Hi all,

For the truck in my signature (300-I6, Duraspark II, manual tranny) I am experimenting with using manifold vacuum for my vacuum advance, as opposed to "ported" vacuum.

Manifold vacuum is high at idle (20" of Hg in my truck), so using it for vacuum advance gives a high timing advance at idle (in my tuck, 10 deg BTDC static timing plus ~18 degrees vacuum advance = 28 deg BTDC total advance at idle).

What I have found is that that this high timing advance at idle causes a consistent "miss" at idle that can't be tuned away with carb settings. This miss is characterized as a consistent (but irregular in periodicity) "engine stumble" (engine shake plus drop in rpm and drop of 1" Hg vacuum). Engine recovers, only to stumble again later.

Putting my timing light on the coil wire during this shows that there is an occasional "miss" in the rapidly firing light.

The only thing that cures this condition is reducing total (static plus vacuum) timing advance at idle to ~18 deg BTDC or lower. If I do that, the stumble goes away and timing light on my coil wire shows no more missing (just a steady machine-gun firing of the timing light).

With timing light on spark plug wire #1, my timing marks are nice and steady at idle and at higher rpms - they do not jump around erratically so the distributor seems fine.

Using spare parts, I have stepwise swapped out the coil, coil wire, and ignition module to no effect. I have cleaned my module-to-harness and distributor-to-harness wiring connections to no effect. I have even blocked off my EGR valve to no effect.

Reducing timing advance at idle seems to be the only thing that cures this miss/stumble. It goes away if I significantly reduce advance at idle.

What could cause this miss that occurs ONLY in idle and ONLY with high timing advance?

Bad spark plug or gap? Transmission/clutch problem that causes a slight load on the engine even in neutral?

 

Matt, my engine is tuned using manifold vacuum, only because the carburetor I have has one vacuum port which is supplied by the venturi. With this, there is ~5-7" Hg @ idle and the vacuum signal never exceeded something like 11-15" Hg.

I am using Motorcraft SP452 sparkplugs. These are Platinum and are specified for the later fuel injected engines. These extend further into the combustion chamber.

My ignition system is controlled by a MSD 6AL box and the sparkplug gap is .050". According to The Frenchtown Flyer, opening the gap effectively advances the timing. Perhaps the ability to fire the sparkplug multiple times while under 3000 RPMs helps to overcome the misfire @ idle?

When I first wired the MSD box, the leads from the distributor were crossed, which advanced the timing significantly. The engine would badly misfire as soon as vacuum was added. I ran for quite some time without vacuum advance until I figured out my mistake [duh! ] This was discussed while at the 2014 OK GTG and what was decided was that the timing was so far advanced, that the spark was going to the wrong terminal in the distributor cap.

I have found with the MSD ignition that my distributor cap and rotor need replacing at about 20,000 miles. My theory is that with multiple firings, the 20K miles is more like 40-60K miles with a standard ignition?

The only problem I have encountered is that when decelerating down a hill with the engine revving higher [1700-2000 RPMs] and the throttle closed, the truck wanted to lurch forward. My thought was that the total advance is too high at that point with manifold vacuum @ 23+". This has been effectively controlled by inserting a reducer in the vacuum line to the distributor. This slows the transition from low to high vacuum and acts like a buffer.

Maybe some of this information will help or will spur others to assist you!

 

Thanks David,

I have come to believe that using manifold vacuum for timing advance is better than using "ported" - mainly after reading the attached article.

Ported vacuum (or venturi) give little to no vacuum advance at idle. Idle is a time when the engine should be able to handle a lot of advance. The article I attached says: "“Ported Vacuum” was strictly an early pre-converter crude emissions strategy and nothing more. Don’t believe anyone who tells you that ported vacuum is a good thing for performance and drivability – it’s not. Anyone with 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."

I believe that part of your excellent gas mileage (https://www.ford-trucks.com/forums/1...-9l-300-a.html) is due to the fact that you are successfully using manifold vacuum to control your vacuum advance. I would like to emulate that, but my system can't seem to handle full vacuum advance at idle without missing. It is a minor miss, but nevertheless irritating to a perfectionist like me. In theory, my engine should be able to handle 18 degrees of vacuum advance at idle, like yours does.

Perhaps my spark plug gap is incorrect or I have a faulty spark plug?
I am using Motorcraft BSF42C (SP-420) copper spark plugs, gapped to 0.044".

As for the ignition setup on my truck - it is all stock components and wiring harness so I don't think there is any way I could have wires crossed.

 

Given what you said above - is it possible that I have my distributor rotated counterclockwise (advanced) so far that I have "skipped" plug firing sequence and the timing I observe using a timing light attached to the cylinder #1 plug wire isn't even showing the actual real engine timing? Would the engine even run like that? I don't know if I have stated this possibility correctly or using correct terms, but perhaps you can get the idea of what I am suggesting.

 

I don't think so. Your truck was running fine until switching to manifold vacuum, right? I listed that information just to show that with timing advanced too far, missing can occur.

Have you checked the condition of your rotor and distributor cap?

Your 20" Hg measurement at idle is similar to what mine was early on. I might even have had 20.5", but would experience pinging. Hmmm, I just remembered your truck still has EGR and mine does not. That is why mine was prone to light - moderate load pinging. Anyway, I had to retard the static timing to 19 - 19.25" Hg. The 20+" Hg vacuum equated to approximately 18 degrees BTDC. This is what I ran until finding that the leads from the distributor were crossed and with zero vacuum advance. It actually worked well, except the starter sporadically dragged. Sometimes, using a timing light can be inaccurate if the harmonic balancer is old and has slipped.

I think that you would benefit from having your distributor "recurved", based on the truck's current set-up. Mine was purchased with the knowledge of the altered parameters of my personal truck. I know of someone who is highly recommended for this type of alteration and uses a Sun diagnostic machine. PM me if you want his contact information.

 

Ported vacuum has been around since the beginning, it isn't some sort of emissions era engineering atrocity foisted on everyone. My 1950 Motors manual for example, says a vacuum port above throttle plates is used to keep a steady idle. Ported vacuum goes back to at least the 1930s. While I enjoy reading these tuning articles as well they should do their homework before saying things like that. It makes me wonder a little bit about the other stuff.

With that out of the way, what people often don't realize is that the vacuum signal is identical between ported and manifold at anything other than idle. That's the whole idea, but another common misconception is that manifold vacuum (somehow) works exactly the opposite to ported.

Manifold vacuum, at idle, is useful to keep combustion temperatures down. If you have a high compression radical cammed motor you already know about manifold vacuum anyway. Most people with stock engines find ported a better choice.

One thing to keep in mind that any weakness in an ignition system shows up under stress. One way to do that as a test is to put the truck in high gear at low speed - maybe around 10 miles an hour. It will start bucking if the wires are bad for example.

A V8 wants about 50 degrees or so of timing at steady cruise on the highway, around 36 mechanical total "all in". Don't worry so much where the initial timing has to fall to get there. Most engines like more than 10, less than 20. If you want to run manifold you'll have to experiment with this. Regardless how you connect the vacuum most distributors are going to need the mechanical advance limited from stock or factory setting.

If it's missing at idle it may be the extra advance is also exposing a problem in the wires, cap, rotor etc. There's no particular reason for a miss as such.

 

The author of that article is a retired GM engineer writing for corvette enthusiast magazine. I figured that indicated that he had done his homework. But maybe not.

Either way, if ported and manifold vacuum signals are identical as you say everywhere except at idle (and we all agree on that), then I can't see the theoretical disadvantage of using manifold. It gives you vacuum advance at idle, when engine load is low.

No offense, and I thank you very much Tedster for your advice. And yours too David. I will check my distributor cap, rotor, and spark plugs. They are all quite new Motorcraft brand parts but you never know there could be a small fault somewhere.

 

Who is a retired GM engineer? So what?

Have to be careful cutting and pasting, posting and reposting things from the internet. There's good stuff there in those articles, but anyone who claims ported vacuum is an emissions era development or invention is smoking their socks that's all I'm saying. I'm sure he's forgotten more than I'll ever know even so. It is true that manifold vacuum will mean cooler engine temps at idle, and stop and go.

It's also true that ported vacuum was a way of reducing certain emissions at idle, NOX iirc. And then they did all sorts of stupid stuff on top of that, at the expense of performance, and mileage, and reliability, and cost etc.

The tell was, smog era engines would even default to manifold vacuum if an overheating condition was indicated. The only take away is you'll have some adjustments to make, it's not really a matter of simply swapping hoses over and calling it good.

 

There are many many different engine setups and how they are tuned. Different distributors, different vacuum advance cans, all kinds of different components can be manipulated to give a certain result.

I think part of your problem is you are using the same components, and are just moving the vacuum to ported or manifold vacuum. It's going to be more complicated than that as you have found out.

In the end, I don't think you are going to end up with any advantage. Why are you worried about the idle performance? And that's really all you are affecting by moving the vacuum source for the advance. Once you open the throttle it's all the same. Ported actually works better for a automatic tranny vehicle that has a low stall converter as most stock vehicles do.

 

Thanks David. I might do that. But first I want to determine how my current distributor is curved. I will do it by disconnecting vacuum advance and observing with a timing light at what rpm the centrifugal advance begins, and at what rpm the centrifugal advance is "all in". Plus observe how many degrees of advance occur over that range.

I may look for a timing sticker/strip to stick over my stock "saw-tooth" indicator, because it can be hard to see/count the teeth. I have to wear my reading glasses to see it very well.

 

Thank you Tedster, your advice is very helpful. I'm still figuring it all out - and was honestly trying to learn from a valid source - the article I attached to my earlier email - written in "Corvette Enthusiast" magazine by a retired GM engineer. When I started looking on the internet for info on using ported versus manifold vacuum, I found many opinions. Some said one was better, some declared the other. I quickly saw that most these were circumstantial opinions based on people simply "swapping hoses" and then basing their declarations on what they personally observed without diagnosis.

I am not interested in "simply swapping hoses over and calling it good". I carefully measured the vacuum signal from my ported, venturi, and manifold vacuum sources during both idle, just off idle (throttle barely cracked open), and high rpm. I saw the differences, and also knew the range of my vacuum canister through its adjustment. I know how to set static timing with the distributor, and how it changes vacuum.

My static timing is set at 10 deg BTDC. Ported vacuum gives me just that at idle. I wanted more advance at idle for reasons given in the magazine article I posted. So I tried manifold vacuum, knowing it would give me full vacuum advance at idle, at the setting I had made on my vacuum canister. Then I found that my engine can't handle that much advance (10 deg static + 18 deg vacuum) at idle without exhibiting a slight miss.

So I turned my vacuum advance canister down (counterclockwise) a lot, until the vacuum advance at manifold vacuum was only giving me +12 degrees vacuum advance at idle (and thus 10 static + 12 vacuum = 22 total degrees advance at idle). Now the miss is almost entirely gone - I can only detect it as a "phantom" stumble that I must concentrate to observe.

A friend of mine here (1986F150six) has a similar engine to mine, and he uses manifold vacuum at 14 degrees static + 18 degrees vacuum = 32 degrees @ idle, and has no "miss".

My basic original question was what would cause my engine to "miss" in idle only at high vacuum advance. I thank you for you answer! Maybe it is a bad component in the ignition system as you suggest.

 

Thanks Dave F, you are always helpful and I value your advice. I didn't want to over-complicate my original question. But I have measured the vacuum signals from my ported, venturi, and manifold sources under different conditions. Previously I was using venturi vacuum (5" Hg at idle, up to 17" at high rpm with no load (parked in neutral)). I was doing that so I could get a little vacuum advance at idle. But I had a problem with that port - it exhibited a characteristic of a rapid transitory "blip" up to 22" Hg just as the throttle was cracked open. This was advancing my spark timing a lot just off the line, resulting in poor power off the starting line and a hot engine-restart off the line stumble. But manifold vacuum drops a lot initially just as the throttle cracks open, and then increases and stabilizes at steady throttle (or idle) at 21" Hg or so in my engine. Using it has eliminated my off-the-line power drop. I like it!

Also, I wanted to use a vacuum source that gave me as much vacuum advance at idle as possible, because I wanted the benefits of using manifold vacuum advance claimed in the magazine article I posted above: better "idle cooling, throttle response, engine efficiency, and fuel economy".

 

Engine vacuum ALWAYS drops when throttle is opened, and then comes back up.

If you are using ported you won't notice what wasn't there to begin with, but it will rise to the same level as the load or speed levels off.

Your problem is, by decreasing the amount of vacuum advance so as to run it at idle, now there probably isn't enough vacuum advance on the "other end" that is, at steady cruise on the highway. V8 can see 50 degrees BTDC or more, lightly loaded, at highway speeds. Vacuum advance is what does that. If you've cranked it way back, then your idle is setup good, but fuel economy will take a huge hit.

I'd be inclined to back off on your initial or crank timing as necessary, and adjust the vacuum to the maximum, to give it as much timing at idle as it will take, light springs, and 34 mechanical "all in" by 3000. This is a little bit different arrangement than many use when ported is connected. This is why just swapping hoses around, as you note, won't really work.

 

Thanks, you give great advice. It may come to that.

Or, alternatively, I could figure out why my engine is missing in idle at high vacuum advance and fix that. Because it theoretically should not since idle is a low-load running condition. Then I would have high vacuum advance under BOTH idle and low-load highway speeds.

That is why I asked in my original post what might cause a miss in idle that occurs due to increased timing advance. If I can solve that, I can utilize high vacuum advance in both idle and light load driving, as 1986F150six does.

 

Concentrate on determining the source of the miss. Can you pull a plug wire at a time at idle and narrow it down to a single cylinder. Observe the engine at night for arcing or crossfire. Have a spray bottle of water handy and maybe see if that might make it worse when wires and boots and distributor sprayed. I like to run a dollop of dielectric grease in the plug and distributor boots, keep the outside clean and free of dirt.

For all that, 32 degrees sounds like a lot at idle. Any engine will start to "hunt" or wander once the advance is too far. Is your friend certain his damper is accurate? The outer steel ring are commonly known to slip off their axis and render the marks inaccurate. I would think that 25 degrees at idle on a 6 banger is a lot.