Holley Carb: Ported vs Manifold Vacuum Advance (CARB BASICS PART 1.5)

insights into running manifold vs ported vacuum to settle and disputes

[markschmitt]

Very good! You did a great job explaining the details and clearing the air! Manifold Vacuum is the correct termination for your Vacuum Advance. Set your Total Advance with the VA unplugged and the Engine RPM a little past where the Mechanical Advance stops Advancing. For a Small Block Chevy, this will be somewhere between 30 and 36 degrees. Then you hook your VA back up to the Manifold Vacuum, and set your Carb Idle and Idle Screws. The only other thing to consider is your Mechanical Advance Curve which is set with the Weights and the Springs in the Distributor and under the Rotor in most cases. You want light Springs for a light car with lots of horsepower, and Heavy Springs for a heavier vehicle with lower horsepower. You typically want your Mechanical Advance to be all there by 3, 000 to 3, 500 RPM. If you change the Mechanical Advance Springs and/or Weights, you will need to repeat the process to set your Total Advance. Oh... and if you've got issues with a Large Cam Grind bouncing your Vacuum Advance at idle, you might need to cap it off and run without it. Like 289 said, Ported VA will give you too much Total Advance and could cause engine damage. Party on...

[angelcuriel]

Msd speds thousands on research they would know, but ur knowledge is very useful

[NAS-Performance]

Great video as usual 😃👍👍.
I actually shipped my Holley 850 out to Max Air Carbs for a total rebuild/custom repainting. A couple of weeks ago, I shut the 455 motor OFF ... fuel was pouring into the intake manifold. 👎👎

[CrazyPetez]

Agree, manifold vacuum is best if engine will run with it. The 355cu in SBC in my ‘69 Corvette, 600:Holley double-pumper, has full vacuum advance of 14* at idle, which the engine really likes. Ported vacuum didn’t cut it.

[truthandjustice]

Correct! Ported Timing was an early and poor emissions idea, and Manifold vacuum is the right way to go. Before anyone disagrees with that... Go and do some research! Good video.

[thisisyourcaptainspeaking]

It depends on which vacuum canister you have on your distributor. If you've got a "modern" canister that's all-in at say 14", and it's connected to manifold vacuum then vacuum advance drops out during throttle tip-in, that's not good. If you've got an "pre-emissions" vacuum canister that's all-in around 5", then it's designed to be connected to manifold vacuum.
So choose the correct vacuum canister and connect it accordingly so it operates as intended.

[JuanAvila-blml]

Good video.. I been trying to find more info to set my car.

[thatkyleguy]

Thanks for this video. I have a question though. If I want to use manifold vacuum for the advance, what should I set my initial timing at idle at? Should I have manifold port capped off while setting it? I have a mild cam 350 bored 60 over. Doesnt run hot idle is just a little roudy. Double hump heads. Stock hei dizzy. It's in a k10 chevy truck. Thanks in advance.

[tyscott]

Is the port vaccum fitting replaceable? I broke mines off trying to mount it to the engine.

[goldenhippie]

I'm confused.. I swear you have another video that you say you rather use ported vacuum.. am I mistaken?

[johngauchat]

Please make a video on vacuum signal and how to improve it.

[jamesdavis]

I have a crate 383 stroker in my 77 Monte Carlo that came with mechanical advance. Also installed 700R4 tranny & 3.73 rear gears. The cam Lift: .500 / .510, Duration @ .050: 225 / 231, Centerline: 108, Fair idle
The vacuum advance cannister is connected to distributer but crimped (not hooked up to any vacuum). As it is, my timing settings are maxed out for maximum engine output. Car runs perfect! If I hook up vacuum advance to manifold, can I simply leave those timing settings alone?

[shogunMR]

hey buddy awesome videos ... Question.. on my 302 with a Holley 600 where would i put my transmission (c4 modulator hose) vacuum hose and thanks

[chessman]

I set my SBC(auto) to manifold vacuum to improve the idle performance. It worked great!  However, I noticed when I decelerated the engine braking resulted in full vacuum advance and that resulted in detonation.   Is there any way around that?

[eslpaKo]

Greetings, I have a Holley 4150 0-4778C installed in a 1965 Ford 289, I had to acquire an automatic transmission TCI 511200 C4, now the point is WHERE in the carburetor you can connect the hose from the transmission vacuum modulator valve? I see only 1 port that is for the distributor timing, some other videos recommend do not make a share connection in "T", Can you help?
#2: For the booster´s hose vacuum, what kind of valve is PCV? I have the intake manifold Street Dominator, but I need you tell me exactly what´s the name of any fitting, valve or device that can be installed in the back of it for the booster´s hose
In advance, Thanks for your concern & prompt response

[sohc]

I have a 66 Mustang with early 80's 302, Holley Ultra 650 DP, and a decent cam that produces vacuum of 12-13 in.Hg. at 850 rpm idle. Timing is 16 degrees initial+20 degrees mechanical advance=36 degrees total at 3200 rpm. I have tried to run vacuum advance, both ported and direct but the engine just doesn't like it. With vacuum advance connected my dial back timing light showed 50+ degrees at 3000 and engine sounded rough. This is a weekend car and I'm not concerned with mpg. Is there any particular reason I should alter the tune just to run vacuum advance?

[biggirlbathingsuits]

The standard setup, before emission controls, vacuum advance was only ported vacuum and its purpose was to improve fuel economy. then with emission controls, some companies got tricky with two hose vacuum advance cans, one ported and one manifold vacuum or like some Fords, the vacuum advance was switched to manifold vacuum. The distributor has to be made for using manifold vacuum, both the vacuum can and the mechanical advance. Performance and pre-emission control distributors should normally only use ported vacuum advance, unless they are specially made for super chargers or turbo chargers. Racing distributors typically have no vacuum advance. Fuel economy isn't much of a factor and the mechanical advance is all in at relatively low RPM and more advance from vacuum would just complicate things.

[ryderducan]

I have a 1975 ford f150 when I sick on the vacuum canister it retards the rotor, this leads me to believe I can only run ported vacuum with that style distributor. Plz someone correct me if I'm wrong

[scottspurlee]

Where, exactly, is the hose connected for Ported Vacuum? You point frequently to a tap on the upper part of the carburetor, I'll presume that's it. Where do I pick up manifold vacuum? Is it somewhere on the carburetor? On the manifold itself somewhere? Thanks!

[osto]

Hi there. I have a question about timing my 351 cleveland. Engine had a mild cam, holley double pumper and runs mallory ignition with adjustable vacuum cannister. Engine was built when i got the car but i have been having ignition issues so i have been trying to get it right. And now after adjusting timing and vacuum cannister my timing is at 18 degrees at idle and 34 at 3000 rpm and ported vacuum to the cannister. Car finally runs great and good throttle response. Just wanted to hear if those setting sounds right. Am fairly new to this so i figure it is better to ask for advice.... Dont hear any pinging or knocking so i hope it is okay... Hope anyone can tell me if my settings are okay...???