# Timing Question (working on it right Now)

Discussion in 'The Garage' started by Justin Fleming, Sep 5, 2005.

1. ### Justin Fleming1/2 ton statusPremium Member

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I have an 86 k5 stock 305. I cant hardly see the timming indicator. But my question is there is a peep hole, and then 4 notches ahead of the peep hole. I dont know how many degress those marks stand for........

The peep hole and notches are all factory connected to the timing chain cover

Anyone info would be awsome as I am running in and out of the house working on this beast.

The stock setting says 4 degrees in drive with vaccum advance disconnected.

thanks

Justin

2. ### Justin Fleming1/2 ton statusPremium Member

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still need help

Anyone???

Also when I set the timing, vaccum line disconnected and plugged,Normal operating temp,in drive....according to the sticker it should be set to +4 degrees. Once I plug the vaccum line in shouldnt I see the timing jump a ways like 30 something?????

thanks

3. ### BrianDamage1/2 ton status

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not sure about the marks. Mine have always had degree marks, but noit sure how my K5 is (never had to mess with it)

you can take white paint and paint the marks, then wipe off excess with a rag to make the marks stand out and be easier to read

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Take the diameter of the balancer and multiply times pi , this gives the circumference......Example:Using 8" balancer----8" x 3.14 = 25.12", now we know there is 360 degrees in a circle so 360 / 25.12 = 14.33 degrees per inch of circumference, divide by 4 and you got roughly 3.5 degrees per 1/4" of circumference or 7 degrees every 1/2" ..........

Get your tape measure out and some nail polish....voila

Now, we don't check timing while the transmission is in gear, idling in park is much safer.

Disconnect the vacuum line from the advance canister and plug the line, use the light and set the base timing, dial-back-to-zero lights are nice because you can turn the dial to the specified setting and rotate the distributor until the light strobes at TDC on the balancer.

If the timing chain is stretched you just wasted your time anyway so use a vacuum guage hooked up to manifold vacuum and advance the timing until highest vacuum is reached then back off just a touch and lock it down, remember that ported vacuum is used for the vacuum advance canister

5. ### Justin Fleming1/2 ton statusPremium Member

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Timming

Currently the truck has emissions stripped, I have the vaccum for the distributor feed from the intake vaccum. when I disconnect the vaccum line and plug, set the timing to about six degrees. When I plug the vaccum line in shouldnt I see the timing mark advance a ways??? It doesnt seem to move but maybe a degree or two....the truck idles good and is running smooth, I just want to finish the fine tuning and lock the dist down.

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JUSTIN, Justin, justin.......with the vacuum advance plugged into manifold vacuum the the distributor is advancing the timing at idle speed to whatever amount in degrees the canister is rated for, this is not what we want to happen.
You must use a ported vacuum source off the carburetor,
vacuum advance canisters have a number stamped on the bracket that indicates the amount of advance the canister provides when vacuum is applied.

7. ### Justin Fleming1/2 ton statusPremium Member

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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.

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.

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.

8. ### Justin Fleming1/2 ton statusPremium Member

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Bump to top

anyone else have any insite??

9. ### 78blazaerRegistered Member

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OK I must admit, I am one of those guys that didn't know much about how the vacuum advanced really work. Thanks for the info. One question though, manifold vacuum is below the throtle plate correct? If so can I use any of the carb vacuum ports below the throttle plate to set my timing using a vacuum gauge? Will the port to the PCV valve a good source (or is it too big)? BTW I am working on a Qjet.

10. ### dirtwarrior17Banned

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therefor if you idle changed when you plugged in the hose then yes it should advance a couple degrees.

basically the more rpm the more advance... you should see the timing jump if you stab the throttle.

*EDIT*

damn... i feel like im doing homework... How long did it take to write that beast of a post?

IIRC you need to take a vacuum reading from the manifold... not the pcv or carb. there should be a vacuum line behind the carb that comes directly out of the manifold... Its been a while since i've worked on my rig so i forgot where that vacuum line is goin but i believe it goes to the back of the carb.

oh yeah if you don't feel like doing math autozone has timing markers that bolt to the timing cover... should be 4.99 or something close to that.