Vacuum advance 101
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More Turbo
Joined: Dec 2013
Posts: 647
Likes: 10
From: SOUTHBURY CT
Vacuum advance 101
Good read on this topic,
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.
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.
Laughing Gas
Joined: Nov 2002
Posts: 1,198
Likes: 9
From: Fairmont
That was a very nice explanation.
As an engine builder, we are spoiled
in that when we dyno an engine, it is WOT.
We don't even think about vac adv
All we're looking for is best power band.
I have always been a fan of manifold vac adv
and I could not understand why anyone would use ported.
It just didn't make sense, but there were so many people
that absolutely swear by it.
Again, I you did a nice job of working through the
different operating scenarios.
I am probably going to "borrow" your Vac Adv 101
and share it with others.
As an engine builder, we are spoiled
in that when we dyno an engine, it is WOT.
We don't even think about vac adv
All we're looking for is best power band.
I have always been a fan of manifold vac adv
and I could not understand why anyone would use ported.
It just didn't make sense, but there were so many people
that absolutely swear by it.
Again, I you did a nice job of working through the
different operating scenarios.
I am probably going to "borrow" your Vac Adv 101
and share it with others.
FTE Legend
Joined: Jul 2010
Posts: 32,874
Likes: 44
From: Northeast, OK
Good job! I hope it is ok to share it with others, especially the 80-86 forum folks where I hang out most of the time.
As I was reading this I initially wanted to quibble about the ported vs manifold vacuum issue as my experience has been quite different and I've not been able to get a reliable idle on even a stock engine, especially one with an auto tranny, using manifold vacuum. However, by the end of your explanation I think I see that my experience is tainted by emissions-era vacuum cans and that with a proper can I can get the idle stable - and enjoy a better idle with lower heat transfer, and maybe better part-throttle running. Is that what you are saying?
As I was reading this I initially wanted to quibble about the ported vs manifold vacuum issue as my experience has been quite different and I've not been able to get a reliable idle on even a stock engine, especially one with an auto tranny, using manifold vacuum. However, by the end of your explanation I think I see that my experience is tainted by emissions-era vacuum cans and that with a proper can I can get the idle stable - and enjoy a better idle with lower heat transfer, and maybe better part-throttle running. Is that what you are saying?
Laughing Gas
Joined: Dec 2012
Posts: 801
Likes: 7
From: Toledo, Ohio
. This would be a good article to put on CrankShaftCoalition.com/wiki if it's not already on there somewhere...
. I like ported vacuum because I'm a fan of low idle RPMs, 500-600 RPMs, especially when dealing with the brutal mechanical engagement clutches in marine outdrives, and that can be hard to achieve with a lot of ignition advance at idle...
. I like ported vacuum because I'm a fan of low idle RPMs, 500-600 RPMs, especially when dealing with the brutal mechanical engagement clutches in marine outdrives, and that can be hard to achieve with a lot of ignition advance at idle...
FTE Legend
Joined: Jul 2010
Posts: 32,874
Likes: 44
From: Northeast, OK
Buzz - That fits with my experience as well. Plus, with manifold vacuum and an auto tranny you get a different, and sometimes unstable, idle speed in neutral vs in gear since the vacuum changes. And, any change in vacuum changes the advance, which changes RPM, which changes vacuum, which changes the advance, which...…
Postmaster
Joined: Mar 2013
Posts: 3,066
Likes: 7
From: Over There
FTE is crazy glitchy lately, quote isn't working, but this is one of my favorite subjects, argued it many times, so here goes.
First winginit's post is as close to blatant plagiarism as you can get in an internet forum. It's a complete copy and paste of an article written by John Hinckley (not the one that shot Reagan) MANY years ago and copied into many forums since.
Second, in all but it's final conclusion that manifold vacuum is better it's completely wrong in it's theory that it's all about lean vs. rich mixtures. To you I'm just some guy on an internet forum, and that was written by the Corvette hall of fame inductee John Hinckley, but that doesn't make it correct. FWIW, in my years in the industry I've found that Corvette guys are often some of the dumbest mechanically.
TMI- please don't, you're better then that. I've been impressed by your work with 335s, don't lower yourself to this idiocy.
Gary and Buzz, your posts are at the crux of the ported vs. manifold issue, though Buzz I find your logic backwards.
Buzz, what you need is maximum off idle torque and response correct? For this I have no doubt that manifold vacuum advance would work better for you. That is unless you are running very low compression, and/or a significant start retarder. When I talk about this subject I push a new timing spec, ideal idle timing. This is timing at idle which will produce the lowest and strongest idle and is often best found by adjusting for maximum manifold vacuum. I suggest you don't trust me, try it. You'll find that if you unplug the vacuum advance, make sure idle is kept low enough that the centrifugal advance doesn't start, and adjust both timing and carb just by ear for a low, smooth, and strong idle. What you'll find that it's at a point where manifold vacuum is highest and timing is in many performance engines well over 20 deg advance depending on compression and cam.
I suspect you'll read that thinking ok, high manifold vacuum makes since, but over 20 idle timing, that's crazy. That's cause for most idle timing is the spec you use to set the timing with a factory spec somewhere between like 4 and 12, but over 20 damn. Well if you tried it out and do find that it does in fact idle best with a very high timing advance you'll come to find one larger issue. You can't start it with that much advance, unless your compression ratio is small and your starter very strong the high advance will cause the engine to kick back or stop as soon as a cylinder fires making it impossible to start. The solution to this is manifold vacuum advance, set the start timing at a point where it will start well likely in the 15 area and add another 10 or so with vacuum advance to get to that over 20 number at idle.
Next issue is ping, if you ever did get it to idle and start with over 20 initial you'll find that it pings right away off idle, 25 ish is too much for open throttle low RPM. Again the solution is manifold vacuum advance. If set like above your idling at 20+ snap to WOT and the timing drops back to the 15 area which is perfect for off idle WOT.
Gary, your stable idle issue is by far the most common with trying to do manifold vacuum at idle, this is even worse on Fords. Fords in particular the DSII have A LOT of vacuum advance, too much really in most cases. So do GMs often stock but the GM can easily be limited with a common aftermarket part, not so on the Ford. The solution is two fold, experimentation with different vacuum cans, I wish I had part numbers for you but don't handy, secondly and most important, you must make sure that at idle manifold vacuum is as high as possible, it's not in the centrifugal advance, and that the vacuum advance is at full stroke. Do those three things and idle will be stable and with an auto as low as practical. Done right there will be minimal(depending on converter) change in idle speed from park to drive.
Ok why, I guess I need to get into why as I stated earlier that the lean vs. rich thing was total bunk. It's also backwards FWIW slightly lean burns faster then slightly rich, not the other way around as the article states, but this is a very minor difference, not enough to effect timing significantly enough to effect how we use vacuum advance. Get this, and this might just blow your mind but the amount of vacuum advance you need depends entirely on how much manifold vacuum you have. Kinda works out nice that way huh, like someone had that in mind when they first came up with the idea of vacuum advance.
The single biggest thing to effect burn rate is cylinder pressure. Higher cylinder pressure = closer fuel and O2 molecules = faster burn, makes since no? Like gun powder in a cartridge vs. poured out on a table, with me so far. OK well the single biggest thing to effect cylinder pressure is manifold pressure, more manifold pressure(less vacuum) = more cylinder pressure = faster burn. So logic follows more manifold vacuum = more vacuum advance, and what do ya know, that's exactly what the vacuum advance does.
Really all in all I don't really need to argue with winginit's plagiarized article, the engine does that for me.
EDIT, After re-reading what I wrote I should add a bit about why ideal idle timing is often well over 20. This is almost entirely the same logic as above, cylinder pressures are very low at idle. The burn takes so long to spread that it must be started very early to ensure that it's well utilized, that cylinder pressures peak right after TDC and burn and expansion is complete before the exhaust valve opens. Two very important secondary benefits you'll find tend to prove this. You'll find that if you for example in most engines advance the timing from the common low teens on up as I suggest, while doing so you'll find you have to close the throttle to lower idle speed back down, and turn in the idle mixture screws to maintain mixture. So your maintaining the same idle speed with less air and fuel. You'll also often find that the exhaust note gets quieter and exhaust colder cause the burn is complete and started to cool from expansion prior to the exhaust valve opening.
That is unless you want that rough flame spitting choppy idle, then by all means set the initial at 10 or so and use ported to make it driveable.
First winginit's post is as close to blatant plagiarism as you can get in an internet forum. It's a complete copy and paste of an article written by John Hinckley (not the one that shot Reagan) MANY years ago and copied into many forums since.
Second, in all but it's final conclusion that manifold vacuum is better it's completely wrong in it's theory that it's all about lean vs. rich mixtures. To you I'm just some guy on an internet forum, and that was written by the Corvette hall of fame inductee John Hinckley, but that doesn't make it correct. FWIW, in my years in the industry I've found that Corvette guys are often some of the dumbest mechanically.
TMI- please don't, you're better then that. I've been impressed by your work with 335s, don't lower yourself to this idiocy.
Gary and Buzz, your posts are at the crux of the ported vs. manifold issue, though Buzz I find your logic backwards.
Buzz, what you need is maximum off idle torque and response correct? For this I have no doubt that manifold vacuum advance would work better for you. That is unless you are running very low compression, and/or a significant start retarder. When I talk about this subject I push a new timing spec, ideal idle timing. This is timing at idle which will produce the lowest and strongest idle and is often best found by adjusting for maximum manifold vacuum. I suggest you don't trust me, try it. You'll find that if you unplug the vacuum advance, make sure idle is kept low enough that the centrifugal advance doesn't start, and adjust both timing and carb just by ear for a low, smooth, and strong idle. What you'll find that it's at a point where manifold vacuum is highest and timing is in many performance engines well over 20 deg advance depending on compression and cam.
I suspect you'll read that thinking ok, high manifold vacuum makes since, but over 20 idle timing, that's crazy. That's cause for most idle timing is the spec you use to set the timing with a factory spec somewhere between like 4 and 12, but over 20 damn. Well if you tried it out and do find that it does in fact idle best with a very high timing advance you'll come to find one larger issue. You can't start it with that much advance, unless your compression ratio is small and your starter very strong the high advance will cause the engine to kick back or stop as soon as a cylinder fires making it impossible to start. The solution to this is manifold vacuum advance, set the start timing at a point where it will start well likely in the 15 area and add another 10 or so with vacuum advance to get to that over 20 number at idle.
Next issue is ping, if you ever did get it to idle and start with over 20 initial you'll find that it pings right away off idle, 25 ish is too much for open throttle low RPM. Again the solution is manifold vacuum advance. If set like above your idling at 20+ snap to WOT and the timing drops back to the 15 area which is perfect for off idle WOT.
Gary, your stable idle issue is by far the most common with trying to do manifold vacuum at idle, this is even worse on Fords. Fords in particular the DSII have A LOT of vacuum advance, too much really in most cases. So do GMs often stock but the GM can easily be limited with a common aftermarket part, not so on the Ford. The solution is two fold, experimentation with different vacuum cans, I wish I had part numbers for you but don't handy, secondly and most important, you must make sure that at idle manifold vacuum is as high as possible, it's not in the centrifugal advance, and that the vacuum advance is at full stroke. Do those three things and idle will be stable and with an auto as low as practical. Done right there will be minimal(depending on converter) change in idle speed from park to drive.
Ok why, I guess I need to get into why as I stated earlier that the lean vs. rich thing was total bunk. It's also backwards FWIW slightly lean burns faster then slightly rich, not the other way around as the article states, but this is a very minor difference, not enough to effect timing significantly enough to effect how we use vacuum advance. Get this, and this might just blow your mind but the amount of vacuum advance you need depends entirely on how much manifold vacuum you have. Kinda works out nice that way huh, like someone had that in mind when they first came up with the idea of vacuum advance.
The single biggest thing to effect burn rate is cylinder pressure. Higher cylinder pressure = closer fuel and O2 molecules = faster burn, makes since no? Like gun powder in a cartridge vs. poured out on a table, with me so far. OK well the single biggest thing to effect cylinder pressure is manifold pressure, more manifold pressure(less vacuum) = more cylinder pressure = faster burn. So logic follows more manifold vacuum = more vacuum advance, and what do ya know, that's exactly what the vacuum advance does.
Really all in all I don't really need to argue with winginit's plagiarized article, the engine does that for me.
EDIT, After re-reading what I wrote I should add a bit about why ideal idle timing is often well over 20. This is almost entirely the same logic as above, cylinder pressures are very low at idle. The burn takes so long to spread that it must be started very early to ensure that it's well utilized, that cylinder pressures peak right after TDC and burn and expansion is complete before the exhaust valve opens. Two very important secondary benefits you'll find tend to prove this. You'll find that if you for example in most engines advance the timing from the common low teens on up as I suggest, while doing so you'll find you have to close the throttle to lower idle speed back down, and turn in the idle mixture screws to maintain mixture. So your maintaining the same idle speed with less air and fuel. You'll also often find that the exhaust note gets quieter and exhaust colder cause the burn is complete and started to cool from expansion prior to the exhaust valve opening.
That is unless you want that rough flame spitting choppy idle, then by all means set the initial at 10 or so and use ported to make it driveable.
Thread Starter
|
More Turbo
Joined: Dec 2013
Posts: 647
Likes: 10
From: SOUTHBURY CT
FTE is crazy glitchy lately, quote isn't working, but this is one of my favorite subjects, argued it many times, so here goes.
First winginit's brilliant post is as close to blatant plagiarism as you can get in an internet forum. It's a complete copy and paste of an article written by John Hinckley (not the one that shot Reagan) MANY years ago and copied into many forums since.
Second, in all but it's final conclusion that manifold vacuum is better it is completely wrong in it's point that it's all about lean vs. rich mixtures. To you I'm just some guy on an internet forum, and that was written by the Corvette hall of fame inductee John Hinckley, but that doesn't make it correct. FWIW, in my years in the industry I've found that Corvette guys are often some of the dumbest mechanically.
TMI- please don't, you're better then that. I've been impressed by your work with 335s, don't lower yourself to this idiocy.
Gary and Buzz, your posts are at the crux of the ported vs. manifold issue, though Buzz I find your logic backwards.
Buzz, what you need is maximum off idle torque and response correct? For this I have no doubt that manifold vacuum advance would work better for you. That is unless you are running very low compression, or a significant start retarder. When I talk about this subject I push a new timing spec, ideal idle timing. This is timing at idle which will produce the lowest and strongest idle and is best found by adjusting for maximum manifold vacuum. I suggest you don't trust me, try it. You'll find that if you unplug the vacuum, make sure idle is kept low enough that the centrifugal doesn't kick in, and adjust both timing and carb just by ear for a low, smooth, and strong idle. You'll find that it's at a point where manifold vacuum is highest and if after you put a timing light on it you'll find that timing is in many performance engines well over 20 deg advance depending on compression and cam.
I suspect you'll read that thinking ok, high manifold vacuum makes since, but over 20 idle timing, that's crazy. That's cause for most idle timing is the spec you use to set the timing with a factory spec somewhere between like 4 and 12, but over 20 damn. Well if you tried it out and do find that it does in fact idle best with a very high timing advance you'll come to find one larger issue. You can't start it with that much advance, unless your compression ratio is small and your starter very strong the high advance will cause the engine to kick back or stop as soon as a cylinder fires making it impossible to start. The solution to this is manifold vacuum advance, set the start timing at a point where it will start well likely in the 15 area and add another 10 or so with vacuum advance to get to that over 20 number at idle.
Next issue is ping, if you ever did get it to idle and start with over 20 initial you'll find that it pings right away off idle, 25 ish is too much for open throttle low RPM. Again the solution is manifold vacuum advance. If set like above your idling at 20+ snap to WOT and the timing drops back to the 15 area which is perfect for off idle WOT.
Gary, your stable idle issue is by far the most common with trying to do manifold vacuum at idle, this is even worse on Fords. Fords in particular the DSII have A LOT of vacuum advance, too much really in most cases. So do GMs often stock but the GM can easily be limited with a common aftermarket part, not so on the Ford. The solution is two fold, experimentation with different vacuum cans, I wish I had part numbers for you but don't handy, secondly and most important, you must make sure that at idle manifold vacuum is as high as possible, it's not in the centrifugal advance, and that the vacuum advance is at full stroke. Do those three things and idle will be stable and with an auto as low as practical. Done right there will be minimal(depending on converter) change in idle speed from park to drive.
Ok why, I guess I need to get into why as I stated earlier that the lean vs. rich thing was total bunk. It's also backwards FWIW slightly lean burns faster then slightly rich, not the other way around as the article states, but this is a very minor difference, not enough to effect timing significantly enough to effect how we use vacuum advance. Get this, and this might just blow your mind but the amount of vacuum advance you need depends entirely on how much manifold vacuum you have. Kinda works out nice that way huh, like someone had that in mind when they first came up with the idea of vacuum advance.
The single biggest thing to effect burn rate is cylinder pressure. Higher cylinder pressure = closer fuel and O2 molecules = faster burn, makes since no? Like gun powder in a cartridge vs. poured out on a table, with me so far. OK well the single biggest thing to effect cylinder pressure is manifold pressure, more manifold pressure(less vacuum) = more cylinder pressure = faster burn. So logic follows more manifold vacuum = more vacuum advance, and what do ya know, that's exactly what the vacuum advance does.
Really all in all I don't really need to argue with winginit's plagiarized article, the engine does that for me.
First winginit's brilliant post is as close to blatant plagiarism as you can get in an internet forum. It's a complete copy and paste of an article written by John Hinckley (not the one that shot Reagan) MANY years ago and copied into many forums since.
Second, in all but it's final conclusion that manifold vacuum is better it is completely wrong in it's point that it's all about lean vs. rich mixtures. To you I'm just some guy on an internet forum, and that was written by the Corvette hall of fame inductee John Hinckley, but that doesn't make it correct. FWIW, in my years in the industry I've found that Corvette guys are often some of the dumbest mechanically.
TMI- please don't, you're better then that. I've been impressed by your work with 335s, don't lower yourself to this idiocy.
Gary and Buzz, your posts are at the crux of the ported vs. manifold issue, though Buzz I find your logic backwards.
Buzz, what you need is maximum off idle torque and response correct? For this I have no doubt that manifold vacuum advance would work better for you. That is unless you are running very low compression, or a significant start retarder. When I talk about this subject I push a new timing spec, ideal idle timing. This is timing at idle which will produce the lowest and strongest idle and is best found by adjusting for maximum manifold vacuum. I suggest you don't trust me, try it. You'll find that if you unplug the vacuum, make sure idle is kept low enough that the centrifugal doesn't kick in, and adjust both timing and carb just by ear for a low, smooth, and strong idle. You'll find that it's at a point where manifold vacuum is highest and if after you put a timing light on it you'll find that timing is in many performance engines well over 20 deg advance depending on compression and cam.
I suspect you'll read that thinking ok, high manifold vacuum makes since, but over 20 idle timing, that's crazy. That's cause for most idle timing is the spec you use to set the timing with a factory spec somewhere between like 4 and 12, but over 20 damn. Well if you tried it out and do find that it does in fact idle best with a very high timing advance you'll come to find one larger issue. You can't start it with that much advance, unless your compression ratio is small and your starter very strong the high advance will cause the engine to kick back or stop as soon as a cylinder fires making it impossible to start. The solution to this is manifold vacuum advance, set the start timing at a point where it will start well likely in the 15 area and add another 10 or so with vacuum advance to get to that over 20 number at idle.
Next issue is ping, if you ever did get it to idle and start with over 20 initial you'll find that it pings right away off idle, 25 ish is too much for open throttle low RPM. Again the solution is manifold vacuum advance. If set like above your idling at 20+ snap to WOT and the timing drops back to the 15 area which is perfect for off idle WOT.
Gary, your stable idle issue is by far the most common with trying to do manifold vacuum at idle, this is even worse on Fords. Fords in particular the DSII have A LOT of vacuum advance, too much really in most cases. So do GMs often stock but the GM can easily be limited with a common aftermarket part, not so on the Ford. The solution is two fold, experimentation with different vacuum cans, I wish I had part numbers for you but don't handy, secondly and most important, you must make sure that at idle manifold vacuum is as high as possible, it's not in the centrifugal advance, and that the vacuum advance is at full stroke. Do those three things and idle will be stable and with an auto as low as practical. Done right there will be minimal(depending on converter) change in idle speed from park to drive.
Ok why, I guess I need to get into why as I stated earlier that the lean vs. rich thing was total bunk. It's also backwards FWIW slightly lean burns faster then slightly rich, not the other way around as the article states, but this is a very minor difference, not enough to effect timing significantly enough to effect how we use vacuum advance. Get this, and this might just blow your mind but the amount of vacuum advance you need depends entirely on how much manifold vacuum you have. Kinda works out nice that way huh, like someone had that in mind when they first came up with the idea of vacuum advance.
The single biggest thing to effect burn rate is cylinder pressure. Higher cylinder pressure = closer fuel and O2 molecules = faster burn, makes since no? Like gun powder in a cartridge vs. poured out on a table, with me so far. OK well the single biggest thing to effect cylinder pressure is manifold pressure, more manifold pressure(less vacuum) = more cylinder pressure = faster burn. So logic follows more manifold vacuum = more vacuum advance, and what do ya know, that's exactly what the vacuum advance does.
Really all in all I don't really need to argue with winginit's plagiarized article, the engine does that for me.
I recently installed a pertronix ignition kit , and noticed my vacuum advance was not working. I went to Nappa to buy another one and they told me i don`t need it with electronic ignition so i went home and started researching and came across that article and thought other people would like to read it and i would be able to located it again if i wanted to reference the artificial
I think an Apology is in order.
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Postmaster
Joined: Mar 2013
Posts: 3,066
Likes: 7
From: Over There
HEY PAL. I didn`t plagiarize anyone ones writing.
I recently installed a pertronix ignition kit , and noticed my vacuum advance was not working. I went to Nappa to buy another one and they told me i don`t need it with electronic ignition so i went home and started researching and came across that article and thought other people would like to read it and i would be able to located it again if i wanted to reference the artificial
I think an Apology is in order.
I recently installed a pertronix ignition kit , and noticed my vacuum advance was not working. I went to Nappa to buy another one and they told me i don`t need it with electronic ignition so i went home and started researching and came across that article and thought other people would like to read it and i would be able to located it again if i wanted to reference the artificial
I think an Apology is in order.
Absolutely not, it read as if YOU had written it, that YOU were the former GM engineer "involved in the development". NOWHERE did you mention it's a copy/paste or accredit who actually wrote it, or at a bare minimum just post where you found it. Clearly you wanted people here to think that YOU wrote it when you didn't. I'm even willing to bet you found it in another forum, as it's been posted in many forums many times, that that post DID accredit it to another and you "failed" to copy/paste that part when you copy/pasted the rest. Then of course there's the simple fact that it's completely wrong, that makes it even worse. YOU, BUDDY, owe an apology to everyone on this forum that read that post, and I'd say John Hinckley but the article is so wrong I don't think it warrants it.
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More Turbo
Joined: Dec 2013
Posts: 647
Likes: 10
From: SOUTHBURY CT
Dude your winded up pretty right, I did get it from another forum after searching . It has been around for years I'm sure. I'm just a guy fixing up a old truck . I copy and pasted so I can reference it on my profile . Your a big talker behind the keyboard . You accused me of that to my face would get you spanked!
Lead Driver
Joined: Jul 2011
Posts: 8,678
Likes: 191
From: On the Edge of the Desert
Regardless who's article it is, it's good. I don't give a flying F who wrote it.
Back on track here, that's a lot of information to take in at once. And I'm still a bit confused on something.
Why do you want such high timing at idle via manifold vacuum? I have my dizzy hooked up to ported vac, with initial timing set at about 14-15*. It runs pretty darn good like that. Starts easy, has pep.
The way I'm seeing this, once the throttle plates are open, ported and manifold vacuum are the same. Or not? I'm not sure now.
If they are, the. The only difference would be performance at idle. So what's the difference at idle?
Back on track here, that's a lot of information to take in at once. And I'm still a bit confused on something.
Why do you want such high timing at idle via manifold vacuum? I have my dizzy hooked up to ported vac, with initial timing set at about 14-15*. It runs pretty darn good like that. Starts easy, has pep.
The way I'm seeing this, once the throttle plates are open, ported and manifold vacuum are the same. Or not? I'm not sure now.
If they are, the. The only difference would be performance at idle. So what's the difference at idle?
FTE Legend
Joined: Jul 2010
Posts: 32,874
Likes: 44
From: Northeast, OK
Yes, for the most part ported and manifold vacuum are the same at some point as the throttle plates open. Just when that is varies by carb. And, a few carbs had a ported vacuum that never reached equality with the manifold vacuum, but most do.
As for your question of why worry about it, I agree. My MPG isn't going to vary enough to make a difference if the idle is a bit faster due to having manifold vacuum. And with ported vacuum I know that at idle I'm not going to have any feedback loop going where a change in load, whether it is dropping the transmission in gear or the A/C compressor kicking on, is going to change the RPM, which changes the vacuum, which changes the advance, which changes the RPM. So I'm happy with the setup I have.
As for your question of why worry about it, I agree. My MPG isn't going to vary enough to make a difference if the idle is a bit faster due to having manifold vacuum. And with ported vacuum I know that at idle I'm not going to have any feedback loop going where a change in load, whether it is dropping the transmission in gear or the A/C compressor kicking on, is going to change the RPM, which changes the vacuum, which changes the advance, which changes the RPM. So I'm happy with the setup I have.
FTE Legend
Joined: Jul 2010
Posts: 32,874
Likes: 44
From: Northeast, OK
Yes, but under operating conditions, not just in the driveway. That way you can tell at what point, in throttle, RPM, and MPH the two are equal. I would be very interested in that.
Lead Driver
Joined: Jul 2011
Posts: 8,678
Likes: 191
From: On the Edge of the Desert
Right, I already have a manifold vac gauge.