Determining top dead center
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| + | *'''FACT AND FICTION CONCERNING VACUUM ADVANCE....''' | ||
| + | *The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound *engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your *distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or *otherwise. | ||
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| + | *Here's an interesting article on vacuum advance written by a GM engineer: | ||
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| + | *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. | ||
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| + | *TIMING AND VACUUM ADVANCE 101 | ||
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| + | *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. | ||
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| + | *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. | ||
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| + | *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). | ||
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| + | *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. | ||
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| + | *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. | ||
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| + | *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. | ||
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| + | *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. | ||
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| + | *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. | ||
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| + | *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. | ||
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| + | *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. | ||
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