Wish we could give ‘thumbs up’ here. A lot of people in this thread are giving some good information! And just for the record - I do agree with MOST of what Rob was saying too. When it comes to setting your car’s timing, there are multiple ways to do it and get reasonable results. Rob’s method actually works better on a car with a mean cam. It’s the only way to deal with a car that has low vacuum and a lopey idle.
This is so fun!!
If you dial it down to the point where it’s not too advanced for cruise with vac advance, then your idle is going to be problematic, because timing will be too low with the blades closed.
Not sure what you mean above. What I do is slow down the vacuum advance start point. Since we are not idling at this point, the ported vacuum has kicked in. There is no change in timing at idle and it is running at the preset 12 to 15 degrees. You may have meant dialing back the initial timing, but that is not what is done.
Using straight manifold vacuum in most cases “over advances” the motor at idle. With 8 to 12 degrees of initial timing and then stacking full vacuum (11 to 12 degrees at max vacuum) advance on it produces 20 to 24 degrees of advance at idle. Then as soon as you open the throttle the engine is treated to partial loss or full loss of the advance back to the initial timing setting. Exactly what the engine does not want. Ported vacuum had nothing to do with emission controls in the late 60’s. It was the best method of transition from the idle circuit to the low speed demands of the engine by increasing advance by allowing vacuum to the vacuum advance. This was also in tandem with the accelerator pump and the power valve. All engineered to transition from the stop light to moving with no hesitation.
Chevy, Pontiac, and Mopar all used some form of ported vacuum to the vacuum advance even back in the 40’s and 50’s. Again, how the factory distributors where curved and designed to work. The venerable Quadra jet on the small block Chevy had ported vacuum to the vacuum advance. so did the Carter’s and Rochester, as well as the Autolite series.
Now all of this is off the table with a high performance build. I never use a distributor with vacuum advance on build with large cams and low vacuum. My current 351 Boss crate motor stroked to 460 with a nasty hydralike roller cam uses a billet MSD distributor that is restricted to 19 degrees of total advance. All in before 2,500 rpm. It is set at 16 degrees initial with a total of 34 to 35 degrees of total.
Rob
Most engines like something over 20 degrees to run smoothest at idle. If you run ported vacuum, and you turn the mechanical up to say, 22 degrees, it will idle gloriously. But when the blades start to open, instead of reducing the timing, you suddenly add about 14 degrees of vacuum advance! That means at cruise, you’re now suddenly at more like 36 degrees! That’s way over-advanced, even for a car at very light throttle. Your car is down on power, and whether you hear it or not, you’ve got problems with ping/detonation. Additionally, at WOT, when the vac advance goes away, the ignition is still too far advanced to produce peak power, and prevent detonation, for the same reasons.
If, instead, you use a baseline for mechanical advance that works well at WOT (and you can test this easily by disconnecting the vac advance and finding out what makes your engine happiest), you will typically be around 8-12 degrees BTDC. That’s not enough timing to allow a smooth idle, however. Adding in manifold vacuum, which is available at all times and throttle positions, you gain another 12-16 degrees of advance, which puts your idle exactly where you want it.
As you transition from closed-throttle to open-throttle, the timing falls back to baseline, allowing your engine to make peak power at the already-established maxiumum for Wide-Open-Throttle, or - based on manifold vacuum. At anything less than full throttle, it adds just enough timing to keep your combustion events in line with the amount of air and fuel available.
Tuning your engine to run fine with “No vacuum advance” at idle and then “toss in a bunch more timing!” at cruise leads to some weird stumbles and problems for most cars. It also frequently leads to dieseling when you turn the car off, because the combustion chamber was so hot at cruise that things are a bit glowy, and the engine has an easy ignition point regardless of the ignition being on or off.
Think of it this way: Your mechanical advance and baseline mechanical timing should be set to provide maximum wide-open-throttle power. Adding or taking away from that will just degrade performance.
But for part-throttle operation, the vac advance adds a correction factor, allowing the thin mix to be lit sooner, when cylinder pressures are low, and combustion events are sluggish. It doesn’t make sense for this correction factor to be turned off at idle, when mix is thinnest and the engine needs the most advance.
Some engines want more vacuum advance (low-compression engines especially!) and some want less. That’s why it’s important to adjust your mechanical timing curve first, and then adjust the amount of vacuum advance in order to make your distributor give your engine exactly what it wants at all times.
Having run a dual-point Accel distributor for years on my first Cougar, and later going to vac advance, I can say emphatically that any street car will run better with vac advance than without. Crisper throttle response, more average power, and much better behavior overall. I won’t run without it again.
Lets say I have 12* initial timing with a all factory engine that has 10.5 to 1 compression its a 68 big block with an automatic so single advance distributor at what rpm would be be ideal for the ported vacuum to start to pull on the breaker plate I understand about all in by 3000 but when is the best transition from initial to vacuum which is something I can adjust.
Generally with the throttle blades opening; as soon as you get beyond the idle circuit the vacuum advance sees engine vacuum. It starts out low because when you are accelerating the carburetor blades are more open than not. If you are at a steady state cruise the vacuum advance sees a high vacuum number. The centrifugal advance only cares about RPM, so it limits maximum advance until such time as it is warranted. This is why on a street driven car you want the centrifugal advance to not go to full number until it is over 3200 RPM. At that engine speed the fire must be lit earlier than it is at idle speeds in order to achieve maximum BSFC and maximum power.
There is a certain amount of vacuum leakage as well as high vacuum above the throttle blades when decelerating, so the vacuum advance is going to be at full advance during some phases of deceleration, which is good for engine cooling and fuel economy.
Ported vacuum isn’t operated by RPMs. It’s just a hole in the venturi that is above the throttle blades when they’re closed, and then exposed to vacuum once they rise above it, as you step on the gas. So at idle, it allows zero vacuum to the distributor, and then at part throttle (or anything past idle) it delivers full vacuum from the manifold.
Virtually all cars have a distributor that advances timing by RPMs. It’s called centrifugal advance. As RPMs go up, the ignition needs to fire sooner so that the mix has time to burn. Of course, computer controlled cars can sometimes lack the mechanical version of it, and just use a dedicated system to add or subtract timing based on RPM, but it works out to be the same thing.
Mechanical advance doesn’t know or care anything about how much throttle you’re running; it just pays attention to RPMs. Vacuum advance adds a way for your engine to sense load, and more importantly, set off the spark sooner when the air/fuel mix is thin (high manifold vacuum) because it burns more slowly in those conditions.
Don’t take my word on this about manifold vs. ported vacuum for distributor timing. Here’s a great article from a guy who knows even more about it than me:
https://www.chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm
The article makes the typical Chevy guy mistakes. Mis - spelled words, bad grammar, and the wrong idea that ported vacuum was caused by trying to meet tailpipe emission standards. News flash: ported vacuum was around in the 1950’s and it makes the car run better. It gives the distributor one additional data point of knowledge to use when making a decision about when to advance the timing or not. Ported vacuum lowers the total advance slightly when decelerating, it also lowers total advance at idle. It’s all good, and has not one thing to do with emissions.
One of the guys who I have learned a lot from over the years is a Mopar guy, Richard Ehrenberg SAE. He writes a column for Mopar Action for a living. He also won the One Lap of America competition in a Dodge Dart with a small block Mopar engine, running vacuum advance connected to the ported vacuum fitting on the Holley carburetor.
Ported vacuum and manifold vacuum are essentially equal at all times except at idle.
So what we are really talking about is idle quality. Nothing else changes.
It is really easy to experiment with this and see what you get with your car. Give it try you won’t hurt anything.
I have tried it. With a stock engine (289, Autolite 4v, C4, 18 inches of vacuum at idle) and automatic transmission running manifold vacuum instead of ported vacuum it raises the idle speed enough that you have to stand on the brakes to keep the car from creeping. It’s not terrible. If you drop the idle speed it makes it harder to start when it is hot. It’s not terrible. It did seem more likely to stall with the AC on, and produced a slight bog pulling out from a light.
With a Comp Cams 268H in a 390, Holley 4V, 4 speed, I had better results. On ported vacuum, at idle(650 RPM) it was only making about 11 to 13 inches of vacuum. The higher idle speed was no issue with the 4 speed, but I did back it down to 850 to 925. Idle vacuum went up to about 16 inches. My power disc brakes actually worked better with the higher vacuum. But it still had that little flat spot right when you hit the gas. I ended up going back to ported vacuum and just raising the Idle RPM to about 875 to 900. Idle speed was much more stable with only mechanical advance.
I would like to hear from others and see what they find. My gut feeling is that in stock applications or close to stock the Ford engineers have it right. WIth a cam and more radical builds it is unlikely that running anything stock is going to really be optimal.
Where I was getting to was where the ford engineers were setting up vacuum advances to start using the spacers and spring in the vacuum advance adjusting feature. With a vacuum tree and its three points, distributer, portrd vacuum, and manifold vacuum where and when do these vacuums apply ?
With regard to the Chevy article, he really isn’t talking about our cars at all. That is to say this forum is dedicated to '67 to '73 Cougars. You just can’t generalize between Chevy and Ford… if you doubt it just ask a Chevy guy where cylinder number 1 is located.
Our Cougars don’t suffer from lean idle issues. EGR didn’t start at Ford until 1973 and even then the Ford EGR valve was supposed to be closed at idle. If we were talking smog era cars of the mid '70’s pre catalytic converter period it would be different. This does not apply to our cars: “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.”
Most of what he writes about basic theory is correct: “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” “centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm.”
But this doesn’t apply to Ford: “After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements,” “these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC).” “ported vacuum was strictly an early, pre-converter crude emissions strategy, and nothing more.”
A little history: If we go back to the post war Y block (1954) and the Mileage Maker overhead valve six (1956) you find a completely different vacuum advance strategy called the Load-O-Matic distributor. It was a pretty clever idea. It used a venturi vacuum port that effectively measured how much air was moving through the carb. This was a pretty good proxy for RPM. This was important because there was no mechanical advance. This vacuum signal was compared with manifold vacuum which was a proxy for engine load. These two factors acted on a vacuum valve that looks like the power valve in a Holley to modulate the vacuum signal to the distributor. The down side of the Load-O-Matic is that the venturi signal is very weak, and the system responds slowly and results in mediocre performance. (I have an elderly six cylinder Econoline that came with one of these distributors, one of the last vehicle Ford built using this system).
Ford transitioned to mechanical advance distributors with ported vacuum operated vacuum advance systems. While there may have some Ford product some where that ran manifold vacuum to the distributor, all of the Windsor V8s (260, 289, 302, 351) all Passenger car FEs (352, 390, 427, 428) and all 335 and 385 series Ford V8s used ported vacuum. It may be true that GM and others might have switched to ported vacuum as an emissions strategy but that was not the case at Ford.
Manifold vacuum is not the devil. From 1968 on there was a thermal switch that would switch the distributor to manifold vacuum at high temps to speed up the engine at idle to increase cooling fan and water pump speed. So yes, Fords can and do run with manifold vacuum. But when the engine cools off, they are right back on ported vacuum. I suspect that with the right modifications you can run either manifold or ported vacuum with good results.
The vacuum tree, if I am understanding you correctly, is the thermal switch located in the thermostat housing area. When the engine is at normal operating temp, or cold, the ported vacuum is directed to the distributor. At high temps the switch directs manifold vacuum to the distributor to increase idle speed and thereby water pump and fan speed to assist cooling.
Thanks Bill.
Went asleep for a couple days……. I have read the “Chevelle Stuff” crap before… More tomorrow… As usual, Royce is right on!!
Rob
I’m just going to throw one more thing out here, and then I’ll let this well-beaten horse rest in peace.
Ported vacuum has three advantages. It can stabilize the timing at idle on a car that has an unstable idle. It can reduce nitrous oxide emissions, which are fairly noxious. And it can heat up the exhaust, allowing still-burning fuel to blow out the manifolds to reduce warmup time.
Manifold vacuum allows for a smoother idle and better idle fuel economy. It allows for more linear throttle response coming off idle. It helps reduce cylinder temps at cruise which helps prevent exhaust valve recession and dieseling, and it greatly reduces underhood temps during in-town or stop-and-go traffic.
Both types of vacuum advance require different timing scenarios in order to allow an engine to take full advantage.
Obviously, the debate over which is “better” still rages on - but I know which works best for my car. Your mileage may vary (pun intended) =)
Manifold vacuum allows for a smoother idle and better idle fuel economy. It allows for more linear throttle response coming off idle. It helps reduce cylinder temps at cruise which helps prevent exhaust valve recession and dieseling, and it greatly reduces underhood temps during in-town or stop-and-go traffic.
This poor horse is going to get another beating! I have digested all said since my last post. The best part is we all learn by these debates!
Back to your initial timing of 8 to 12 degrees and manifold vacuum. This results in an idle timing of 24 to 20 degrees BTDC depending on the vacuum advance total timing when all in. On a stock engine and as pointed out this may defeat the ability to slow the hot curb idle speed down to say 700 rpm. What can result is the near complete closing of the throttle blades completely defeating the air mixture screws. The “transfer slots” in the throttle plate must be in at least a 50/50 position to effectively support the air mixture screws. This can and will create a hard stumble off idle as you transition to the low speed circuit. People do not understand how greatly the idle circuit can affect the transition to the low speed circuit. I have worked with many 2 and 4 barrel carbs and ship across the country. The opening of the secondary circuit (back throttle plates) is critical in achieving the proper 50 /50 or 60/40 split of the primary transfer slots on a 4 barrel carb.
Of course all of the above can be masked by an overly rich setup on the carb. If you ever wonder why your air screws can turn in and out and have little affect is a symptom of poor carb adjustment. All of this can be affected by using manifold vacuum. Again, we are talking mild engines or stock engines. To much advance at idle is the culprit. Ported vacuum as Royce said was the choice from the 1950’s up. And it had NOTHING to do with emission control.
Again, at part throttle opening, the total advance of a ported controlled engine set at 12 to 15 degrees initial is actually more advanced than a manifold controlled vacuum advance motor set at 8 degrees initial. This is a fact and as it was stated more advance equals cooler engine temps, better mileage, and more power. A stock engine does not need 24 degrees of advance to idle properly. If it does, it has other problems. Add the fact that the ported engine under moderate acceleration added some vacuum advance timing while the manifold controlled vacuum actually subtracted timing under the same condition is the exact opposite of “more linear” throttle response.
Summary. Ported vacuum on stock and mild engines. More initial timing that stock specs. Adjustable vacuum advance or springs and shims for a factory vacuum advance. I have a jar full of springs and shims for factory Ford vacuum advances. Slow the vacuum advance down a bit to prevent “light load pinging” under moderate acceleration or throttle loads. Keep the total at 34 to 38 degrees based on engine compression.
I specialize in engine tuning and carb rebuilding across the country. I wet test every carb I work on a similar engine and video the engine at hot idle and note all settings. I have never found and stock/mild 60’s engine that prefers manifold vacuum. The engineers at the big three knew what they were doing. I got my first Ford in 1971. Start Medicare this February.
Rob
Rob thanks for sharing your years of experience as this type of tuning takes many years of trial and error to master. Would using a initial timing on the higher side say 12 to 16* help slow down or eliminate running on after ignition shut off ? Also is the transition slot in at 50/50 when the slot is a square or does it need to be taller more exposed than a square shape ?
Preaction,
In my experience, more advance means more run on in high compression engines. So no, this would only exacerbate run on. The late 60’s and early 70’s were the ragged edge of “run on”. The Boss 351 engine and the 429 Cobra Jet 429 were great examples of run on. The factory engineers installed an electric solenoid to “hold” the hot curb idle speed. When the key was shut off the solenoid retracted closing the throttle blades completely close. This stopped engine run on in these “emission controlled” engines that were on the ragged edge of lean to meet standards, yet have performance.
The relationship of the throttle blade to the transfer slot is a bit “ambiguous” to say the least. An oil film will be formed on the throttle plate to “kinda” represent the line of closure so to speak. Using a light below and into the throttle body relationship is helpful. If you have the carb upside down you can see the “edge” of the throttle blade on the throttle body/plate. It should not be above the throttle transfer slot completely nor below it. After some visual you will get the feel of its relationship. This adjustment that can be controlled by initial timing on a ported vacuum engine and if a 4 barrel the secondary throttle stop screw can be critical to controlling engine run on. And if your air mixture screws do not affect idle, you may have the blades to high or to low.
But more initial advance will exacerbate run on, even on a properly adjusted carb. Especially on the above mentioned engines. And other high compression engines of that era of extreme “leanlessness”. Of note the 429 Cobra Jet of 1971 used a Rochester QuadraJet to achieve “emission standards” of the day. To my knowledge the only time a Rochester Carb was install on a 60’s performance Ford. And it was equipped with the electric solenoid to shut the throttle plates. The 429 “Super Cobra Jet” was exempt from this emission standard and ran a Holley. I guess related to its performance category.
What engine are you talking about? I may need to open a thread on carb/timing adjustment and other factors on 1967 to 1973 Ford engines. I have found many interesting tidbits over the years.
Rob
Just as a clarification. The Rochester was first used on the 429CJ in 1970, with a Holley on the 429SCJ when installed in the mid size (Cyclone Spoiler for example). Continued into 1971.
The Holley carburetor was also used on the 429SCJ in the 1971 Mustang. Not sure if there were any 1971 Cougars with the 429SCJ but if so they would have also used a Holley carburetor for better performance.
Rob, Im working on a new to me 428cj that is a all origional motor the initial is set at 6* and the idle adjustment screws do affect the idle so they are in range car starts right up and runs strong but runs on when turned off.