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Author Topic: '68 396 timing suggestions?  (Read 8922 times)
snowballfisher
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« on: October 26, 2005, 08:44:48 PM »

I was wondering if anyone had found a good fuel/timing combination for an old high comp. 396?  I've been told to retard the timing a degree or two to help in cooling.  It's also been reccomended to run a heat range cooler on plugs, and run premium 89-91 and use lead additive.  This was a pretty reliable source but I was just wondering what any thoughts on this might be.  I believe that there's tons of knowledgeable folks on this board, just from the advice that I've already received.
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JohnZ
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« Reply #1 on: October 27, 2005, 12:33:13 PM »

Forget the lead additive (they're useless). You'll note a major improvement in cooling performance, throttle response, and fuel economy by connecting your vacuum advance unit to a full manifold vacuum source (the production configuration had it connected to "ported" vacuum, which provided zero vacuum advance at idle). Retarding the timing will make cooling problems worse - that's what the "ported" vacuum does already; you want all the vacuum advance you can get at idle to reduce EGT.

Ideally, you want "total timing" (sum of initial and centrifugal, vacuum disconnected and plugged) of about 34*-36*, all in by 2800 rpm or so; a good setup is 20*-24* in the distributor (centrifugal) and 10*-12* initial, with a vacuum can that adds 12*-15* and is fully-deployed at idle (connected to full manifold vacuum).   Smiley
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rich69rs
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« Reply #2 on: October 28, 2005, 07:08:29 PM »

I've been restling with the pros and cons of changing from the original carbureted ported vs direct manifold source for the vacuum advance for awhile now.  Haven't made any changes yet - still trying to appreciate all of the potential engineering implications. 

JohnZ, am I thinking clearly with the following???

Let's say that I go ahead and convert over to manifold vacuum as the source for the vacuum canister.  And then we go out on the highway and are cruising at 75 mph @ 3000 rpm.  For the specs you referenced in your previous post, centrifugal advance is all in.  We have initial (static) timing of 10-12 degrees, centrifugal advance of 20-24 degrees which would give us 30 - 36 degrees advance so far.  Iif I'm cruising, throttle partially openend, relatively high manifold vacuum, there is the potential for another 12-15 degrees of vacuum advance (depending on how much vacuum is required to fully stroke the vacuum canister) or 46-49 degrees of total advance. 

Not knowing any better, to me this seems a little too much total advance.  Do you have a suggestion for desirable total advance for the situation when you would be cruising at highway speed and you would have a combination of initial, centrifugal and at least some (if not all) the vacuum advance?

Clearly manifold vacuum to the distributor helps with idle and lower rpm ranges.  I don't want to create an uh-oh (detonation) at highway speeds.

Thanks

Richard
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Richard Thomas
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JohnZ
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« Reply #3 on: October 29, 2005, 10:26:23 AM »

Richard, what you describe is exactly the way it's supposed to work, but it helps to understand the logic behind it; I explained the logic in some detail in an article I wrote in the October/November 2003 issue of "Corvette Enthusiast" magazine (I'm the Technical Editor of that publication), and the cut-and-paste below is a more concise version that explains ignition timing and advance systems and how they work together - kinda long, but you can't explain these systems in layman's terms in one paragraph:

                                            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.
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Ed Bertrand
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« Reply #4 on: October 29, 2005, 11:18:01 AM »

John,

EXCELLENT article. I'm going out right now and move my vacuum advance from ported to full!!

 Grin

Ed
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Jimsl78
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« Reply #5 on: October 29, 2005, 04:18:54 PM »

There is only one vacuum tube connection on my 4053 holley, (and it's ported vacuum.)
If I put a tee in the vacuum secondary tube. will that be the a good place to pick up full vacuum? Huh
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JohnZ
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« Reply #6 on: October 30, 2005, 11:05:00 AM »

No, it's not - the vacuum that operates the secondary opening diaphragm isn't manifold vacuum  at all - it's VENTURI vacuum, created by high-velocity airflow going past a pickup tube orifice in the primary venturi (referring back to my post above, remember that when you put your foot down, manifold vacuum goes to zero, so it can't activate anything). The best place to pick up full manifold vacuum with a 4053 is to tee into the short hose from the nipple at the front of the baseplate that runs to the choke pull-off diaphragm - that's full manifold vacuum. Original Z/28's already have a plastic tee in that hose to feed the vacuum signal to the diverter valve.  Smiley
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Jimsl78
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« Reply #7 on: October 30, 2005, 11:25:54 AM »

No, it's not - the vacuum that operates the secondary opening diaphragm isn't manifold vacuum at all - it's VENTURI vacuum, created by high-velocity airflow going past a pickup tube orifice in the primary venturi (referring back to my post above, remember that when you put your foot down, manifold vacuum goes to zero, so it can't activate anything). The best place to pick up full manifold vacuum with a 4053 is to tee into the short hose from the nipple at the front of the baseplate that runs to the choke pull-off diaphragm - that's full manifold vacuum. Original Z/28's already have a plastic tee in that hose to feed the vacuum signal to the diverter valve. Smiley

Actually that is the tube I meant to describe (thinking about but said somthing else)
Thanks!
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Jimsl78
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« Reply #8 on: October 30, 2005, 12:41:57 PM »

Just tried it, first thing I noticed was the smell of the exhaust at idle wasn't as rich as before, so after I turned the idle down I took it out for a spin. Definitely an improvement! Grin
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rich69rs
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« Reply #9 on: October 30, 2005, 12:46:04 PM »

John:

Thank you very much for the info and the very insightful and helpful article.  I'm going to begin the process of coverting both my '66 Chevelle and the '69RS over to full manifold vaccuum.

Richard

rich69rs
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Richard Thomas
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snowballfisher
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« Reply #10 on: October 30, 2005, 07:06:30 PM »

JohnZ,

I'm really glad I asked the timing question and it webbed out from there into all of this great information.  I thank you very much for taking the time to share with us some great knowledge... and putting it in terms that the average gearhead can grasp.
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