Hilltop Workshop | Vacuum Advance and its CORRECT operation, explained.
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- เผยแพร่เมื่อ 23 ก.ค. 2024
- My contribution to (hopefully) steer the discussion in the right direction, around what seems to be a somewhat misunderstood topic.
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There may be some European cars that somehow use vacuum advance as a primary advance but on Fords from ‘50 to ‘72, the factory spec was to run the vacuum advance on manifold vacuum.
Here’s why:
First, idling with high advance is more efficient. The throttle plates can be closed more, and the engine runs cooler due to more complete combustion within the cylinder itself.
Second, advance needs to be the least at low engine speeds and high engine loads. In other words, you *need* the advance to drop away to the base setting when you open the throttle at or just off idle. If you have advance there, the rapid burning of the high-density charge will cause the flame front to smash into the rising piston, making the distinctive “ping” or rattle of “spark knock.” The mechanical advance then advances the timing the proper amount to keep the flame front meeting the piston at the correct time. This needs to be earlier and earlier because the flame front has a finite speed, while the piston is moving faster and faster.
Third, the vacuum advance needs to be fully deployed at low manifold vacuum levels because the density in the cylinders is very low during this time, and the flame front travels very slowly. The increased advance allows, once again, for the proper meeting of piston and flame front.
There are a couple of other things that don’t apply to old Fords, or cars with Holley carburetors.
The 1960s era T-birds used manifold vacuum.
The ported vacuum (at least in a Holley carburetor and every carb on a Ford) is equal to manifold vacuum at every throttle position other than idle. There is no trickery that uses the Bernoulli principle to make more vacuum at low engine speeds.
The vacuum secondary actuator gets its signal from air passing through the primary Venturi, on the right side of the carburetor. This is the only orifice for this purpose. There is a spring in the actuator that pushes the secondary throttle closed, and its strength is the only thing that controls secondary opening. There are no orifices in the secondary that augment this action.
You made a very nice video, and I can’t guarantee that there aren’t European carburetors that do what you are saying, but there aren’t any Holly carbs that operate that way, and no carbs on Fords. Feel free to provide evidence to the contrary.
I have a ford with 302 with a holly, aalso run better on manifold vacuum.
If i use ported, i have to open the throttle so far as to getting into the transfer slot make the idle mixture srews inoperable.
@@lucsavoie9501 same on my 65 mustang (302, punched to 331). Manifold vacuum advance is what works best for my setup.
Nice!! I also have a 302 in a 77 f100 with decent cam, runs much smoother on manifold.
Please see my video of the 1961 Thunderbird. 👍
This is the absolute best explanation of the vacuum advance anywhere on the net! I am so happy I found this video!!! Thank you for taking the time to make this video!
Thanks, a lot of opinions out there without personal experience.
I've been messing with one of these shitters for a couple weeks now as a home job and I've been messing about with the vacuum advance but set correctly off the port vacuum it runs like a hairy dog but manifold vaccum it goes like a train. Your video made me not doubt my self as I thought i was going crazy as I'm a diesel mechanic and don't often get to dabble in the dark arts of carb
You didn't touch upon the issue of different static advance settings necessary when switching between ported and manifold. One runs less static advance on manifold vacuum and more static advance on ported so that when at idle both methods have about equal advance. Once throttle plates open sufficiently both methods have the identical signal. Well made video, thank you.
No argument - yes, Ported is generally better, but your point Joe is excellent and 100% correct. Many of the TH-camrs fail to mention this! You need to increase your static timing, maybe up to 20-25* if using ported, but that level of static timing can make the motor hard to start. Manifold vac timing allows you to crank at 14-18* for easy starting, then as soon as it fires up, timing shoots up to 25-30* which is where it needs to be. This gives you a huge torque reserve off the line which masks any drop in vac timing as the throttle blades open up. Good video Thanks!
Great explanation and absolutely correct thanks for making the video
The small holes that you are pointing to at 10:47 are in function similar to the “transfer slot”. They are there to provide additional fuel at the beginning of throttle opening. They are part of the idle mixture, not for any vacuum. The vacuum ports are to the left and right.
In your carburetor, next to the vacuum connection for the secondary diaphragm, the other connection goes to “something” and then connects up high at the secondary. What is that “something”? My carburetor version doesn’t have that part. The particular connection goes to the Jet valves, and I am very interested in what ways the Jet valves get clean air only.
Some is them are progression holes/slots; Some of them are vacuum ports.
@@thehilltopworkshop At 6:00 video time, the picture top right shows a rectangular metal with vacuum lines fastened on the carburetor. What is that? That is what I am interested to know. Thanks.
Congratulations. Great video.
Good video helping to explain how ported is really preferred. Problem is that people think ported is emissions only due to history of ported/manifold advance. Also, edelbrock lablels their ported nipple as "emissions" instead of ported which is highly confusing. Modern muscle cars will run better using ported vs. Manifold vac as most distributor advance mechanisms are set up for ported. Distributors setup for full manifold vac need light springs to advance quickly on throttle tip in because manifold vac drops off and advance drops off. Required Cruise rpm total advance is not untypical at 45 to 50 deg which is initial + mechanical + vac advance. This is beneficial to burn the slow burning lean mixtures at low load cruise. Folks running mech advance only distributors are losing 30% fuel economy because they think vac advance is vodoo or emissions. WOT wont use any vac advance (ported or manifold) so dont get confused with specs like 32 deg all in at 3000 rpm. Specs like that are for WOT only applications. I have spent hundreds of hours setting up both ported and manifold vac ignitions and always had better driveability and throttle response with ported using 10-12 initial vs. Manifold with zero initial. Exceptions are engines with very big cams that will not idle without a ton of advance.
As far as im concerned, the only difference between the two is at idle and light cruises, everything else is the same. I can't see where and how it can make a difference in driveability, I'm really not sure what people mean by driveability anyways, i can't tell the difference.
If it only makes a difference at idle, why not go with the one that gives best idle. for me, manifold gives me smooth and quiet idle and cooler exhaust.
Initial is 10, another 10 for vacuum advance for a total of 20 at idle.
Ànd 12 mechanical advance.
@@lucsavoie9501 Of course it makes a difference at idle. If you can't get a decent ("best idle" as you put it) from your engine without using full manifold vac to dump in a ton of advance, then there is something wrong with your engine. Ported vac advance adds no timing at idle so if your engine won't idle well, you need to perform some troubleshooting. If you have a giant cam engine, then you're probably racing and full manifold vac can bring the idle up with adding a ton of static timing which has its own problems with hard starting, too much timing below ~3000-3200 (knocking and pinging) and in some cases, too much total timing. If you have a race engine, then WOT uses no vac advance so who cares. If someone is driving a mild cam muscle car engine around with full manifold vac advance, then they had better have the dist set up for it. Aftermarket dist manufactures assume you're running ported vac advance and their spring rate and vac advance chambers are set up for that which is why the better suppliers provide instructions (and parts) for converting to full manifold.
@@BlueOvalEdge I just watched your video on your explanation of the diferences between ported and manifold. I understand all of that.
Where i think you might be off is when you say that they work opposite.
Sure, the pointer on the gauge moves in opposite directions but in my observations while doing several tests on this topic, when you open the throttle with manifold vacuum, the vacuum does not go to zero, it goes to the same exact level it does with ported. I really don't see any advantage in using ported and many with manifold.
Ps. I didn't mean it doesn't idle well with ported, just much better with manifold.
The misconception is there because most American cars up to 1971 called for Manifold vacuum with some very minor exceptions. In 1971 or so automakers were attempting to save fuel and have a cleaner combustion process for emissions, how do you do that? You add more air but can't because you're essentially at full advance on manifold. With manifold at idle to get the fuel to burn it needed to be richer which in cars in city's spend most of their time (not WOT). So opening the throttle allowed more air in and made a better combustion process (but now your idle is too high which they fixed by retarding the timing). Ported vs. Manifold at WOT doesn't matter; you're still beholden to total timing which is the vacuum advance + mechanical timing The difference is how you split the numbers. Now if you have a fairly high compression motor adding more timing will allow it to run cooler so you aren't overheating in city or stop and go traffic. I think the moral here is that there is no right or wrong it really at cruise you have vacuum advance. I would encourage everyone looking at ported vs manifold videos to buy a wideband O2 and gauge, and tune the car and carb for what the engine likes as today's gas is definitely different than that of 50 years ago
I love you!!!
I don't know what "overwhelming majority" of vehicles were designed to use ported. Pretty much all vehicles pre-emissions in the US used manifold. You WANT advance at idle. The engine will run smoother and cooler as a result. Then when you crack the throttle open, the mechanical advance will start working and the vacuum advance's effects will diminish and cease completely when you lose enough vacuum. At that point, you're running on whatever mechanical advance curve you or the manufacturer has put in the distributor.
Not sure how so many don't understand how this works. Can you make it work with ported or venturi? Sure, but using manifold along with the proper mechanical curve is superior and would have always been what manufacturers used had emissions not been an issue.
And doing it with manifold is also much easier.
Please see my other video featuring factory ported vacuum advance on a 1961 Thunderbird. 👍🙂
@@thehilltopworkshop Sure, I'll watch it, but 1 model isn't "overwhelming majority", which is what I addressed.
"Overwhelming majority" pre-emissions were manifold.
Ported only became commonplace because of emissions. Pretty well-documented if you look around.
Sorry but there are times when manifold vacuum works well for connecting the distributor vacuum advance.
In engines with long duration aftermarket cams with a lot of valve overlap you need a lot of timing to help them idle smoothly, you can use manifold for this and another benefit is it will reduce timing at wide open throttle to help prevent detonation.
Maybe you haven’t built a lot of engines or don’t work on high performance cars that’s why you don’t know this?
Most engines of the style you're describing don't run vacuum advance at all. It is NOT the job of ignition timing to set or adjust the idle speed. If you're using the ignition timing to correct the idle speed, then something is wrong with your tune.
No need to apologise. 😅😉
That is just incorrect. Timing is part of tuning to achieve the highest vacuum at idle. Maybe look up David Vizard he has written many books and explains this concept really well.
As others have mentioned, all engines use to always use manifold vacuum, the only reason ported vacuum became a thing is for emissions. Retarding the timing at idle lowered emissions.
@@shelbydupree6157 K.
While you’ve got some good basic info, where you go wrong is saying ported vacuum is “right” and manifold is “wrong” when every engine is different.
For example It’s common for people to get carried away with cam selection and run too big of a cam on the street. vacuum advance from the manifold will get a better idle.
Maybe a stock engine might like ported vacuum. And that carburetor in the video looks like it’s from a lawnmower btw
@@shelbydupree6157 The fact that you say the carburettor looks like it's from a lawn mower, illustrates why I'm wasting my time. You clearly have zero real world experience.
Go and practise your guitar. 👍
i find the engine runs much better with manifold vacuum.
I've done a lot of experimenting on this topic and found that with manifold vacuum. when accelerating, the vacuum doe's not go to zero, it doe's come down, but to the same level of vacuum as the ported vacuum would be, it can't go any lower and it doesn't overshoot below the ported vacuum, I've proven that.
it's easy to be fooled in thinking that things work opposite but they really don't, if the vacuum didn't drop in manifold vacuum, you would be way too far advance and would cause pingging, it has to drop, but never below where ported level would be.
i find with manifold vacuum, idle is much smoother, engine is quieter and exhaust manifold runs at least 50 degrees cooler, cooling fan runs less frequent at idle.
With ported, at cruise below 40mph, vacuum is at zero and manifold vacuum is at high, therefore ported defeats the purpose of vacuum advance for economy.
As for driveability and performance, there was no noticeable difference.
The benefit of manifold vacuum is idle performance and low cruise where the vacuum is high.
th-cam.com/video/L_hx8I6GzyU/w-d-xo.html
Sorry guy but ported vacuum is an on/off switch not a progressive engaugement. Ported vacuum is zero at closed throttle and exactly the same as manifold vacuum at open throttle. Get two vacuum gauges and put one on ported vacuum, the othe on manifold vacuum. Work the throttle and note the difference. Maybe you will understand.
That is a fundamentally wrong statement and physically impossible.. Have a look at my other video where I have the vacuum gauge connected directly the ported vacuum fitting on the carburettor.
Sorry guy. 👍😉
There are exceptions to using ported vacuum...............yeah.............like EVERY car made before 1969
"EVERY car"? I have a couple in the shop right now that would like a word with you... :/
@@thehilltopworkshop Thats what I get for using an absolute term like every :) What pre 69 cars do you have with stock oem carbs on them that use ported vacuum?
@@kztrekz3329 EDIT: Just uploading a video response for you right now. ;)
At the moment, a 1961 Thunderbird with a factory original 390 FE Big Block. Also, pretty much any GM V8 with a QuadraJet uses ported vacuum.
Single barrel and WW Strombergs - Ported vacuum in pretty much every application.
Webers on countless British and Euros - All ported vacuum.
Zeniths on Dozens of Euros - Ported vacuum.
No, thats not correct by a long shot. Ported vac ignition advance has been used by automobile engines since the 40s. The switch to ported in the 70s was indeed for emissions as well known ,but it is incorrect to make these broad brush 1969 remarks and also incorrect to say or suggest that ported is emissions only. Not correct.
@@BlueOvalEdge If they have been using since the 40s how could they then switch to it in the 70s? Wouldnt they have already been using it?
Nope!
Thank you for putting that in such easy-to-understand terms.
I'm convinced. Can't argue with that.