Sheep]

My distributor has no provision for vacuum advance, the 440 runs just fine…

 

[Kern Dog]

This is making more sense now. So the curves are identical but have different starting points? As per my below diagram?
View attachment 1523687

So if I wanted to try MV the only changes I'd need to make would be to the carburetor (lowering the idle speed and what else?)?

How would MV give better throttle response off idle then? I thought I was starting to grasp this but then I went back through the thread and on page 4, a member has lowered the initial timing because he is running MV, but wouldn't that then reduce the overall timing and reduce the amount of advance being provided at part throttle and therefore make the car more sluggish when cruising?

It probably doesn't help either when someone keeps saying "ported vacuum advance is completely useless" - how is it useless if it's doing what it's meant to do when you are driving? Maybe it's "completely useless at idle" but it's not completely useless.

I hope I'm not the only one confused by this.

I was a Carpenter...your graph probably makes sense to office guys but I don't understand it.

Those pushing the manifold vacuum option haven't explained it in a way that makes sense to me either.
Here is how I see it, feel free to tell me if I am wrong.
Example #1:
Your initial timing is at 18 degrees BTDC, you have ported vacuum that adds NOTHING at idle. The centrifugal advance (mechanical) adds 18 degrees for a total mechanical of 36 degrees. You crack the throttle from an idle and it responds normally as the mechanical advance adds timing.
At WOT, the vacuum source adds nothing to the timing. At part throttle, it adds 20 degrees. You roll down the road at 50 mph with about 56 degrees of advance at light throttle and all seems well.

Example #2:
Your initial is set to 6 degrees BTDC, you have manifold vacuum that adds 20 degrees like the above distributor. This puts me at 26 degrees of timing at idle.
You crack the throttle from an idle and the timing retards.
How could this NOT cause a stumble or hiccup in performance? You go from 26 degrees of timing to some lesser and unknown amount.

 

[66 Sat]

I was a Carpenter...your graph probably makes sense to office guys but I don't understand it.

Those pushing the manifold vacuum option haven't explained it in a way that makes sense to me either.
Here is how I see it, feel free to tell me if I am wrong.
Example #1:
Your initial timing is at 18 degrees BTDC, you have ported vacuum that adds NOTHING at idle. The centrifugal advance (mechanical) adds 18 degrees for a total mechanical of 36 degrees. You crack the throttle from an idle and it responds normally as the mechanical advance adds timing.
At WOT, the vacuum source adds nothing to the timing. At part throttle, it adds 20 degrees. You roll down the road at 50 mph with about 56 degrees of advance at light throttle and all seems well.

Example #2:
Your initial is set to 6 degrees BTDC, you have manifold vacuum that adds 20 degrees like the above distributor. You crack the throttle from an idle and the timing retards.
How could this NOT cause a stumble or hiccup in performance? You go from 26 degrees of timing to some lesser and unknown amount.

The graphs are meant to be identical from idle RPM onwards, in that MV starts high (at idle) and once the throttle is opened immediately drops to a certain point, and PV starts at zero (at idle) and once the throttle is opened immediately rises to a certain point (the same point as MV presumably).

Maybe instead of RPM on the bottom (x axis) I should have written "Throttle opening", so as the throttle opens further vacuum advance reduces.

Apparently the amount of vacuum advance is the same once your past idle, so assuming the initial timing and mechanical curve are the same with both a MV and PV setup then there shouldn't be any stumble with MV, as the vaccum advance would only be dropping to the same point that the PV is rising to.

IF I'm understanding this correctly.

The part I get confused about is that if the above is the case, why the song and dance about MV vs PV, if your car idles ok. Who cares about how much power the car makes at idle? If you have some radical cam that won't idle without 25 degrees of advance then ok, but otherwise what's the point?