which is recommended to use to connect a dist. to? what is the advantages of one over the other? does one provide more power or the other better fuel economy? does one provide higher total timing? why two?
Could depend on which engine, but most vacuum advance designs use ported vacuum. Total timing is determined mechanically by the combination of initial setting and range of advance in the mechanical advance unit. Relative to "higher" total timing, neither ported nor manifold vacuum will give you a greater total advance, but it will sure give you a different curve. Bob
A lot of engine actually use Manifold vacuum,if you look at older cars tune up book it tells you to unhook vacuum hose when setting the timing.Advance at idle usually make it run cooler and at cruse it well allow you to back out of the throttle and save gas at freeway speeds. Try driving your car with two vacuum guages hooked up one to each and seeing what they do.
I posted this once before.......Lots of good info. Hear is some info on ported vs manifold vacuum Quotation thanks to JohnZ 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.
Great info, thank you for taking the time to post all this, which I believe is absolutely correct and spot on. With your involvement with GM and obvious better understanding than most of us on this topic, can you advise how this would apply to a mild supercharged street engine? I believe that Vacuum advance still plays an important role but many people seem to think that a blower motor does not for some reason. Thanks in advance.
Many blown motors could use a vacuum advance as well as unblown race cars. Many blower builders have good info on their web sites many people choose to ignore and lock out the advance
I have always heard to never run the vacuum advance to the "ported" source, but never understood why. Thanks for a clear and concise explanation.
Good Article......might explain why my Dual Point Dist without vac advance runs fat at idle, seems alright once the R's are up.
Unless you are running a new replacement carb, you probably don't have a choice. Most pre-smog engines and carbs don't have any ported vacuum ports. There is no place to get it even if you wanted it.
I had a 68 Mustang with a 289. It had 2 vacuum lines going to it's distributor. Although I can't remember the particular source for both of them. Perhaps they used both as a smog thing? I did remove a washer like spacer inside of this unit, and the car seemed to run better. Perhaps it added a bit more advance.
Does the manifold vacuum still hold true for an automatic trans? Cause when you put it in gear, vacuum drops, thus you'd loose advance right? Maybe I'll get me an adjustable can and try keeping all the advance in while in gear. Sent from my iPod touch using TJJ
By all means try ported and unported vacuum. I think the widespread claim that all pre-emissions engines vac advance got full manifold vacuum is not accurate. Here are a few examples from the good old days. 1958 Edsel factory "Maintenance" (shop manual) - Section 9 Engine electrical -Description and operation 8 cylinder - "The vacuum port in the carburetor throttle bore is located above the throttle plates to prevent spark advance at idle." 6 cylinder - there is no mech advance - it is completely vacuum controlled - there are 3 vac ports in the carb throat and they are all above the closed throttle plate. A "spark valve" on the carb determines whether to use venturi vacuum (varies with air flow) or one of the other 2. 1948 Chevy Truck Shop Manual - Section 12-26 - Electrical system "As the throttle is opened slightly from the idling position, manifold vacuum is applied to the vacuum spark control passage in the carburetor..." Today MSD says this about the 8360 replacement dizzy for Chebbies with points. "The vacuum (advance) line should be routed to a ported vacuum outlet above the throttle plates." However at least One of ACCEL's street distributors does call for "manifold vacuum" http://prestoliteweb.com/Portals/0/downloads/accel/pdf/34000V.PDF >>> First, ported/unported only effects idle timing. <<< As soon as the throttle is cracked open, I'm driving with full manifold vacuum anyhow. I believe Overheating at idle is more likely a cooling system issue like low pump speed, less able to stuff water through an aged radiator, and near zero airflow thru the radiator than increased heat rejection from "retarded" timing. It's not the cylinder head but the exhaust valve that gets really blasted by the high exhaust gas temp that results from retarded from optimum timing. The amount of combustion it takes to keep an engine idling is pretty small, so the exhaust valve's life at high power is much tougher/hotter. I think the reason behind using ported vacuum was and is the quest for a smooth consistent idle by keeping the ignition timing "fixed.". Having the timing change with vacuum potentially adds a feedback loop. Put an auto trans in drive, and the engine speed drops a bit, lowering intake manifold vacuum. Most vac advance curves seem to max out around 12 to 14 inches of Hg, so stock engines idling would add the full 20 or so degrees of vac advance to the initial setting using manifold vacuum.
Any time this question come up do a search under c9's posts. Here is a good thread from 3 years ago http://www.jalopyjournal.com/forum/showthread.php?t=314524&highlight=vacuum&showall=1
In 1965, I remember having a problem finding a vacuum port on my 57 Chevy 283 (I was a kid). This older guy gave me a carb mounting stud that was hollow. I ran my vacuum advance off it, it worked! On the other hand, I had a 305 in an early 80's longbed pick-up and couldn't run ported or non ported vacuum, or it would ping on the Hi-way. I ran it w/o any vacuum advance and it ran great, but I got bad gas mileage.
I just found this as I am working out pinging issues. Thank you for taking the time to type it all up!! I understand now, as well as understanding why my Dad told me to run to manifold vacuum.
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