advance/retard ignition curves- help i don't undertsand
Discussion
guys
ok i'm trying to get my head round ignition timing, sorry if this all sounds stupid, i get the idea, spark starts the combustion before the cylinder hits tdc, so that th fame front travels downwards so that in the time it takes to get to the piston the piston is on it's way down
now have i got this the right way round, as the revs increase you have to advance the ignition, ie the sparks is triggered earlier, because the falme front moves at roughly the same speed, but has to leave earlier to get to the piston at the same time because the piston is moving faster
now the igniton i believe is also advanced the more the throttle is open and i can't undertand why and the advance is less and less at high revs os the throttle is open more so that the advance at high revs
full throttle is less then the advance at high revs low throttle
i also always imagined that the curves would be smooth, but they often seem spikey with random troughs and lumps, why is this
sorry if this is all stupid, i'm just trying to get my head around it
pk
ok i'm trying to get my head round ignition timing, sorry if this all sounds stupid, i get the idea, spark starts the combustion before the cylinder hits tdc, so that th fame front travels downwards so that in the time it takes to get to the piston the piston is on it's way down
now have i got this the right way round, as the revs increase you have to advance the ignition, ie the sparks is triggered earlier, because the falme front moves at roughly the same speed, but has to leave earlier to get to the piston at the same time because the piston is moving faster
now the igniton i believe is also advanced the more the throttle is open and i can't undertand why and the advance is less and less at high revs os the throttle is open more so that the advance at high revs
full throttle is less then the advance at high revs low throttle
i also always imagined that the curves would be smooth, but they often seem spikey with random troughs and lumps, why is this
sorry if this is all stupid, i'm just trying to get my head around it
pk
Perhaps it would help to describe what a conventional mechanical distributor does. Mapped systems do more or less the same thing, although they use different mechanisms and are more flexible.
In a conventional mechanical distributor you have two mechanisms to control the ignition timing. A mechanical spring/weight system advances the ignition as the revs rise. This takes account of the fact that you need to light the mixture earlier as the revs increase. A vacuum actuator connected to the inlet manifold advances the ignition as you close the throttle. This takes account of the fact that the mixture burns more slowly when it is less dense.
The static timing is what you get when neither of these mechanisms are doing anything. The static timing is what you get at low rpm when you give it full throttle, you can also reproduce this by simply disconnecting the vac line to the distributor when the engine is idling. The static timing will typically be around 10 degrees.
Lets leave the vac line disconnected for the moment. As you increase the revs, the mechanical advance will advance the ignition timing. For example it might increase the timing by 20 degrees at 4000 rpm and then hold constant at higher revs. This gives you the timing characteristics at full throttle.
The vac advance mechanism dials in even more advance when the throttle is partially closed. For example it might give you an extra 16 degrees of advance when the throttle is completely closed.
These two mechanisms are cumulative. If you close the throttle at high revs you would get 10 degrees static + 20 degrees mechanical + 16 degrees vacuum = 46 degrees total.
I would expect a mapped system to have broadly similar characteristics, maybe with a few extra ripples to take account of the quirks of a particular engine but without any dramatic differences. If you have a more complicated system with turbos, water sprays, multiple injectors etc you might end up with a more complicated map as the knock characteristics of the engine change with load and revs but I'd still expect to see the same basic shape to the map.
In a conventional mechanical distributor you have two mechanisms to control the ignition timing. A mechanical spring/weight system advances the ignition as the revs rise. This takes account of the fact that you need to light the mixture earlier as the revs increase. A vacuum actuator connected to the inlet manifold advances the ignition as you close the throttle. This takes account of the fact that the mixture burns more slowly when it is less dense.
The static timing is what you get when neither of these mechanisms are doing anything. The static timing is what you get at low rpm when you give it full throttle, you can also reproduce this by simply disconnecting the vac line to the distributor when the engine is idling. The static timing will typically be around 10 degrees.
Lets leave the vac line disconnected for the moment. As you increase the revs, the mechanical advance will advance the ignition timing. For example it might increase the timing by 20 degrees at 4000 rpm and then hold constant at higher revs. This gives you the timing characteristics at full throttle.
The vac advance mechanism dials in even more advance when the throttle is partially closed. For example it might give you an extra 16 degrees of advance when the throttle is completely closed.
These two mechanisms are cumulative. If you close the throttle at high revs you would get 10 degrees static + 20 degrees mechanical + 16 degrees vacuum = 46 degrees total.
I would expect a mapped system to have broadly similar characteristics, maybe with a few extra ripples to take account of the quirks of a particular engine but without any dramatic differences. If you have a more complicated system with turbos, water sprays, multiple injectors etc you might end up with a more complicated map as the knock characteristics of the engine change with load and revs but I'd still expect to see the same basic shape to the map.
Good explanation, although I always believed that the vacuum advance is more or less linear to engine load and throttle opening, ie the harder the engine is working, the more ignition advance you get. To do this, the take-off for the vacuum advance is normally on the "atmosphere" side of the throttle butterfly rather than the "manifold" side.
The manifold side works the other way around - small throttle openings equal high vacuum - most "economy" gauges are simply manifold vacuum gauges. A vacuum advance connected to the manifold side would give the characteristics you describe (ie high advance with throttle closed), but this effectively retards the ignition under load, and I don't think that was the objective.
Maybe it's me (again!)
The manifold side works the other way around - small throttle openings equal high vacuum - most "economy" gauges are simply manifold vacuum gauges. A vacuum advance connected to the manifold side would give the characteristics you describe (ie high advance with throttle closed), but this effectively retards the ignition under load, and I don't think that was the objective.
Maybe it's me (again!)
As the name suggests 
the vacuum advance gives you advanced ignition under vacuum. It is often connected to a throttle edge tapping, meaning you get no depression at full throttle, gradually increasing as you close the throttle, and then disappearing at idle as the throttle plate passes the tapping when it is fully closed. If you have a throttle edge tapping you may also have a vacuum delay valve in the vacuum line to stop the vacuum advance kicking in abruptly as you open the throttle from idle.
the vacuum advance gives you advanced ignition under vacuum. It is often connected to a throttle edge tapping, meaning you get no depression at full throttle, gradually increasing as you close the throttle, and then disappearing at idle as the throttle plate passes the tapping when it is fully closed. If you have a throttle edge tapping you may also have a vacuum delay valve in the vacuum line to stop the vacuum advance kicking in abruptly as you open the throttle from idle.Peter's explanation is as usual brilliant ..
one cavaet tho is that most vacuum take offs are just on the atmosphere side of the butterfly and not the manifold side, so at closed throttle there would normally be no vacuum advance trimming .. that vacuum in the vacuum advance line happens just as you open the throttle and the butterfly edge passes across the vacuum take off drilling which then becomes on the manifold side of the butterfly edge ..
This means that at idle you can get static timing of ten degrees or similar, but at very light cruise it can add another 15-20 plus degrees of advance. This decays away as throttle is opened as the manifold depression gets less and less as the throttle becomes less of an inlet restriction.
one cavaet tho is that most vacuum take offs are just on the atmosphere side of the butterfly and not the manifold side, so at closed throttle there would normally be no vacuum advance trimming .. that vacuum in the vacuum advance line happens just as you open the throttle and the butterfly edge passes across the vacuum take off drilling which then becomes on the manifold side of the butterfly edge ..
This means that at idle you can get static timing of ten degrees or similar, but at very light cruise it can add another 15-20 plus degrees of advance. This decays away as throttle is opened as the manifold depression gets less and less as the throttle becomes less of an inlet restriction.
I need to take apart an old dizzy 
I always thought that you needed more advance if the throttle was open, eg if you're motorway cruising and suddenly put your foot down (let's assume revs stay the same in that 0.2s) more fuel/air more time required to burn fuel, more advance needed >> this sentence is bollox, see my next post
The vacuum in the manifold at small throttle openings controls the advance, but the vacuum actualy retards the engine
I seem to remember having this discusion before, but can't remember being told to get back in my box
45deg sounds like a bloody lot
>> just re-read V8s and trackcars comments, I think we might be saying the same thing, it depend where you take the pressure reading as to whether it increases as the throttle opens or closes, hence it can either be a vac advance OR a vac retard. This will have a huge effect on static timing
>> Edited by Incorrigible on Monday 26th September 16:33
>> Edited by Incorrigible on Monday 26th September 16:48
I always thought that you needed more advance if the throttle was open, eg if you're motorway cruising and suddenly put your foot down (let's assume revs stay the same in that 0.2s) more fuel/air more time required to burn fuel, more advance needed >> this sentence is bollox, see my next post
The vacuum in the manifold at small throttle openings controls the advance, but the vacuum actualy retards the engine
I seem to remember having this discusion before, but can't remember being told to get back in my box
45deg sounds like a bloody lot
>> just re-read V8s and trackcars comments, I think we might be saying the same thing, it depend where you take the pressure reading as to whether it increases as the throttle opens or closes, hence it can either be a vac advance OR a vac retard. This will have a huge effect on static timing
>> Edited by Incorrigible on Monday 26th September 16:33
>> Edited by Incorrigible on Monday 26th September 16:48
Sorry, brain fade.
Some of what I said was right, but fundamentaly you need MORE advance with a WEAKER mixture, ie cruising or small throttle openings
So if the vaccum is produced by pre throttle air flow, it's a vaccum retard. Throttle opens, more air, more vaccuum
But if the vacuum take off is in the plenum then you get more vacuum as you close the throttle, more vaccum, ignition advanced
Some of what I said was right, but fundamentaly you need MORE advance with a WEAKER mixture, ie cruising or small throttle openings
So if the vaccum is produced by pre throttle air flow, it's a vaccum retard. Throttle opens, more air, more vaccuum
But if the vacuum take off is in the plenum then you get more vacuum as you close the throttle, more vaccum, ignition advanced
Depression upstream of the throttle is negligeable compared to manifold depression, it is also unpredictable because it depends on the state of the air filter. I don't know of any engines that use upstream depression to control the ignition timing.
A conventional vacuum advance unit advances the ignition when there is vacuum in the manifold i.e. under light load conditions.
A conventional vacuum advance unit advances the ignition when there is vacuum in the manifold i.e. under light load conditions.
With the engined loaded, i.e. low manifold vacuum/ wide open throttle, the volumetric efficiency is high. This means a higher dynamic compression ratio, which makes the mixture burn faster, hence less advance needed. Part throttle required more advance because the less compressed mixture burns more slowly.
You asked for it.
Timing Mech vs Vac advance Good Read
This is a reprint from another board author unknown.
As many of you are aware, timing and vacuum advance is one of my favourite 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 optimise 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 optimising 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 carburettor 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.
_________________
Timing Mech vs Vac advance Good Read
This is a reprint from another board author unknown.
As many of you are aware, timing and vacuum advance is one of my favourite 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 optimise 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 optimising 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 carburettor 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.
_________________
great article 
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