Should I Use Ported, Manifold Vacuum Advance or Mechanical Timing Advance?
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- Опубліковано 6 вер 2024
- Ignition or timing advance is often the key to making the most horsepower possible. Deciding on whether you should use ported vacuum advance, manifold vacuum advance, or bypass it all together and use mechanical timing advance - heck - you can even lock in the mechanical advance use use only one timing setting throughout the entire band.
If you are going from manifold vacuum to ported vacuum - ported vacuum can cause your timing to increase at the wrong point, which would cause detonation.
Whether you are setting up a small block chevy, Pontiac V8, small block ford, or dodge engine the idea is basically the same. MSD, Accel, and a host of other distributor manufacturers operate basically the same way as well.
Ported vacuum advance gives you the minimum advance at idle while increasing the timing curve during the increased load.
Manifold vacuum advance gives you full vacuum advance at idle and drops out during a load to return to the mechanical advance. This gives engines better and cooler idling properties.
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Both manifold vac and ported vac work the same and produce the same performance past idle all the way up to redline. You set base timing with vac unhooked. So when at WOT, your ignition timing reverts back to mechanical advance only because there is no vacuum present. (This is what you want to prevent pinging/detonation). Manifold vac provides extra timing at idle too, where ported waits until just past idle to apply it. They are both the same after that. Some engines run smoother, cooler, and quieter at idle when hooked to manifold vac. It is okay to have this extra timing from both mechanical and vacuum advance at idle because there is no load on the engine and there is low cylinder pressures. The engine doesn't care. In fact it perks up vacuum signal with the extra advance. This also allows for you to close the throttle plates more while setting idle speed and achieve better function of idle mixture screws. The point of vacuum advance is to squeeze in more timing under light throttle/ light load conditions for better fuel efficiency and performance. At WOT, it is all gone.
All I wanted was to get my car to start quickly and run reasonably smoothly without doing any harm to my brand new Blueprint Engines 383 stroker. Sice "Blueprint" requires that you operate without vac. advance; I've spent the last two weeks on set up without getting what I wanted. Research never answered my questions as well as your succinct comment did.I will run the engine with v.a. plugged into base of carb .This allows for considerably more advance(if needed); which I only need at start up. Thanks for the info; much appreciated. BTW. I must add that at this time there is quite a nasty exhaust leak in one of the primary pipes. Don't know how much this affected my set up; but it will be fixed soon; and re calibrated as necessary... thanx again.
Ported vac ONLY works at idle, it plays NO role at any other throttle position. Meaning.....at ANY other throttle position other than idle, ported and manifold vacuum are the SAME. My 67 Fairlane with original Autolite 2100 has NO ported vacuum hookup. Why? Because they didnt exist until about 68 when the EPA started flexing their muscle. Its introduction was purely for emissions, NO other reason. So if youre concerned with polluting the environment at IDLE........go ported, if you want your engine to run well at IDLE......go manifold.
I concur. My mild 318 loves being on manifold vacuum.
@@Tk-ou9ec I have never seen a use for ported vacuum other than to waste fuel and performance. Always run manifold unless engine does not have enough vacuum to support it.( Usually 8" or below) and then just use mecanical.
Quick question I have a 1941 Pontiac flat 6, hose that goes from my distributor heads to my carburetor, is that considered ported or manifold...?
And should I change it to the manifold.?
Nailed it! 67 Wildcat. Base timing is at 2 degrees so kinda dumb to run no advance at idle!
There shouldn't be any vacuum on a ported source at idle. The vacuum signal on a ported source gets stronger off idle up until a certain point then goes away as you approach WOT. Full manifold vacuum is present at idle and goes down as you open the throttle and drops to no vacuum at WOT
Ported vacuum advance is for smoggers. Vacuum advance is based on load. Lean mixtures need more lead; rich mixtures need less.
Manifold vac on all my classic cars and daily drivers. Best performance, fuel economy, less heat on exhaust and engine bay.
Same with my mild 318. Runs great on manifold vacuum.
Quick question I have a 1941 Pontiac flat 6, hose that goes from my distributor heads to my carburetor, is that considered ported or manifold...?
And should I change it to the manifold.?
@@trivigonzales Depends if the hookup point on your carb is above or below the throttle blades. Above = ported; below = manifold.
Id be surprised if it was hooked to ported as prior to 1968 basically every production motor with a vac adv canister was hooked to full manifold signal.
Ported vac was introduced for emissions reasons in the late 60s / early 70s to pull ALL vac based advance out at idle/coast conditions (0% throttle) to increase temperatures in the exhaust to burn off extra hydrocarbons. Less timing = spark happens later = combustion happens later in the cycle = lots of heat (and wasted energy) goes out the exhaust hence heating up the exhaust system = burning off more bad stuff (and consequently in the 70s also making catalytic converters actually functional as they need immense heat to do anything).
As soon as those throttle blades open ported vac should essentially be identical in every way to manifold vac.
Hence using manifold is the way to go - benefits at idle of cooler exhaust temps = cooler engine and bay temps; reduced fuel consumption at idle and no/low throttle cruising conditions; and snappier throttle response.
Long winded response I know but its also a complete myth that vac advance has anything at all to do with performance. On wide open throttle regardless if youre running ported or manifold your signal is gonna be 0.0inHG - so the canister wont do anything :) If it is doing something at WOT, there's something wrong hahaha
@@roccoloops thanks for all your knowledge I really appreciate it. It was hooked up to the side of the carburetor but I moved it to the manifold and adjusted all timing and runs much better the response it great , only issue is it runs just a little more warmer then before
@@trivigonzales Most welcome mate! Thats great news glad its worked out! How much warmer are we talking? Should be cooler if anything -- its a good idea to check how much vacuum advance the canister is supplying with a full signal -- depending if you have an OEM or aftermarket distributor some have adjustable vacuum canisters which need to be adjusted according to what the engine wants to pull at idle :)
If youre not having any issues with pre detonation and its running better Id count that as a major win!
Definitely one of those subjects that people argue over which is better, ported or manifold. You could easily make a hour long video covering all the details of this topic. My 67 Pontiac had an original California vacuum advance setup. It used a dual vacuum canister using both ported and manifold fighting against each other for emission purposes. I've since installed and run a standard canister running the normal manifold vacuum like the rest of the Pontiac cars in the other 49 states did back in 67.
So strange they would do that from Cali. I guess they’ve always been a pain in the butt when it comes to emissions.
Not a real difference OTHER than at idle. If you're having idle troubles and the butterflies are out of the transfer slots, try manifold vacuum. Experiment to see what your engine likes. Keeping in mind base timing might also need a tweak.
Definitely need some type of vaccum advance for economy. Cases can be made for either manifold or vaccum. Give your engine what it wants. I dont see a locked dizzy as streetable unless you have a manual or high stall. Its gonna be idling too high. It would be snappy because you have the instant advanced timing off idle but how much of that is excessive and not needed timing. Most factory or even stock aftermarket dizzys have slow mechanical advances. If you curve them to come in sooner off idle so they start coming in at 1000-1200 and all in by 2500 or so, you can use the vaccum advance to fill in whether off idle or at cruise. If your smoothly accelerating off idle, both vaccum advance methods are still advancing. Ported vaccum adds timing til a certain point with throttle angle then goes away nearing WOT. By that time the mechanical is all in. With manifold vaccum advance it doesnt just disappear under smooth acceleration only at WOT, as do both manifold and ported. If you smoothly accelerate, you open the throttle slightly but its not WOT. At the same time the engine is speeding up but the throttle is not WOT so vaccum increases and the vaccum advance is still advancing the timing. So both are kind of doing the same thing. Combine that with a quick acting mechanical advance and you can achieve the timing you need for both part throttle acceleration and WOT acceleration. At cruise, depending on cruise speed and engine speed and gearing all determine how much timing your engine wants. Could be up 50+ degrees if you engine is reving higher with low load. If you have an adjustable vaccum can or a can with the advance you need you benefit from way better economy.
Your missing the point of vacuum advance. It's for mileage under very little load. If your bringing more advance under load your going to break rings and pistons. You need a lot of advance under a lot of vacuum for best mileage then you want it to go away under heavy load low vacuum and just be mechanical advance.
Hooray. So much misinformation in this video. Actually, it's just plain wrong.
I like my MSD mechanical with the light spring kit to get the timing all in around 2500-2700 rpm. 18 deg Initial timing, 18 deg mechanical timing. OH, don't forget about the timed vacuum port on the Holley primary metering plate. Thanks for sharing.
If you run vacuum advance to put in another 8 to 10 degrees at part throttle you will get substantially better mileage. With manifold vacuum it will go away at wot and run just like it does without vacuum advance.
@@jnljnl8485 Kinda wished I did have vacuum advance on the Ford 460 big block, but I don't drive it daily, pro street car to car shows and a couple of local cruises. Thanks for your input.
Ported vacuum advance is necessary for engine braking. Normally manifold vacuum is the best for idle low consumption, but while going downhill, the high rpm advance added to vacuum advance, not only the engine doesn't brake but it will provoke detonations. The ported vacuum outlet in the carburetor is just behind the throttle butterfly so that at idle there is no vacuum hence no advance but just a little bit of throttle, the full vacuum is applied to provoke maximum vacuum advance to get it lower with further loading. By this, when using engine brake at idle throttle the engine gets retarded ignition.
Depends entirely on the setup cam , compression vac #s timming seting headers??? A lot of things one does not necessarily fit another its a tuning aid thats ir
My 75 Dodge 440 truck loves No vaccum advance at all hooked up....slighty advanced on the distributor and both ported pipe on carb and nipple on distributor plugged off....and runs great and still very snappy off the line and smooth running:)
I have a 65 Mustang and have followed tons of videos for setting timing. When I hook my dizzy to ported, the timing says it’s at 30 degrees base.
When I unhook and plug it says it’s around 12 (where it should be).
I’ve noticed if I run ported, the car doesn’t idle too well. But if I hook it to manifold it idles great and the cam is very noticeable where as with it hooked to ported, you can barely tell it’s cammed.
Car also starts right up when hot when it’s hooked to manifold. When it’s hooked to ported, it sort of struggles to start.
Is the timing supposed to look that way when it’s hooked to ported? It’s got a curved dizzy and a handful of mods with a Holley Street demon carb.
It’s a higher compression engine too. Im thinking maybe this setup just prefers manifold at idle.
Nice! I bought an Edelbrock 1405. I will install it on a 302v8 with the HEI electronic ignition. It is better for my standard motor to connect the vacuum to the Manifold connector???
Quick question I have a 1941 Pontiac flat 6, hose that goes from my distributor heads to my carburetor, is that considered ported or manifold...?
And should I change it to the manifold.?
My vacuum advance canister has two nipples for two vacuum lines. Should I do one ported and one manifold? Thank you.
After waiting 2 years for an answer....no