my 4 barrel holly carb using the ported vacuum for my vacuum advance has no reading on my vacuum gauge as i run the RPM up. what gives? is it possible the base gasket is incorrect? engine idles with steady 10" manifold vacuum. or could it be a carb issue?
The "ported" vacuum connection only has vacuum at low throttle openings. If your distributor's vacuum advance is calibrated for that vacuum source(rather than full manifold vacuum) it will be fine.
The ported vacuum is taken just above the throttle plate (s) so I doubt the base gasket is the problem. Perhaps the throttle body to carb gasket, but I'm not sure it is routed thru there. Why not just use manifold vacuum? The ported vacuum was put in by the mfg's for smog purposes. Have you seen any 50's-60's cars (carbs) with ported vacuum access?
This is a manifold vacuum chart. This is always an argument but I've always used manifold vacuum for a vacuum advance. You don' get high vacuum readings shown in this chart using a ported vacuum port designed for smog motors.
Just checking but the vacuum port on the passenger side of the primary metering block is for your vacuum advance connection. Next to the idle mixture screw.
http://www.jalopyjournal.com/forum/showthread.php?t=698266 Heres some issues I had with my holley, including vacuum issues...lots of good info was give to me!
Yeah...I know that but as I said on my previous post it looks like his manifold at a steady 10 pounds is low.That's why I posted the chart.
the motor is a 427/425 corvette, it is ported vacuum in the correct vac.port. 10" at idle is very smooth. still no vacuum from that port when i throttle up. i will check the timing and post. thanks for all your input
You won't have vacuum when you open the throttle, that is the premise on how it works. When it is at idle the vacuum is highest and advances the distr. That setup didn't have enough vacuum 10" is about right, you probably have to help one of the light buckets to close, right?
PSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSST Obviously Chevy guys so here is a news flash . Many of the rest of us use ported vacuum and have for many years. We consider it normal. Secondly why use vaccum at all on a mdified engine? You dont eed it as the curveis already too long. I always use mech only with a modified advance curve. Tis a long subject and takes a lot of typing to explain but you should know very few marine engines use vaccum . Most are totally mechanical including the bowtie based ones.
I posted this before.......Lots of good info on why to use vacuum advance. Hear is some info on ported vs manifold vacuum Quotation thanks to JohnZ G.M engineer 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.
Answer..... yes. Ported vacuum was around long before anyone was concerned with emissions. Unless you have an extremely radical cam, vacuum advance will improve fuel economy even on a performance engine. If the mechanical advance curve isn't right for the setup, that needs correcting whether you have vacuum advance or not.
At 15 MPG from my 426 max wedge (Can or IMP gal) I am content. As a tuneup specialist for many years I understand its function well on a production engine and have used it a time or too on Hi Po chevies but with my own brand it more often then not gets in the way of good performance. However tis true it can be used to fill in the holes so to speak to get the best of both worlds however when I build a street piece I would almost never have a mild enuf engine to make it practical . Usually I want it to start and all in by 3000. Sometimes though despite the best plans one discovers that some engines simply dont want the extra and it gets in the way of a decent running engine (but not always.) That is for sure. With manifold vacuum any extra provided by vacuum drops out at WOT anyway. Depends on the engine. BTW Many marine engines do not use vacuum though and some have very nice ignition timing curves. Don
Boat engines, industrial engines, pump engines, generator engines, truck engines, race engines.... none of those normally run at low loads as a car cruising down the road does. On those kinds of applications a vacuum advance is just unneeded weight and another potential cause of failure.
thanks for all of your input, great article by (themoose) valuable lesson really tells us the importance of the vacuum system
I had an early Holley 700 DP with no ported vac nipple on the meterting block, according to Holley it was a 1965 plate and they were not made with them at that time...not to say all were like that but.... I ran a mech mallary with no vac advance, with a shorter curve. I am running a single point with quicker springs and a B-10 can which adds about 10 more vac connceted to vacuum port on base of carb. No difference between how each one ran except it runs cooler with the vac.
You won't find a ported vacuum port on hardly any car back then. Ford had the odd vacuum advance and requires the special carb of that time to make it work. 49-56. I always laugh when I see guys say that they need ported vacuum for a car with 3 Rochester carbs. I have to ask them exactly where do they expect to get their ported vacuum? You ain't getting it from a Rochester carb!
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