I picked up my new Hot Rod and was reading the article on Driveability and the statement was made that if you don't have 14-16 inches of vacuum at idle don't run a vacuum advance. What do you guys think.
Still true in most respects... 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. __________________
It (manifold vs ported) depends on a lot of factors but bottom line if it runs better with manifold run it that way, if it idles better ported run it that way. It only effects idle. As soon as you touch the throttle they are the same....
That's not always true.... Depending on where the 'ported' vacuum is picked up, it can be exactly opposite to manifold vacuum. A lot of the 'ported vacuum' lines were originally controlled with a temp switch to prevent overheating. One little thing the aftermarket carb makers don't tell you is most of those 'vacuum' fittings on their carbs are there for emissions purposes only, and by law they can't advise you to defeat them. I've always had the best results with manifold vacuum; you may have to experiment with initial timing and idle speed a bit for the best idle, but overall performance will be better.
That would be venturi vacuum, not ported or timed. We are speaking of vacuum ported or timed to throttle opening. So I have set my mechanical to 20 and I run 18 initial. I now throw 20 more vacuum advance on top of my 18 initial and it is 38 at idle. There are many cars which would rather not be at 38 degrees at idle. So if hooking up manifold causes your car to miss at idle, I would recommend going to ported/timed. To find what timing your car wants at idle, disconnect the vacuum advance and rotate the distributor until you find max RPM or high vacuum. Record this number. Now look at your initial plus your vacuum and figure out which combination gets closes to your ideal idle timing. Example 28 degrees = best idle. Initial is 8, you need manifold vacuum to get close, say 8 plus 16. But if your initial is 18 and your vacuum is 20 thats 38 and will probably not idle well. Use common sense and tune your car, there is no one size fits all answer. jm2c Hoop
I have read that Timing and Vacuum Advance 101 a few times before, and it never gets old. I have found that a LOT of people don't understand how to use vacuum advance, and having them read this article helps. Thanks for re-posting.
regarding hoop98's advice,I tried to buy a Echlin vc1810 unit,the counterman said vc1810 is an obsulete number. Said he couldn't cross it to a current number,can anyone help me? Greg thanks
Borg-Warner V329 Niehoff DR305 Wells DV1810 Standard Motor Prod. VC177 Number***Start******Dist Degrees********Max VC-24A***8-11*********8*************16-18 VC-171***7-9**********8*************14-16 VC-177***3-5**********8*************5.75-8 VC-181***5-7**********8*************11-13 VC-183***5-7**********8*************13-15 8 Distributor = 16 crank degrees
Emissions side of things: true venturi vacuum is rare. Ford and Chrysler Corp used venturi vacuum to open EGR exhaust gas recirculation valves but had to use vacuum amplifiers to do that. In my job 95% of carbs I see have no venturi vacuum port. There would be no use for it, usually. Venturi vacuum is very weak... That leaves full manifold vacuum and ported vacuum available at the carb. These vacuum connections are what run 99% of vacuum operated devices. Generally speaking vacuum advance, EGR, EVAP evaporative emissions systems are run off ported vacuum. Yes I occasionally see full manifold vacuum to vacuum advance but in my experience that is not the norm. Generally speaking brake booster, PCV positive crankcase ventilation, air injection control, MAP manifold absolute pressure sensor, TAC thermal air cleaner, choke pull offs run off full manifold vacuum. Ported vacuum is sourced from a slot or small hole near the throttle blade. At idle the blade is below the passage. Off idle the passage is exposed to manifold vacuum. Of course at WOT no vacuum is present anywhere unless there's a vacuum tank. However, 99% of vehicle operation for most of us takes place not too far above idle. There's still vacuum present at that situation. I agree that at WOT vacuum advance isn't contributing to total advance. Unless you're a street outlaw you are usually below WOT and vacuum advance applies to total advance. TVS thermal vacuum switches are temperature controlled vacuum switches and come in lots of variations. In a performance situation I would expect these to be missing or bypassed. An exception would be those vehicles still subject to emission " smog" testing. In the emissions era (65-late 80s), retarding timing over a previous pre-emissions setting is usually to reduce the NOX nitrogen oxides emission, although it lowers HCs hydrocarbons and CO carbon monoxide, too. Hence the " NOX" kits sold in CA that effectively bypassed vacuum advance or reset initial timing. I agree that some early attempts by the manufacturers to reduce smog made a really poor running car. However you can't call PCV, air injection or EGR a band-aid. They are all still around and they work. HAMB brings together people who know everything there is to know about anything automotive in collective or group. I only know about smog check and emissions repair because I've done it for 22 years in CA. Something like 30k+ smog checks and repairs. Oh, and run a vacuum advance!
The Jalopy Journal
