Vacuum advance units VC31? VC25? VC221? What does this mean?
Moderator: Team
Vacuum advance units VC31? VC25? VC221? What does this mean?
Hey All
I am trying to get a vacuum advance for the distributor on a 70 F100 powered by a 302.
Trying to keep the appearance somewhat stock, I am looking for a vacuum advance that will pull the plate over a total of 30 deg.
I am wondering if the part numbers on these vacuum units translate to the amount of advance they will provide.
The confusing ones are the VC223 etc..I am assuming there is a retard and advance and somehow the part number is indicating both?
Help!
Thanks all..I know this is not a super performance question, but it will help keep the stock look with this warmed up engine.
I am trying to get a vacuum advance for the distributor on a 70 F100 powered by a 302.
Trying to keep the appearance somewhat stock, I am looking for a vacuum advance that will pull the plate over a total of 30 deg.
I am wondering if the part numbers on these vacuum units translate to the amount of advance they will provide.
The confusing ones are the VC223 etc..I am assuming there is a retard and advance and somehow the part number is indicating both?
Help!
Thanks all..I know this is not a super performance question, but it will help keep the stock look with this warmed up engine.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Of the few Dura Spark II dizzy canisters I have tested, they only moved around 11 degrees.
This was done with a mightyvac and a timing light. The difference I ran into was at what vacumm range it happened at.
.
This was done with a mightyvac and a timing light. The difference I ran into was at what vacumm range it happened at.
.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Distributors with vacuum/mechanical advance are application specific, the factory distributor for example on a 302W Falcon is quite different to the 302W Ford 500 truck engine. The factory spent a lot of hours curving ignitions for all eventualities.
Some years back i was involved in doing a charity rally type car which were supposed to have relatively unmodified as fitted engines, the best money we spent was getting it onto the dyno, the tuner spent most of his time on the ignition and very little time on the carburetor, the outcome was chalk and cheese.
There should be a spec sheet on those part numbers you have quoted.
Cheers.
Some years back i was involved in doing a charity rally type car which were supposed to have relatively unmodified as fitted engines, the best money we spent was getting it onto the dyno, the tuner spent most of his time on the ignition and very little time on the carburetor, the outcome was chalk and cheese.
There should be a spec sheet on those part numbers you have quoted.
Cheers.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
If the engine is modified with a bigger cam [ reduction in idle & cruise vacuum ], trying to use factory VA units is fraught with danger. They will more than likely be erratic in operation with the reduced vacuum. An adjustable aftermarket unit should be used.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Standard Motor Products ( SMP ) VC31 should be an adjustable unit with 3/32 allen wrench
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
The nice thing about Ford distributors is, units 1957-1982 (approx) fully interchange including internal parts. The bad part is, NONE of the piece-parts are available separately from anyone, so do NOT lose any! I am unaware of any listing that tells you what results come from what parts. Many pricy aftermarket distributors use OEM Ford parts with a custom aluminum body and bearings where Ford used simple bushings.
Big & little caps interchange up to 1982 as well. After 1982 (electronic) dizzys also interchange up thru the '90s and most will back-date their guts into early bodies, but Ford went to a big-cap body after '82 so there can be fitment problems. Every single engine model has its own vac-adv setting (including zero!), and each also has its own centrifugal adv weight and spring set. An old-style distributor machine is invaluable for this kind of fussy tuning work.
How a vacuum advance works in a given engine is dependent on where the vac adv unit is adjusted since ALL are adjustable with a 3/32" allen wrench inside the vac adv. hose spigot to a needle valve. They also interact with the centrifugal adv, parts, which are mostly reversible: they have a number and adv. profile both right side up and a different one upside down. Each weight has a travel limit-and-stop post that can be adjusted by careful grinding or welding. As-stock, they have a rubber sleeve on the post to limit travel but most rot away quickly from lube-oil & ozone fumes.
Big & little caps interchange up to 1982 as well. After 1982 (electronic) dizzys also interchange up thru the '90s and most will back-date their guts into early bodies, but Ford went to a big-cap body after '82 so there can be fitment problems. Every single engine model has its own vac-adv setting (including zero!), and each also has its own centrifugal adv weight and spring set. An old-style distributor machine is invaluable for this kind of fussy tuning work.
How a vacuum advance works in a given engine is dependent on where the vac adv unit is adjusted since ALL are adjustable with a 3/32" allen wrench inside the vac adv. hose spigot to a needle valve. They also interact with the centrifugal adv, parts, which are mostly reversible: they have a number and adv. profile both right side up and a different one upside down. Each weight has a travel limit-and-stop post that can be adjusted by careful grinding or welding. As-stock, they have a rubber sleeve on the post to limit travel but most rot away quickly from lube-oil & ozone fumes.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Also keep in mind that you have a radical different vacuum curve, depending on if you use ported vs. manifold vacuum.
Personally I prefer manifold for several reasons, but there is a ton of opinions out there.
Frank
Personally I prefer manifold for several reasons, but there is a ton of opinions out there.
Frank
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Other than having advance at idle ( manifold vacuum ) or not with ported vacuum could you explain what you meant by "a radical different vacuum curve,"
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Not only at idle, but there is also much more advance at cruising speed, where the throttle is almost closed.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Just for comparison, 2005 Escalade.
Code: Select all
Mean Best Timing Spark Advance Vs. Load Vs. RPM
gm/cyl. RPM
400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800 5200 5600 6000 6400 6800 7200 7600 8000
0.08 48.02 51.01 54.02 58.02 58.02 58.02 56.02 56.02 52.02 52.02 52.02 56.02 56.02 54.02 54.02 54.02 54.02 54.02 54.02 54.02
0.16 35.01 37.01 44.00 48.02 48.02 48.02 46.02 46.02 42.00 42.00 42.00 46.02 46.02 44.00 44.00 44.00 44.00 44.00 44.00 44.00
0.24 24.00 28.00 35.52 38.42 39.60 40.81 40.00 40.00 38.42 37.21 38.42 40.00 41.21 41.60 41.01 41.01 41.01 41.01 41.01 41.01
0.32 18.00 24.00 30.00 32.00 34.00 36.00 36.00 36.00 36.00 34.00 36.00 36.00 38.00 40.00 40.00 40.00 40.00 40.00 40.00 40.00
0.40 15.49 21.49 27.52 29.52 32.51 34.00 33.52 34.00 33.52 32.51 34.51 34.51 35.52 37.52 37.52 37.52 37.52 37.52 37.52 37.52
0.48 13.01 19.01 25.01 27.01 31.01 32.00 31.01 32.00 31.01 31.01 33.01 33.01 33.01 35.01 35.01 35.01 35.01 35.01 35.01 35.01
0.56 13.01 18.51 24.00 26.51 29.52 31.01 30.51 31.52 30.51 30.51 31.52 32.51 32.51 34.51 34.51 34.51 34.51 34.51 34.51 34.51
0.64 13.01 18.00 23.01 26.00 28.00 30.00 30.00 31.01 30.00 30.00 30.00 32.00 32.00 34.00 34.00 34.00 34.00 34.00 34.00 34.00
0.72 12.00 16.00 20.00 24.00 24.00 22.00 24.75 27.56 28.42 27.34 28.86 29.54 30.68 28.68 28.68 28.68 28.68 28.68 28.68 28.68
0.80 9.01 14.00 19.01 23.01 23.01 21.01 23.76 26.57 26.81 24.68 27.71 26.77 29.34 27.67 27.67 27.67 27.67 27.67 27.67 27.67
0.88 8.00 13.01 18.00 22.00 22.00 20.00 22.75 25.56 25.80 23.67 26.73 25.78 28.35 26.68 26.68 26.68 26.68 26.68 26.68 26.68
0.96 7.01 12.00 17.01 21.01 21.01 19.01 21.76 24.57 24.81 22.68 25.71 24.77 27.34 25.67 25.67 25.67 25.67 25.67 25.67 25.67
1.04 7.01 12.00 17.01 21.01 21.01 19.01 21.76 24.57 24.81 22.68 25.71 24.77 27.34 25.67 25.67 25.67 25.67 25.67 25.67 25.67
1.12 7.01 12.00 17.01 21.01 21.01 19.01 21.76 24.57 24.81 22.68 25.71 24.77 27.34 25.67 25.67 25.67 25.67 25.67 25.67 25.67
1.20 7.01 12.00 17.01 21.01 21.01 19.01 21.76 24.57 24.81 22.68 25.71 24.77 27.34 25.67 25.67 25.67 25.67 25.67 25.67 25.67
Heat is energy, energy is horsepower...but you gotta control the heat.
-Carl
-Carl
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
There still seems to be some misunderstanding about ported or manifold vacuum . A high percentage of carbureted vehicles came from the factory wth vacuum advance hooked to ported vacuum. This did not limit how much vacuum advance there was at cruising speed," where the throttle is almost closed."
The port in the throttle body is uncovered once off idle giving the same vacuum as manifold vacuum.
The carbs that had an EGR valve vacuum port are located differently. If you hook your vacuum advance to this port you may have an oddly acting vacuum advance. Maybe this is where the confusion comes from.
I have never seen a carb with a ported vacuum advance that did not deliver full vacuum advance at cruise speed.
There were some factory carbs that were intended to idle with nearly closed throttle blades. I restored one of these ( Carter ) a few weeks ago. The port was so close to the throttle blade that .030" would expose it. The carb incorporated Idle Bypass AIR for a nearly closed throttle blade. And yes this was the OE carb for a high performance engine. 1969 340 Cuda.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Here is - straight from the horses mouth - a guy who was involved developing that stuff.
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whiz-bang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, drivability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whiz-bang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, drivability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Many of the stock vacuum ports that the factory used for the vacuum advance were on the side of the carb. With out tracing the port, one would think that it is above the butterflies and therefore ported vacuum. It is not. The vacuum port on the Rochester 2 bbl and many 4 bbls goes straight down through the carb base below the butterflies which is manifold vacuum. Just because it goes into the side of the carb it does not mean that it is ported.rgalajda wrote: ↑Tue Jan 31, 2023 5:06 am
There still seems to be some misunderstanding about ported or manifold vacuum . A high percentage of carbureted vehicles came from the factory wth vacuum advance hooked to ported vacuum. This did not limit how much vacuum advance there was at cruising speed," where the throttle is almost closed."
The port in the throttle body is uncovered once off idle giving the same vacuum as manifold vacuum.
The carbs that had an EGR valve vacuum port are located differently. If you hook your vacuum advance to this port you may have an oddly acting vacuum advance. Maybe this is where the confusion comes from.
I have never seen a carb with a ported vacuum advance that did not deliver full vacuum advance at cruise speed.
There were some factory carbs that were intended to idle with nearly closed throttle blades. I restored one of these ( Carter ) a few weeks ago. The port was so close to the throttle blade that .030" would expose it. The carb incorporated Idle Bypass AIR for a nearly closed throttle blade. And yes this was the OE carb for a high performance engine. 1969 340 Cuda.
Like the 97, there is no port on the Rochester carb to get ported vacuum. The hot rod replacement carbs are made to adapt to many different applications and supply both types.
Yes there is manifold vacuum on the advance at idle. That is why the book tells you to disconnect the vacuum line and plug the line before setting the initial advance with a timing light. If you keep the light hooked up, the mark will jump up off of the timing tag when the vacuum is reconnected. That is the way that GM designed it. Of course the advance retards the timing the slit second that the butterflies start to open so it is no big deal. All of the GM engines from the 50s and 60s were designed this way. They all tell you to disconnect and plug the advance line when setting the timing. If you don't plug the line it won't start because of the vacuum leak.
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Yes ,I have read that before. I run manifold vacuum myself. It sounds like we are on the same page understanding this.
Except where you wrote : "Of course the advance retards the timing the slit second that the butterflies start to open so it is no big deal. "
Except where you wrote : "Of course the advance retards the timing the slit second that the butterflies start to open so it is no big deal. "
Re: Vacuum advance units VC31? VC25? VC221? What does this mean?
Another thing, the vacuum source that is used for ignition control should be independent of all other vacuum take offs, and close to the carb base.
Also, preferably not run off a common tree placed a distance from the carb.
Cheers.
Also, preferably not run off a common tree placed a distance from the carb.
Cheers.