Most engines have between degrees of ignition advance at idle. This is referred to as initial timing. What's the difference between mechanical and centrifugal advance? As an engine revs up, we need to allow even more of a head start for the spark plug in order for complete combustion to occur.
For this reason, a mechanical advance is built into most distributors. As the distributor spins faster and faster with engine RPM, centrifugal forces fling out weights inside the distributor housing, moving a cam mechanism and advancing the timing.
This mechanical also known as a centrifugal advance is an extremely reliable and simplistic approach to controlling engine timing at given engine speeds. It can be adjusted by changing the stiffness of the springs on the distributor's weights, and the amount of mechanical advance can be increased or decreased based on stop-bushings in the mechanism.
We make mechanical advance sound pretty great- and in theory, it is - but there is a major problem with it as the only source of ignition timing compensation. Mechanical advance relies on one input, and one input alone: RPM. It cannot take into account engine load, fuel mixture or any of the many other variables that dictate ideal ignition timing.
For that reason, it is best paired with another form of ignition advance: you guessed it, the vacuum canister. If you were to put a timing light on a car going down the highway with the vacuum advance properly connected, you would be extremely surprised to see somewhere around degrees of ignition timing.
Ping city? Detonation central? Not on a flat stretch of highway. In that situation, what many might deem a radical amount of timing is actually quite beneficial to engine performance. Timing that could potentially damage an engine at wide-open throttle WOT can actually help it achieve significant mile per gallon improvements on the highway. You see, lean fuel mixtures burn very slowly and, at cruise, the engine should be approaching a stoichiometric ratio of right around The added ignition timing from the vacuum advance allows the lean cruise mixture to achieve as complete a burn as possible during the power stroke and maximize engine efficiency.
But how does the vacuum advance know when to engage? As a car cruises down a flat stretch of highway, the throttle plates in the throttle body, or carburetor, are barely cracked open as it takes very little horsepower to move a vehicle down a flat stretch of road in high gear. With the engine turning highway rpms of between rpm and the throttle cracked ever so slightly, manifold vacuum shoots way up. This negative pressure exerts a pulling force on the diaphragm inside the vacuum advance can which has a mechanism linked to it to advance timing.
Lets say you encounter a hill or go to pass another car while cruising down the highway. As you apply more throttle, air rushes through the carb, into the intake manifold increasing pressure and pushing the diaphragm in the vacuum can right back out, retarding timing back to wherever it would normally be, given engine RPM and mechanical advance. Where should the vacuum canister be routed? There's been a lot of debate whether or not the vacuum canister should be plugged into a ported or direct vacuum source.
If my cam was a bit more roudy and produced less vacuum at idle, then the manifold vacuum may not have been an option for me. OP - you basically have the two answers right there.. Second - regardless of the connection - at full throttle there is no vacuum advance because there is no appreciable vacuum. Well, art is art, isn't it? Still, on the other hand, water is water! And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does.
Now, uh Now you tell me what you know. I have always ran ported but that being said, I have left mine un hooked from my most recent motor install. With it hooked up it pulses and chugs the engine while cruising under rpm, above that it smooths out but when slowing and coming to a ligth it chugs and is really annoying. I run mine from the ported and it's a new '72 RR electronic dist running a msd 6al. Everything I've read says to use manifold vacuum for any vehicle that is pre-emissions era.
After emissions kicked in, many of those systems required Ported vacuum to run correctly. However, if you have an emissions-controlled engine that you've removed everything, run manifold vacuum.
It's pretty simple when you think about it. You want most advance during high vacuum situations ie: idle, steady cruising. I've found that hooking up to ported usually causes a bad stumble when transitioning off idle, due to the vacuum coming on and advancing the timing ridiculously quick.
Bloody Mary, Full of Vodka, Blessed art thou among cocktails The factory standard vacuum advance connection on my Dodge Poly is ported vacuum. Check your facts and suppositions. Print Thread Switch to Threaded Mode. Default Style infopop2 moparts RealmHQ sd. DarkDoom ubbcentral. Powered by UBB. 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 degrees "total timing" at high rpm wide-open throttle to get some of the performance back.
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.
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.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Comment Post Cancel. March 21, , PM. Nice informative piece thanks for sharing. March 22, , AM. He is right on a Stock engine, but the last couple Paragraghs he starts hedging when the cam enters the picture.
A proper tuned carb or EFI with the correct timing will not have the issues he is talking about. Proper is the key words. Take mine as a extreme it has depending on the cam timing I am using 2" to 4" of vacuum at a idle. March 22, , PM. Jeff, are you saying that with a very radical cam you're better with ported vacuum or that if properly tuned you don't need vacuum advance at all?
Barry Donovan. I think it came from a blow through ea Previously boxer3main the death rate and fairy tales cannot kill the nature left behind. Lets see here, I don't think they are thinking this through here on the added timing that will happen at cruise when the manifold vacuum is high I know on my '99 Z28 that at about 50mph while under light throttle it will register pinging on the Knock Sensor and it's only pulling about 24 degrees of advance at that point But hey real race cars don't have vacuum advance anyways IMO if you want that much timing at idle, lock the distributor out and set your Total Timing as your Base Timing and forget about it Hell you want all your timing in by rpm anyways, so if you have a rpm stall what good is having a timing curve in the first place Originally posted by racingsnake View Post.
OK, so we don't get too far off topic, is the amount of vacuum in ported proportional to the amount of air going through the venturi and effected by such or a manifold vacuum function when the throttle plates start to open and retards was manifold vacuum falls?
Last edited by Huskinhano ; March 22, , PM. March 23, , AM. If I read you right Yes. Ported climbs as more air goes threw the carb, Manifold Vac will drop as the blades open but if the Carb becomes a restriction then it will build again, think part throttle at highway speeds.
Then when you let off it will climb even more. At WOT it will climb only if the carb is to small for Max performance when it becomes a restriction. March 24, , PM. Found the info I was looking for on a AMC forum. I was slightly wrong on my understanding of ported vacuum. Ported is basically manifold vacuum but not functioning at idle while the throttle is closed. What I thought how ported worked is how venture port vacuum used some times.
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