Wayne, thanks for the tip, might do that.

 

That's not ported vacuum, it's manifold vacuum. No good for distributor advancement.

 

Ported vs. Manifold - After my own issues and research, including conflicting info direct from Holley techs, this is what I found and believe:

TIMING AND VACUUM ADVANCE 101
John Hinckley aka John Z

"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.

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."

 

You’re doing fine. The fact that you’re willing to ask questions means that you care about the results. I learn something every time I’m with my 2 year old granddaughter. I have 63 years on her, and she’s smarter than me.

 

That John Hinckley write up is very informative.
Also, the Thread Starter mentioned he had 190* at Thermostat, and then 200* at the water pump inlet. Hmm _ _ _
If I read those words correctly, I'm not seeing any coolant temp drop thru the Radiator ?

Dash Temp gauge shows 220 Degrees, T-stat housing shows 190 degrees @ the temp sending unit and 200 degrees at the water pump inlet/outlet.

Here is a scenario of my AMC 304 V-8, year '77:
This engine has been overheated for decades. It was my Jeep CJ-7 that I traveled the famous Rubicon Jeep Trail for 18-summers. The jeep sat idle mostly the remainder of the year. Seems the electric oem temp gauge quit working and I told the wife if we see Steam coming out from under the hood, it needs a Cool-Down.
The last year of the Rubicon challenge was 2001, and the jeep engine was seldom run while parked indoors until 2022.
I pulled the jeep out of mothballs to take it on a weekend outing with other jeepers and got it running best it would.
Headed to the weekend fun run, the outside temps were 104* and the first long uphill grade into the coastal mountains the engine temp slowly increased from 190-200* on flat ground to a steady increase to 230 on that long pull. Once engine temps reached 230, it quickly jumped to 240 on my elect gauge, and BELCHED COOLANT VICIOUSLY.
I read Over Heating articles until I was blue in the face.
Out of curiosity, I took the jeep to the local radiator shop explaining to the owner my Grief. I kept the engine running for him to scruitinize with his heat temp gun.
I think the engine temps were stable at idle near 190* with outside temps of maybe 100*, The heat gun showed about a 15* drop in coolant temps from radiator inlet to outlet.
After he was done, he looked at me and said "your overheating problems are Elsewhere.
Sure enough, I had found an article that said if all leads have been exhausted for curing an overheating issue, there is a crack somewhere or a stretched Head Bolt.

When cleaning the engine, and looking it over closely, _ _ _ I discovered a lower head bolt with the Head Missing ! _ _ a compromised head bolt.
Replacing that head bolt cured the runnability temperature problem, and the system is stabliized.
The oem radiator is a two row with a very small upper tank for air-pocket compression volume.
The quirky thing about this oem cooling system is that the stabilized level of coolant that the system is happy with, measures 1 to 1-1/2 inches below the top of the cooling tubes in the radiator Cold. If I add coolant to the system to bring the level above the tubes, that excess now gets pushed out once at normal running operation.
Once checking the coolant level cold, the stabilized level measures the 1" or slightly more below the top of the radiator vertical tubes.

Now, after reading that article that Wayne attached, I may need to copy the text to a Notepad file.
Utilizing the Manifold vacuum rather than the Ported vacuum seems logical as the author John mentions.

I may have to fiddle around with the vacuum advance on my '78 Ford 429. I've been running mechanical advance only, with the vacuum advance disconnected.
All i can remember is way back in 1993, I had a terrible time with Pinging on accel. Now, I may have to revisit, and try running Manifold Vacuum.

Note - I "bookmarked this Thread with a note to myself to see post #18. _ _ for further digesting at a later date.