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Author Topic: 69 1111480 distributor  (Read 24534 times)
melav8r
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« on: June 24, 2008, 05:41:48 PM »

Anyone know which vacuum advance control pod to use as a replacement in my DZ 302's 480 distributor?
« Last Edit: June 24, 2008, 06:01:49 PM by melav8r » Logged
JohnZ
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« Reply #1 on: June 24, 2008, 07:57:15 PM »

Yup - use a NAPA/Echlin VC-1810; it starts moving at 4" Hg., and is fully-deployed at 8" Hg.. Don't connect it to the stock distributor vacuum port - that's "ported" vacuum (no vacuum at idle). Cap off the port and tee the distributor line into the choke pull-off diaphragm hose - that's full manifold vacuum.
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melav8r
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« Reply #2 on: June 24, 2008, 08:02:59 PM »

Thanks John, found a link in Team Camaro mentioning the VC-1810/B28, same one I assume? It does however call for VC-680/B1 for the 69-70 302, not enough vacuum early enough with the B1?
What about springs while I'm at it?
thanks again.

http://www.camaros.net/forums/showthread.php?t=22139
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JohnZ
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« Reply #3 on: June 25, 2008, 11:46:14 AM »

Any '67'-'69 Z/28 should use the VC-1810, as all had the "30-30" cam and only produced about 9"-10" Hg. vacuum at idle, thus needing a vacuum advance that's fully-deployed at 2" Hg. below that. The '70 Z/28 used the LT-1 cam, which produced more vacuum at idle.

Spring usage can only be determined by curving the distributor on a Sun machine - the "curve kit" springs vary a lot.
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L78 steve
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« Reply #4 on: June 30, 2008, 12:12:33 PM »

The VC-1810 is ideal but NAPA tells me it is NLA.
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69 Z/28 Dover White X33,ZL2,PS,M20,Std.int.04C
67 SS/RS Mt. Green 1W,2LGSR,3SL,4K,5BY,07C
JohnZ
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« Reply #5 on: July 01, 2008, 09:56:13 AM »

The VC-1810 is ideal but NAPA tells me it is NLA.

If you can find them, the VC-1810 also crosses to the following:

AC-Delco #D1316A or D1312C
Niehoff DR305
Borg-Warner V329
Standard VC-177
GM #1116236
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« Reply #6 on: July 01, 2008, 03:40:42 PM »

I'm wondering if anyone has used and noticed that the B28 only gives 10 degrees of advance.I altered one to replace a 355 to give the full 15 degrees.Steve.
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69 Z/28 Dover White X33,ZL2,PS,M20,Std.int.04C
67 SS/RS Mt. Green 1W,2LGSR,3SL,4K,5BY,07C
JohnZ
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« Reply #7 on: July 02, 2008, 12:06:25 PM »

I'm wondering if anyone has used and noticed that the B28 only gives 10 degrees of advance.I altered one to replace a 355 to give the full 15 degrees.Steve.

There's a fair amount of variation to specs in all of the vac cans - it's a good idea to check them out with a Mityvac; they should add 15*-16* of advance.
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asm69
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« Reply #8 on: July 04, 2008, 12:10:43 PM »

John,

Didn't GM engineer the distributor spark advance to be connected to a ported vacuum source?

A ported vacuum source would provide increasing spark advance as the engine rpm rises and as
the manifold vacuum decreases.

Wouldn't connecting the distributor vacuum advance to manifold vacuum provide a spark advance that is
full advance at idle and diminishing spark advance as rpm increases and manifold vacuum decreases?

My understanding of the distributor spark advance system is that the distributor weights along with the
vacuum advance work together to provide a total spark advance according to engine needs.

asm69
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Sauron327
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« Reply #9 on: July 04, 2008, 02:18:04 PM »

 Read John's info under "Maintenance" Page 11.
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fireZ
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« Reply #10 on: July 05, 2008, 04:11:21 PM »

I just read that article you mentioned and am trying it as soon as I am done typing.I hope it works for my 68 Z as well as it sounds it should. Again many thanks John ZZ
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1968 Z28 LA Built
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Sauron327
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« Reply #11 on: July 05, 2008, 06:16:39 PM »

 Just wondering how well it worked. I made the change to my car and noticed an immediate performance increase. Still have a tweak or two but I am very pleased w/ John's advice.  Scott
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fireZ
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« Reply #12 on: July 05, 2008, 06:22:15 PM »

The smell of fuel at idle seems to be gone,and a noticable differance in earlier power. What have you had  to tweak? I have not touched anything yet after doing the vacumm line changes John mentioned.
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Sauron327
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« Reply #13 on: July 05, 2008, 06:57:54 PM »

 Initially all I did was switch the v. line and as you said I too noticed an increase in lower rpm power and responsiveness. And through the entire curve for that matter. The tweaks I mentioned I have not done yet. They pertain to an off idle stumble.John's v. line suggestion helped this as well. Which I was able to also improve with timing, dwell and mixture adjustments. Finite tuning adjustments are areas that I do not possess extensive knowledge. And my motor is a far cry from the high-perf. 302 ( Only a 327 standard bore .010 over, stock pistons, 63 vette heads, mild cam, hyd. lifters, 69 GM Z/28 intake, 670 Street perf. carb., stock exhaust man., 2/12 in. exhaust.  Scott.
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JohnZ
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« Reply #14 on: July 05, 2008, 10:25:34 PM »

John,

Didn't GM engineer the distributor spark advance to be connected to a ported vacuum source?Wouldn't connecting the distributor vacuum advance to manifold vacuum provide a spark advance that is
full advance at idle and diminishing spark advance as rpm increases and manifold vacuum decreases?
asm69


Yes, ported spark was the design condition, as a crutch for emissions. Yes, the full manifold vacuum connection provides full advance at idle, which is EXACTLY what you want. My "timing 101" paper below explains how timing and advance work together, and explains the "ported" vacuum aberration.

                                            TIMING AND VACUUM ADVANCE 101

                                                           John Hinckley


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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