Right now I am using a stock mopar electronic vacuum advance distributor with MSD 6AL box and MSD blaster 2 coil. I am considering going to the MSD pro billit with no vacuum advance. Now comes the question. Is it better to use a vacuum advance distributor on a street car with a mild performance setup or no vacuum advance? I know with the stock distributor I can set total timing at 34* at 3500 and rev the motor on up and it will keep advancing to the 50* or so. I am assuming that the MSD pro billit with no vacuum advance would lock in at 34* total and stay there. Is this correct?
Use or don't use vacuum advance on distributor???????????????????
- Thread starter duster360
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SirDan
Well-Known Member
If it's working properly it, wont advance on top of your mechanical advance under hard throttle. it should only advance under light loads. this helps emissions and mpgs slightly. I think it can also be adjusted as well. But like I said it only advances under light throttle, so at WOT it will only advance mechanicly. If you really wanted to delete it off the stock distributor you just take the vacuum line off and plug it. I hope this helps.
I found that my 68 340 S Barracuda ran best with the stock Mopar electronic distributor with the original light spring and one from the Mopar recurve kit worked great with the vacuum advance blocked off. It would have 17° anvance at idle and 36° at 2500 rpm and seemed to not have any more advance after that. It ran great that way.
moper
Well-Known Member
The vacuum advance, as noted, only affects part and light throttle driving. It is designed to maximize fuel mileage and keep plugs cleaner over their life time. Properly set, it does not negatively affect the engine at wide open throttle or idle, and in fact can add 10-30% more mpg depending on the setup and tune. It is a feelable difference when you're driving and I always run them when possible. There is a point where the vacuum characteristics of an engine package make it impossible to adjust the advance to work with it. That's usually with larger cams or engines that are overcammed. At that point, it's better to cap the line and run without it.
Dart Sport 360
Wanna Race?
Where should the vacuum line be connected? Right now I have mine disconnected and the mechanical is set at 34*. I find it smells really rich at idle so I wouldn't mine trying to hook it up.
Should I recheck my total advance after I hook up the line?
Should I recheck my total advance after I hook up the line?
It needs to be connected to a ported vacuum inlet. One that has little or no vacuum at idle, but increases vacuum as the throttle is opened. Yes, you should recheck your timing once it is connected, as you will likely have too much total timing once you reconnect the vacuum advance.
For those of you interested.......
This was written by a former GM engineer as a response to a similar question on a Camaro board:
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. 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 arent fully-deployed until they see about 15 Hg. Manifold vacuum, so those cans dont 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 thats 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 dont understand it, they're on commission, and they want to sell "race car" parts.
__________________
This was written by a former GM engineer as a response to a similar question on a Camaro board:
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. 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 arent fully-deployed until they see about 15 Hg. Manifold vacuum, so those cans dont 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 thats 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 dont understand it, they're on commission, and they want to sell "race car" parts.
__________________
OldmanRick
Well-Known Member
Thanks Revhendo. Appreciate the work that went into that. Glad to know it wasn't my emagination back in the day when i used manifold vac. and juggled the springs for curve along with a jet size or two up and felt a huge improvement on those early/mid 70's cars.
Very enlightening article. Found new stuff out. Thanks.
skep419
No show, all go!
Who wants to rely on the vacuum advance at idle (manifold vacuum). Set the base and run ported vacuum done deal.
Turns out I should have read the article before posting. hahaha
My car has less than 5" of vacuum at idle so manifold vacuum isn't an option.
Turns out I should have read the article before posting. hahaha
My car has less than 5" of vacuum at idle so manifold vacuum isn't an option.
I found that about a year ago or so on a Jeep forum. It's the first time I've ever seen it explained to where I could understand it completely and it made sense. I wish I knew who wrote it so I could thank them.
Me too. I understood some of the function, but not the reason for manifold vacuum. I suppose, it's because I was raised doing service work on everything that had ported vacuum from the factory, so that's how I always "assumed" it should be. You know what they say about assumin stuff. lol Thanks for that. Very useful information.
Dart Sport 360
Wanna Race?
So where exactly would I hook mine up to get manifold vacuum?
Anywhere on the baseplate. I would double check it with a vacuum gauge just to be sure though.
ir3333
Well-Known Member
.. that's why most say i tried it and it didn't work...problem is, it is not always plug and play but may require tuning to your build.
.been playing with mine and w/vac advance mileage is at 19 from 16.9 without advance.
..i think that article can be googled under vacuum advance 101.
.been playing with mine and w/vac advance mileage is at 19 from 16.9 without advance.
..i think that article can be googled under vacuum advance 101.
I think I will try the manifold vacuum on my 360 magnum. I played a bit with timing, I se it about 15-17 idle and 34-36 with the mechanical advance. At idle, the engine is a little rough with the vacuum advance connected to the spark vacuum and seemed to run smoother with the timing advanced manually.
Great article!
Great article!
great article but he forgot one thing. his excuse for advanced timing is correct lean and rich greatly affect burn speed but what was never mentioned was compression. when you are at part throttle lean high vacuum conditions cylinder fill is very low compression can get as low as 50lbs and at this compression burn speed is vary slow. at wide open throttle the cylinder is hopefully filling to its high fill potential making the compression very high speeding the burn speed tremendously. all this is all in the same direction as the article but its all involved and needs to be thought of if you want to lurn and know what is going on inside your engine. another thing to think of... if your initial timing is low like 6 or 10 if your run full time vacuum and step down on the throttle your timing will drop a long way off and cause a hesitation. so if your not running a high initial timing say 20 or maybe higher you may feel a stumble. race car dont run them because 2 reasons there always wide open and dont need advance.(they would use them under yellow cause it helps keep the plugs clean helping throttle response when green comes out and saving fuel) this brings the other reason they dont use them, there illegal rule makers dont want to police them...and leaking and helping make hp on restricted engines. conclusion: if you dont run vacuum advance your missing out. run it full time vacuum but even if you use ported vacuum use it its still better then not using it
moper
Well-Known Member
It's also interesting in that Chrysler had ported vacuum in the 50s on the Hemis. So the conclusion that it was only pre-cat fix is BS as far as I'm concerned. If the engineer in question could be verified I'd feel better about the explanation. I don't see it as entirely plausible given the reality of it's use decades prior to invention of convertors. But that's just me...
skep419
No show, all go!
Tell me if this is right. Say it take 15" of vacuum to fully deploy the vacuum advance. As long as you had over that under load (at idle in gear) you would be better off hooking to manifold vacuum. Once the throttle blades open ported/manifold vacuum are the same? however if you have a large cam and low idle vacuum. ported would be better because the timing wouldn't be consistant.
I think the best thing to do is take the article into consideration. As many variables that come into play with different builds, there' no way one answer can be a blanket. Best thing to do is try ported and manifold vacuum AND with no vacuum advence at all. Super tune it all three ways. Whichever it runs best, use it.
It looks like a fella by the name of John Hinckley wrote it.
[ame]http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Timing101Article.pdf[/ame]
[ame]http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Timing101Article.pdf[/ame]
OwdKasd
Well-Known Member
I am running all MSD with no Vacuum advance on Billet distributer. My car Runs Great for me. I don't race it I just cruise around like an old lady but I have no problems whatsoever. I'm sure my 318 is a bit overcammed so I don't think Vacuum advance would work out too well.
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