I stumbled across C9's incredible post on distributor timing in the tech archives, in which he says that manifold vacuum should be used for the vacuum advance on a sbc. Later in the post 1oldtimer says that ported vacuum should be used. What are some other opinions out there, and does anybody have any reasons why, one way or the other. If ported vacuum should be used, then how can I pull vacuum from early holley carburetors that don't have vacuum fittings?
I allways thought ported vacuum was there for the soul purpose of supporting timeing advance.........IE no vacuum at closed throttle......am I wrong Idunno ?
I couldn't find the thread, but as I remember, 1oldtimer never came back to answer a couple of questions. To add a little info, GM developed the ported vacuum bit to make the engine run hot so as to improve the nox/smog levels. I understand they licensed the idea to FoMoCo and ChryCo as well. I'm fairly sure the article has some additional info on vacuum levels from each vacuum source under real-world driving conditions. If the additional info isn't one of the posts after the Got Time article I still have it in my files and can post it.
See the other post reply for the reason on ported vacuum. Correct that there is zero vacuum at closed throttle with ported vacuum. Once the engine is spun up a bit ported vacuum follows manifold vacuum fairly close.
I think part of the confusion comes from where Chevrolet got their manifold vacuum. Many of the stock vacuum ports that the factory used for the vacuum advance were on the side of the carb. With out tracing the port, one would think that it is above the butterflies and therefore ported vacuum. It is not. The vacuum port on the Rochester 2 bbl and many 4 bbls goes straight down through the carb base below the butterflies which is manifold vacuum. Just because it goes into the side of the carb it does not mean that it is ported. Hook up a vacuum gauge to it if you don't believe me. Like the 97, there is no port on the Rochester carb to get ported vacuum. The hot rod replacement carbs are made to adapt to many different applications and supply both types. Yes there is manifold vacuum on the advance at idle. That is why the book tells you to disconnect the vacuum line and plug the line before setting the initial advance with a timing light. If you keep the light hooked up, the mark will jump up off of the timing tag when the vacuum is reconnected. That is the way that GM designed it. Of course the advance retards the timing the slit second that the butterflies start to open so it is no big deal. All of the GM engines from the 50s and 60s were designed this way. They all tell you to disconnect and plug the advance line when setting the timing. If you don't plug the line it won't start because of the vacuum leak.
Btw: The single barrel carbs used on the Chevy inline 6s from the 1930s until 1962 and I believe the later model inlines as well until the 70s, always used ported vacuum just above the closed butterfly for the vacuum advance. On those engines it advances ignition timing for part throttle like accelerating and cruising, together with the distributor centrifugal advance. Vacuum line doesn't have to be disconnected for setting timing with a light, as vacuum at the advance is 0 at idle.
ported vacuum bit to make the engine run hot so as to improve the nox/smog levels. ... with the retarded idle spark timing used for emission purposes. They: 1. made the problem (TDC spark at idle), which is what makes the car run hot 2. realized it wouldn't work with full vacuum (it would just advance and let the engine run faster) 3. ported delays the advance so the car is cleaner Full vacuum is much better if you like your idle speed to change continuously when you put it in gear, turn on the lights, or use the heater controls. I don't.
So what are we talking here, 50 rpm? A hundred? Not that big a deal I'd say. Maybe you need a bigger engine. Or you could tell us a little bit about your car.
Ported vacuum will make your engine run hotter at idle and some times it will make you engine stumble on hard acceleration right off of idle, I said some times. So don't respond that you don't have that problem, not every body will. I run manifold vacuum on all the cars that I build, and it works just fine and keeps the engine temps down at idle.
Also with manifold vacuum and auto trans /A-C etc. putting it in park to shut it off led to dieseling/run on. Thats when idle shutdown soliniods started being used early -mid 60s. Add any kind of performance cam and you can have a 500 RPM netural/ drive difference. Personally I like manifold vacuum and run it whereever I can but some apps. it just don't work out. And,yes it does make a good bit of difference in cooling at idle. Some emissions systems use a temp sensor to provide manifold vacuum at idle to the dist. when the engine temp goes above a certain point. What does that tell you ??
I don't know what would give you those symptoms. I've had an ass load of Chevelles, Corvettes, Pontiacs and others that I can't think of right now that all ran the stock GM full vacuum advance and I never experienced any of that. I never ran a wild cam though. Mine were mostly mild stockers.
IIRC, ported vacuum will not decrease as much (or at all) during acceleration, so you will have full vacuum advance during acceleration, which may lead to detonation. FWIW, I believe ported vacuum reads at the narrowest portion of the venturi, so that the faster the air flows past, the higher the vacuum reading. Which means that the vacuum is related more to engine speed rather than load. Cosmo
ported vacuum reads right above the throttle plate at idle, which is not in the venturi at all, and should never have any more vacuum than a normal vacuum connection below the throttle plate.
I think whether it's ported or manifold, when you're at wide open throttle, they both fall to almost zero. If you still had vacuum at wide open throttle with a vacuum advance distributor, your timing would wind up way too advanced and you'd blow a hole in a piston or something. I always time my hot rod engines at full centrifugal advance (up around 3000 rpm) to get around 34 or 36 degrees advance. I do that with the vacuum advance disconnected and plugged up. I use a timing tape on the harmonic balancer or use a timing light with a dial on the back. Then I hook up the vacuum advance and readjust the idle speed and everything's fine. I don't care what the advance is at idle, because it can be all over the place depending on what the idle rpm is, what springs and weight are in your mechanical advance, how much vacuum you have, etc. Sometimes it actually runs better hooking the vacuum advance to full manifold vacuum. Sometimes hooking the vacuum advance to full manifold vacuum can make the engine seem more perky at lower rpms and make the car more driveable. It does little good on a hot rod engine to try to tune everything to factory specs if you're using a hot cam with a bunch of lope and a high idle. What really matters is the total centrifugal timing at wide open throttle. If you only time it at idle, you risk blowing up the engine when it's revved up at WOT.
Ok, here's part of the timing article I wrote: "I got curious about a comment I heard about manifold and ported vacuum going to zero at WOT (Wide Open Throttle) and ran a little experiment. The car - 32 roadster - weighs 2400#, engine is an overbored 455 with 462 cid, 9/1 compression ratio, Edelbrock Performer intake, Carter 750 cfm competition carb with electric choke added later and a Crower Compu-Pro #1 cam which has about 262 & 266 degrees advertised duration intake and exhaust with 112 degree lobe centers. It’s a smooth cam and the car when warm idles @ 19" vacuum. The dash carries a large (2 5/8") S-W vacuum gauge which indicates the same as my vacuum/pressure test gauge. Advance is 8 degrees initial and all in at about 2600 rpm with a total of 32 degrees. Vacuum advance is about 16 degrees and sourced from Manifold Vacuum (MV). The car runs very well on 87 octane in summer and winter and does not overheat in traffic. Firing the car from dead cold and on the elec choke, MV reads 18-19" and idle is around 900-1000 rpm. Ported Vacuum (PV) read 12" on startup. Once the engine warmed up, MV reads 18.5 - 19" and PV reads zero at about 500-600 rpm. Cruise at 40 mph with a light throttle setting on a flat road gives you 18.5 - 19" MV and just about the same on PV. Rolling the throttle in about half way shows 8 - 10" of vacuum on both MV and PV during light acceleration. Once at 60 mph MV read 18 - 18.5" vacuum (keep in mind this is a very light car) and PV read 10". Flooring the throttle at 40 mph or 60 mph brought the MV down to 1" or so and PV to zero. At idle with a fully warm engine, MV reads 18.5 - 19" and PV reads zero. The lack of additional timing at idle is what creates an overheating problem in the GM engines. It takes time to burn the lean idle mixture and additional advance is required to get the process underway early and avoid overheating. Exactly the same thing (overheating) would happen with the timing severely retarded in an engine under load at a higher rpm level. There’s a lot of confusion out there about timing, both centrifugal (mechanical advance) and vacuum advance as well as the vacuum sources to use. The key thing is to realize they are two different systems that work together to give optimum spark advance for a particular condition and key on rpm as well as load. To my way of thinking perhaps there would be less confusion if the vacuum advance cannister was called the vacuum retard cannister. I’ve been amazed at the lengths some go to, to cure an overheating problem that can be solved in most cases simply by selecting the correct vacuum source. Granted, most of my experience has been in cars with small engine bays and many times not the biggest radiator in the world, but I note, the bigger cars have the same amount of timing and overheating problems as the small car guys do and for some reason many car owners avoid doing something as simple as swapping vacuum sources to cure overheating and prefer to throw money at the problem. As far as spinning up a little experiment, I’m not trying to prove anyone wrong here, just got curious, had some free time and those are the results I came up with. An additional note; for those of you who live at a higher altitude than where these tests took place, you’ll find that your vacuum levels at no-load (idle) rpms will read lower. To the tune of a 1" vacuum loss for every 1000' of altitude. The tests took place at 350' altitude and manifold vacuum at idle read 18.5 - 19". After moving to Sunny Arizona and ending up at 3300' altitude the manifold vacuum now reads 15.5". Highway figures and under load vacuum levels remain the same. This combined article was written from research on timing figures and real world experience. You can learn a lot by taking the time to set up some inexpensive instrumentation and taking the time to run a few simple tests. What you’ll gain is a better understanding as to what’s going on with your engine and gain a small bit of education about the particular thing you’re researching. Learning from books is one way to do it and there’s nothing quite like taking advantage of what smart, experienced and educated people have done and written down for your educational pleasure. What I’m talking about is the stubbornness and unwillingness to learn from those who’ve been down the road before us. It’s amazing sometimes to talk to an individual who thinks factory engineers don’t know much. Thing to recognize is that factory engineers know a helluva lot more than we do and get into sophisticated areas that the great majority of us know nothing about. Keep in mind too, factory engineers are constrained by the bean counters, the necessity to build a vehicle that is useful to the majority and seldom are let loose to pursue a dream or even an interesting idea. When they do get the freedom to investigate particularly interesting areas, the results can be astounding. The name, "Zora-Arkus Duntov" should ring a bell...."
Sorry, but that's not true. Ported vacuum may be close to zero at idle depending on how low of an idle speed you have, but it does make a difference in timing usually if you try to set the timing without disconnecting the vacuum advance line and plugging the vacuum line. I always plug that line up when I set the timing. You might get away without plugging that line if you have a totally stock low performance engine, with a totally stock carb, and at the factory specified idle rpm.
Maybe not true in your case, but I apparently talked about the early chevy 6s in particular. Both my engines are not stock and idle just under 500 rpm. I connected a vacuum gauge to the ported vacuum of the second carb and get 0 vacuum at idle, believe it or not. In fact, when I tested both vacuum advances on the bench before installation, I confirmed the shop manual specs of 4-6 Hg of vacuum are required before the advance starts moving. Not all engines vacuum advances and connections are equal, which is my point and can easily be confirmed and measured.
I'm going to sign off here, since it's pointless to argue this. The reason why the comments seem to come from both directions is that many favoring manifold vacuum are reporting what they actually saw, and extending this into the physics of how ignition operates. This is called a "post hoc" error: mistaking something that occurred prior to the event for the cause of the event.
You're an interesting guy, but tossing out complex stuff and then hiding behind the curtain of anonymity doesn't cut it....
I really hope this thread gets straightened out. This is a REALLY good topic and something I've been reading about lately. I'll be following this thread with the hopes of coming to a conclusion on my particular setup (which I won't even start on). I'm running early 94's with a vacuum advance ignition and am up in the air about poking holes in my manifold or getting later ported 94's. Someone step in and save this. Some good points here. I would ALSO like to hear more from Panic. Speak up man! Break-down some of your vast knowledge for us.
Yeah, I believe you. Sorry if I came across like a jerk. My experience is mostly with hopped up V8s, and it always did make a difference on those. I remember working on a totally stock engine about 35 years ago and wondering why the manual said to disconnect the advance hose while setting timing, because on that one engine, it didn't seem to make a difference at all on the timing. But on all of the hopped up cars I've worked on, it did make a difference. If you're hooked up to manifold vacuum and not ported vacuum it REALLY makes a difference. Sorry if I sounded rude. I was just trying to help set the record straight for all of the guys with hopped up engines who are trying to get their engines tuned up. I'm always having to explain this to guys I know who can't get their engine tuned up right, and it's tough to convince them sometimes that it does make a difference sometimes when they have it in their head that it doesn't make a difference on some old car they worked on once. With a hot cam, it does matter.
C9, do not worry about this "panic" guy ^^^^^^^^^^^^^^^^he is on several forums and is an "expert" even though nobody has ever seen any of his cars or bikes? I believe he has even got tossed off a board or two?
The part about ported timing being for emissions at idle is true but it seems to me that C9 and Rusty Bolts have pretty much got the issue nailed.
I'm guessing you have or had something like an older Corvette with leaky vacuum accessories. Turning on lights or using heater controls would only change how the engine runs if it gave you a loss of vacuum -- through cracked hoses or leaky diaphragms, like the diaphragms that flip open the headlights on a Corvette. A leak there would make your idle go screwy because it would add a vacuum leak and change your fuel air mixture at idle. If all of those vacuum accessories were leak free, I can't see any reason why the engine would run differently just by turning on the headlights or playing with the heater. Pulling a diaphragm open should only make a very brief blip in the level of manifold vacuum -- probably not even noticeable because the engine is continually sucking air whenever it's running. If you have power brakes, the diaphragm in those is huge compared to the little diaphragms in headlight openers or vacuum powered heater controls. I don't think anyone can feel their tuneup go to hell every time they hit the brakes. I can see how it might make a difference in how the engine runs when you shift into gear though. I guess that's something to think about for anyone trying to decide whether they really want to use manifold vacuum instead of ported vacuum.
This debate always comes around. Thats why I dont get involved in it. Mechanical distributer all the way. Pontiac 400 10-1 compression. 12.5 second 1/4 mile in a 3810 lb car with non ported heads and pump gas. 1.67 seconds 0-60 ft times and 0-60 mph in 3.6 seconds. 14 degs initial, 35-36 fully advanced at 2800. NO vacuume advance. Oh also idles in 90+deg weather in heavy traffic and doesn't get over 175 degs. Who needs stinkin vacuume advance? I dont need no stinking vacuume advance!!!!
I use a vacuum gauge to tune my car, since I don't know any of the cam specs to do it by light. My motor is a early 80's ford 302, Eldebrock intake, unknown cam, Eldebrock 4bbl (square carter type), and duraspark II distributor. I ran for several years with mecahnical distributor, and it ran ok. But when I went to a full vacuum advance duraspark, I suddenly gained a real seat of the pants gain in HP, and my MPG almost doubled. I connected to the full vacuum port of the carb. I ran on the ported at first, and the car ran about 10 degrees hotter. I also saw 0 vacuum on the ported, until it was brought up to about 1200-1500 rpm. On my motor, the full vacuum made the best improvement.
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