Need some help fellars, ‘66 F100, 352, mostly stock, I can’t remember cam size, only thing I can remember is the cam is one size bigger then an RV cam, Edelbrock intake, Holly 600 street demon carb, D.U.I dizzy, headers, D.U.I recommends setting initial timing at 12BTC, which I did, I plugged the V/A line with tape, timed it to 12BTC at about 500 RPM, once that was done I hooked up the V/A line and checked timing, it jumped to 28BTC, what the hell am I missing, without V/A line plugged in it’s at 12BTC, with the line plugged in it’s at 28BTC, it starts right up and runs good, shouldn’t it stay at 12BTC with the V/A line plugged or unplugged? Also I don’t have manifold vacuum so I plugged it into a port on the carb

 

Any vacuum on the line, wether from ported or manifold will move it, and that's ok even at idle. Though ported shouldn't have much.
I hope the following doesn't start an FTE war, and I know Resonateur recently suggested ported vac was the way to go, however the following write up finally cured my questions, and among other things explains why advance is good at idle.
"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."

 

Ok, I appreciate the response, do I need to install an fitting in my intake to get manifold vacuum? It’s an Edelbrock and I believe it has place for a fitting, drinker side rear, I think it’s the #4 intake runner

 

Being you have vacuum on the port you have the hose on, the 28* it shows when hooked up, I would not worry about hooking it to manifold vacuum.

What you may want to do is see how much HG that port is pulling at idle and see what manifold is pulling for HG.
You will also want to see at what HG advance starts and is all in by.
You may find (just numbers used) that advance starts at 5 HG and is all in by 10 HG and the port had 12 HG.
So hooking it to manifold vacuum that has say 19 HG would not add any more advance because it is all in by 10 HG and the port has 12 HG.

I would also say if you are not getting any pinging then you are good.
If you are getting pinging you will need to make adjustments to get it to stop.
Dave ----

 

Looks like you have the info you need for now. 2x on make sure it's not pinging at all. If it happens, it will most likely happen under heavy throttle. Later, when you're more comfortable with things you can look at your timing curve, do some wide open throttle runs with a timer, and optimize your curve for power.

Btw, that's a fairly mild cam if it idles smooth at 500 rpm. Also, if you don't have a vacuum gauge, then you should

 

Ok, I really appreciate the responses guys, I’m going to tinker with it more tonite and do some more tuning, thank you for the help

 

I know that long post of "it must have manifold vacuum" I dont think is 100% right for at least 2 reasons.
If that was the case why from the factory are motors not hooked to manifold vacuum and that goes for GM motors that that guy worked for?

I only read vary little of that post but it said it runs cooler at idle with more timing advance with manifold vacuum.
Now when the motor gets a load, pulling a long hill or heavy load, manifold vacuum goes less and timing retards.
You now have the motor working harder building heat and not retarded the timing that will also build more heat.
Is that why factory runs ported vacuum?

Not all motors run the best with manifold vacuum. Just like not all motors like ported vacuum.
What you need to do is try both and see what your motor likes best and run with it.
My .02
Dave ----

 

I agree, I’m going to try both and see what it likes best, I’m always open to all opinions, I’m not trying to start a war lol just picking up as much knowledge as I can

 

I agree with much of the above. You want to take it easy while figuring out what works for your truck, and make sure you don’t do a lot of pinging en-route.
It’s a bit curious you have significant vacuum on your ported, particularly at 500 rpm where the throttle plates should be nearly closed.

I looked at the Street Demon and see that the rear end of the carb has three manifold vacuum locations. They are labelled ‘Power Brakes’, ‘Direct Manifold’, and ‘PCV’. Any one of these will work for the distributor advance provided you get the plumbing adapted.

I wouldn't just 'try both' - I would do a bit of specific testing to understand some of the basics on how your engine is operating
For a closer look, the tools you want are: Vacuum gauge, Vacuum Pump, Tach, and Timing light.
Vacuum pump like this: https://www.skf.com/mityvac/products...mp-kits/mv8000

Use the vacuum gauge to see what the ported and manifold are pulling at idle. Revving the engine in neutral and watching a vac gauge won’t tell you much since it is not under load. However, watching a vacuum gauge while driving under different conditions will be good information on how the vacuum advance will be engaged during those conditions.

Use the vacuum pump attached to the distributor, and the timing light on the mark, to watch how increased levels of vacuum move the timing mark. You can plot your curve by taking multiple readings and plotting Vacuum reading vs. Timing mark. Sounds like you have a new dizzy – so you should be able to get the curve from the factory specs as well. You might do both and compare them.

Your vac advance canister might be adjustable as well – so you` can watch how adjustments affect the response and the max attainable advance.

Measuring your ported and manifold vac at idle, compared with the vacuum pump test, will show why your readings go to 28 at ported idle

Mechanical advance will affect timing at elevated RPMs, but likely not down in the 500 rpm range. With the vacuum advance plugged, and revving the engine in neutral, with a timing light you can watch how the mechanical advance is working. You can plot RPMs vs. Timing mark to get your mechanical curve – and you also might have access to the factory specs. You can run it up until it stops advancing – and this is your all-in advance at WOT, which is initial + mechanical. At WOT, vac should be essentially 0, so will not affect total advance.

BTW, IMO, 12 degrees initial is a bit high to start. I would start at 10

What’s optimum for your engine? Who knows - depends on compression, cam profile / timing, ignition timing and load. On my truck, I dialed back my vacuum advance to eliminate pinging since the engine pulls a ton of vacuum, and my tranny / gearing - and the way I drive it - has me pulling pretty hard through a wide rpm range at medium throttle. I could ping pulling hard through the mid rpm range.

 

After I got the initial timing set at 12 I bumped the RPM to 800, 500 was a bit low for me and that was when I checked timing after installing the vacuum hose, I do have the tools mentioned except the vacuum gauge, I have one on order, I also scheduled it for a new dual exhaust next week, the current system is about 30 yrs old and ratty as all hell, so I will bump it down to 10 and go from there, I would really like to get the exhaust sealed up so I can actually hear things, it’s obnoxious right now.

Yes, the dizzy is new, I have the same one on my 66 F100 with a 240, there was very little adjustment needed and it runs like a sewing machine, it’s been almost 10yrs, I’m use to points dizzies so this is a new learning experience for me

i appreciate the info, I’ll take all your willing to give me so I can get this thing running good.

thanks again!

 

The good old ported vs manifold vacuum discussion
All engine builders I talked to recommend manifold vacuum, ported was used to help with emission control systems. Can you make the engine run with ported? Yes, absolutely. It will also run without V/A at all. But without load you can advance the vacuum a lot to give the engine more time to fully ignite the mixture and thus, safe on gas.

I would also start with 10* and set the idle to 600-650 rpm, depending on your cam. 500 rpm is a bit low. Set the timing and go on a test drive. Find an uphill road and set the engine under heavy load. If you hear no pinging, advance another degree. Continue until you hear pinging and go back 1*. Now you have your initial timing set.

Adjust the carb for highest vacuum. I usually add 1/8 of a turn to make it more rich. Hook up the V/A to manifold vacuum and you should be good to go. I would also check maximum advance at 3000rpm.
If you want you can do all the stuff above, like adjusting the V/A or the mechanical advance with different springs or changing the slot opening. I personal wouldn't spent too much time on a 352 truck engine.

 

I would like to add something here. Wayne explained things pretty well, and I totally agree use full manifold vacuum. But if this DUI you have was built for you by Performance distributors using your engine specs. By all means set it up like they say. But if it is an off the shelf DUI or any other off the shelf distributor you need to time to maximum centrifugal advance. Because there is no telling what kind of curve is in there.
Plug the vacuum line. Run the engine until it stops advancing hopefully this happens before 3000 RPM, 90% of Ford V8 engines especially modified ones are happiest at 34-36 degrees total advance. Add or subtract from the initial advance until you are in that range.
If the initial advance ends up needing to be over 12 degrees you will have to recurve the distributor or settle for what you have.

 

It was built for me by Performance Distributors, straight from Memphis to my door, I have one on my 240 6 and works great

 

Then you should be good to go. I've had a couple of their Duraspark distributors they were dead on. But that was before they were bought out by whoever owns Comp Cams.