I have not used the vacuum advance on my street strip 440 but after reading a couple articles I'm thinking I would benefit from using it with my Holley 850. If you are using vacuum advances I would like to know whether you guys are using ported or manifold vacuum ports. Thanks!
Manifold OR Ported Vacuum Advance
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earlyrides
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Vacuum advance only operates at light throttle cruising, it's purpose is to improve efficiency and economy. With any significant throttle opening there is not enough manifold vacuum for the vacuum advance to add timing.
The vacuum advance is normally hooked to the timed port that is half way up the side of the primary metering block. This port is exposed to the venturi above the throttle blades when they are closed so there will not be vacuum on the port at idle. Connecting it to a port exposed directly to the manifold will cause full vacuum advance at idle which can cause off idle drive-ability problems.
The vacuum advance is normally hooked to the timed port that is half way up the side of the primary metering block. This port is exposed to the venturi above the throttle blades when they are closed so there will not be vacuum on the port at idle. Connecting it to a port exposed directly to the manifold will cause full vacuum advance at idle which can cause off idle drive-ability problems.
The article below makes the argument for manifold vacuum.
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, 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.
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, 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.
I have hooked my vacuum advance to manifold port on my Holley 850 and I get no advace at idle. I don't understand that but I'm good with it. I'm using the MP electronic iginiton conversion kit by the way. Anyway, I have degreed the harmonic balancer to true top dead center and have marked it correctly and have installed timing tape. My readings are as follows: 12 initial 38 mechanical 58 total and it runs like a scallded dog!!!! From what I've read this is way too much total advance but I'm not pinging with 92 octane fuel. My question is should I work to lower the total or just go with it?
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Big John
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Well... the "Timing 101" is aimed at the small block Chevy. See the "SBC" reference in at least one place. Up to when emissions became more important than performance, (mid to late 70's) Mopars used the vacuum created in the venturi of the carb rather than the manifold. No controls. As air rushes through the venturi, it creates more vacuum as the air passes the distributor advance port. BTW, this is basically the same port that opens the end carbs on a six barrel setup... so don't say vacuum drops off... It doesn't.
Other brands (read GM etc.) may have used vacuum off the manifold. I believe they did, but whatever.... Mopar didn't, so the statement "30-40 years of controlling vacuum advance with full manifold vacuum" and has no relevance and again shows this was not written for the Mopar owner.
Read whatever you want...
The vacuum advance is there for drivability and gas mileage. It allows the timing to advance with the amount of air flowing through the carb venturi. It basically brings the advance on a little quicker than the mechanical will. For a basically stock engine, this is usually a good thing. My own experience and most everything I've read bears that out.
For a modified engine, now you are talking a different animal. Depending on cam shaft, cylinder pressure, fuel etc., vacuum advance may not be the best way to go.
What you have done is basically hook the vacuum advance to the manifold... You say it makes no difference, so I'll bet the vacuum advance isn't even working. The diaphragms in the vacuum pot dry out and leak after a while. Pretty typical problem and easy to fix with a new vacuum "pot".
Other brands (read GM etc.) may have used vacuum off the manifold. I believe they did, but whatever.... Mopar didn't, so the statement "30-40 years of controlling vacuum advance with full manifold vacuum" and has no relevance and again shows this was not written for the Mopar owner.
Read whatever you want...
The vacuum advance is there for drivability and gas mileage. It allows the timing to advance with the amount of air flowing through the carb venturi. It basically brings the advance on a little quicker than the mechanical will. For a basically stock engine, this is usually a good thing. My own experience and most everything I've read bears that out.
For a modified engine, now you are talking a different animal. Depending on cam shaft, cylinder pressure, fuel etc., vacuum advance may not be the best way to go.
What you have done is basically hook the vacuum advance to the manifold... You say it makes no difference, so I'll bet the vacuum advance isn't even working. The diaphragms in the vacuum pot dry out and leak after a while. Pretty typical problem and easy to fix with a new vacuum "pot".
From what I'm gathering with Timing and Vacuum Advance 101 is this isn't referring to vacuum off the manifold literally but the carburetor manifold port (which is referred to as manifold vacuum because it is lower down and on the base of the carburetor) vs ported vacuum (located higher up and off the base of the carburetor) that was introduced in the mid '70's industry wide as a means to regulate emissions of all makes including Mopar. The article was written by an engineer who worked for GM from 1964-1984 and then Chrysler 1985-until retirement in 2001. When I stated I had no advance at all I was referring to idle advance and based on what I've read I should have at close to 25 degree's advance while connected to the manifold vacuum at idle. If I were to connect to ported vacuum I should not see advance at idle either. The advance can is working because at 3500 rpm with vacuum advance connected I'm get 58 degree's with the timing light.
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Big John
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You are not applying one very fundamental piece of engine knowledge... and Earlyrides and I have tried to explain it.
Manifold vacuum (and by that I mean anything downstream of the carb venturi) is high with the throttle blades closed (idle). At WOT, there is little or no vacuum in the manifold. Read that again.
So... you've hooked your distributor vacuum advance to the manifold vacuum source. At idle, the vacuum is the highest and it pulls the diaphragm and advances the spark fully. Open the throttle, engine RPM comes up and the vacuum in the manifold drops. The distributor advances mechanically because the RPM is up and if the vacuum drops enough, the distributor vacuum canister releases and retards the spark. At WOT, the mechanical advance fully advanced and the vacuum is now at full retard (and if you've watched Tropic Thunder, you'll know not to go full retard).
IMHO, given the advance numbers you have stated, the vacuum advance is at full as soon as the engine starts and idles. With 25° timing at idle I would expect that about 12° or 13° of that is the vacuum advance. At 26°, I would expect there to be issues with hard starting (engine not turning over) because of that amount of timing. 58° total sounds to me like a recipe for disaster or at least some hard engine knocking. Pull that 13° of vacuum advance because there's no manifold vacuum and you're down to 45° which may be manageable.
Now let's hook it to the correct port on the venturi. Remember, vacuum at the venturi increases with the air flow going through the carb. The mechanical advance runs as before. As the throttle opens and air flow increases, the vacuum increases and pulls the diaphragm in the vacuum canister and advances the ignition.
Now... the whole "ported vacuum" thing. It has no relevance here. Mopar didn't do anything like this until the mid seventies and that was to reduce emissions. Ported vacuum is a simply a vacuum source that is switched on and off. Basically emissions crap that shouldn't be even considered here.
Venturi vacuum... That's what Chrysler used and I think you may need to understand that IT IS NOT PORTED VACUUM. Read that last part again.
I did a quick search and came up with this. http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf Maybe reading this from another source will help you understand.
Manifold vacuum (and by that I mean anything downstream of the carb venturi) is high with the throttle blades closed (idle). At WOT, there is little or no vacuum in the manifold. Read that again.
So... you've hooked your distributor vacuum advance to the manifold vacuum source. At idle, the vacuum is the highest and it pulls the diaphragm and advances the spark fully. Open the throttle, engine RPM comes up and the vacuum in the manifold drops. The distributor advances mechanically because the RPM is up and if the vacuum drops enough, the distributor vacuum canister releases and retards the spark. At WOT, the mechanical advance fully advanced and the vacuum is now at full retard (and if you've watched Tropic Thunder, you'll know not to go full retard).
IMHO, given the advance numbers you have stated, the vacuum advance is at full as soon as the engine starts and idles. With 25° timing at idle I would expect that about 12° or 13° of that is the vacuum advance. At 26°, I would expect there to be issues with hard starting (engine not turning over) because of that amount of timing. 58° total sounds to me like a recipe for disaster or at least some hard engine knocking. Pull that 13° of vacuum advance because there's no manifold vacuum and you're down to 45° which may be manageable.
Now let's hook it to the correct port on the venturi. Remember, vacuum at the venturi increases with the air flow going through the carb. The mechanical advance runs as before. As the throttle opens and air flow increases, the vacuum increases and pulls the diaphragm in the vacuum canister and advances the ignition.
Now... the whole "ported vacuum" thing. It has no relevance here. Mopar didn't do anything like this until the mid seventies and that was to reduce emissions. Ported vacuum is a simply a vacuum source that is switched on and off. Basically emissions crap that shouldn't be even considered here.
Venturi vacuum... That's what Chrysler used and I think you may need to understand that IT IS NOT PORTED VACUUM. Read that last part again.
I did a quick search and came up with this. http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf Maybe reading this from another source will help you understand.
Thanks Big John! I stated I was idling at 12 degrees and being that I was hooked into manifold vacuum I thought I should be up close to 25 degrees but I'm not. So therefore, in my case, I don't know if it would even matter where I hooked the vacuum but I will experiment with the upper port (venturi) on the Holley 850. This 440 has never ran better since I have hooked vacuum advance up! This is not a full on race car but a 10 second street car on nitrous and 11's without.
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mahoney0_00_3
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ha that's why my chevy runs like sh#t thanks john never thought of that!/just went out and changed it and it runs right thanks AL
Big John
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That's a lot of advance in a sprayed car.
We're out of my territory on that one. My late buddy, Nitrous John Gossen scattered a lot of engines figuring out the right combo of timing and fuel. I never got involved with it other than a couple late night rides. I would error on the side of caution here... In fact you may want some timing retard at high RPM with the nitrous. I'd do it with electronic controls, but again, my experience is very limited.
We're out of my territory on that one. My late buddy, Nitrous John Gossen scattered a lot of engines figuring out the right combo of timing and fuel. I never got involved with it other than a couple late night rides. I would error on the side of caution here... In fact you may want some timing retard at high RPM with the nitrous. I'd do it with electronic controls, but again, my experience is very limited.