What is the difference between Manifold & Ported Vacuum Advance? Which one should I use?

bostoncat68 · Dec 23, 2023

Jim Weise said: ↑

Spark advance-- this is the key..

Your focusing on how it's is being achieved, but the real thing your doing is advancing the ignition timing. Our Buick 455 based engines like about 34* of total advance, when built to street specs. Couple degrees one way or the other maybe, but that is where they run the best, for power.

But that's not what the OP is asking, and not what others are commenting about when they switch to a manifold source for vacuum advance.. The improvements they are seeing are due to more spark advance at idle and low speeds. Virtually any 455 will run better at 15-20 initial advance.. With a stock or real small cam, you might run into difficulties getting the starter to turn it over when hot.. but with most of the cams we are running these days (220* and up at .050) enough compression blows off at cranking speed this won't be an issue.

Two things must be considered here, when answering the question of the OP.

1. Control-- what method of vacuum supply gives you the best control over your spark lead, in various driving conditions. If you chose manifold, then it's going to take some cargo-coil type springs to keep the mechanical advance out of it in off idle, light throttle loads. That detonation is the hardest to get rid of when tuning. Big converters, heavy cars, and small camshafts all make this problem worst. I have seen situations where I had to go to a locked out distributor, to stop that off idle ping, when trying to run manifold vacuum for my advance canister. The problem comes because to keep the car out of the idle into gear "death spiral" you have to achieve all your vacuum advance at a couple inches of vacuum below the vacuum available at idle, so under light throttle take off, your not dropping any vacuum advance.. In certain combos this can lead to an unsolvable off idle detonation event. You limit your control with manifold vacuum.

Ported vacuum, on the other hand, provides a weaker signal, and only at part throttle.. so it's much easier to tailor the vacuum advance canister to provide the 10-15* of additional spark lead that you want for light throttle cruising. And doing this, does not effect the timing at any other speed/load.

2. Purpose... What is the vacuum advance for?
Vacuum advance was added to engines to provide additional spark lead, for more efficient operation under light throttle loads. Most Buick 455 based engine combos like between 45-50 degrees of advance when just going down the road, and I have seen a properly set up full size Buick achieve over 20mpg on the highway. Using the vacuum advance to modify base timing is possible, but it often leads to poor performance, detonation issues, and even engine damage.

So for that reasoning, I am going to say that your going to achieve the best results on a wide variety of combos, by using a ported source for your vacuum advance.

___________________________________

Let's talk about how I set up a distributor here in the shop.. I have had the opportunity to build coming up on 150 455 based engines over the last 20 plus years for Buick guys all over the country. But most of them, I don't have the car here, so I have come up with a basic guide for distributor setup.

First off, these are grouped by idle vacuum.. because two engines with the exact cam, can run differently.. for a variety of reasons. I take the vacuum reading at 900 rpm.

18-22"
Basically stock here.. engines will run better and tolerate about 14-16* of initial advance, total mechanical advance at 34-36* and add about 15* of ported vacuum advance for cruising.

14-17"
Moderate combos... They love around 20* initial, no problem starting them, limit the mech advance to 35* max, add 10* with your vacuum can and ported vacuum.

10-13"
Hot Street/Strip combos... These motors really need the base timing in the 24-26* range.. max at 34, and add 10* with ported vacuum. Enough base timing with these is critical for acceptable street manners.

Most GS cars use the smaller points type distributor, and I recommend it because it is easier to modify than the HEI units. Distributor mechanical modification of the advance mechanism is required in most of these combos. Next time I set one up, I will take pictures, and we can go thru that process and explain it.

Merry Christmas and Happy New year to all.

JW
Click to expand...

Super helpful...

BQUICK · Dec 23, 2023

69WILD said: ↑

I did try switching my modified 72 350 to manifold. It didn't help performance and idle got rough. I am still dumbfounded at the improvement in performance of my stock 69 430 by switching to manifold. I wish I would have done it decades ago instead of adjusting timing issues every year with no improvement......Great information here.
Click to expand...

You probably had a good amount of initial so ended up with too much timing at idle or vacuum can was more than 18deg. Most motors like up to 25deg at idle but more can make for roughness.

69WILD · Dec 23, 2023

BQUICK said: ↑

You probably had a good amount of initial so ended up with too much timing at idle or vacuum can was more than 18deg. Most motors like up to 25deg at idle but more can make for roughness.
Click to expand...

Both my cars can't be advanced very much or detonation takes over. Both va are at about 10* total.....I think I'm going to set advance on both cars as suggested here and add racing gas to my premium next year. I'm confident they will both run and preform better. I only put about 1000 miles on each every year on average so maybe only $500 or so extra a year.

Emanuel Bader · Dec 23, 2023

Jim, I like how you explain things in detail, thanks!

Merry Christmas everyone

Mark Demko · Dec 23, 2023

Manifold versus ported, it comes down to WHEN you want/need the advance

1968_GS400 · Dec 23, 2023

Jim Weise said: ↑

Spark advance-- this is the key..

Your focusing on how it's is being achieved, but the real thing your doing is advancing the ignition timing. Our Buick 455 based engines like about 34* of total advance, when built to street specs. Couple degrees one way or the other maybe, but that is where they run the best, for power.

But that's not what the OP is asking, and not what others are commenting about when they switch to a manifold source for vacuum advance.. The improvements they are seeing are due to more spark advance at idle and low speeds. Virtually any 455 will run better at 15-20 initial advance.. With a stock or real small cam, you might run into difficulties getting the starter to turn it over when hot.. but with most of the cams we are running these days (220* and up at .050) enough compression blows off at cranking speed this won't be an issue.

Two things must be considered here, when answering the question of the OP.

1. Control-- what method of vacuum supply gives you the best control over your spark lead, in various driving conditions. If you chose manifold, then it's going to take some cargo-coil type springs to keep the mechanical advance out of it in off idle, light throttle loads. That detonation is the hardest to get rid of when tuning. Big converters, heavy cars, and small camshafts all make this problem worst. I have seen situations where I had to go to a locked out distributor, to stop that off idle ping, when trying to run manifold vacuum for my advance canister. The problem comes because to keep the car out of the idle into gear "death spiral" you have to achieve all your vacuum advance at a couple inches of vacuum below the vacuum available at idle, so under light throttle take off, your not dropping any vacuum advance.. In certain combos this can lead to an unsolvable off idle detonation event. You limit your control with manifold vacuum.

Ported vacuum, on the other hand, provides a weaker signal, and only at part throttle.. so it's much easier to tailor the vacuum advance canister to provide the 10-15* of additional spark lead that you want for light throttle cruising. And doing this, does not effect the timing at any other speed/load.

2. Purpose... What is the vacuum advance for?
Vacuum advance was added to engines to provide additional spark lead, for more efficient operation under light throttle loads. Most Buick 455 based engine combos like between 45-50 degrees of advance when just going down the road, and I have seen a properly set up full size Buick achieve over 20mpg on the highway. Using the vacuum advance to modify base timing is possible, but it often leads to poor performance, detonation issues, and even engine damage.

So for that reasoning, I am going to say that your going to achieve the best results on a wide variety of combos, by using a ported source for your vacuum advance.

___________________________________

Let's talk about how I set up a distributor here in the shop.. I have had the opportunity to build coming up on 150 455 based engines over the last 20 plus years for Buick guys all over the country. But most of them, I don't have the car here, so I have come up with a basic guide for distributor setup.

First off, these are grouped by idle vacuum.. because two engines with the exact cam, can run differently.. for a variety of reasons. I take the vacuum reading at 900 rpm.

18-22"
Basically stock here.. engines will run better and tolerate about 14-16* of initial advance, total mechanical advance at 34-36* and add about 15* of ported vacuum advance for cruising.

14-17"
Moderate combos... They love around 20* initial, no problem starting them, limit the mech advance to 35* max, add 10* with your vacuum can and ported vacuum.

10-13"
Hot Street/Strip combos... These motors really need the base timing in the 24-26* range.. max at 34, and add 10* with ported vacuum. Enough base timing with these is critical for acceptable street manners.

Most GS cars use the smaller points type distributor, and I recommend it because it is easier to modify than the HEI units. Distributor mechanical modification of the advance mechanism is required in most of these combos. Next time I set one up, I will take pictures, and we can go thru that process and explain it.

Merry Christmas and Happy New year to all.

JW
Click to expand...

Did all the stock vacuum canisters give about 14-18 degrees advance?

So for the stock setup, you need basically a stock vacuum canister and then move initial timing from 0 to 4 degrees BTDC (if it was tuned to chassis manual spec) to 14-16 degrees BTDC?

Then to achieve the total 34-36 degrees of mechanical advance at certain rpm you would need to swap springs? I gather from other posts that for street you want to move the all in point from factory 4600 rpm spec to 2500-2800 rpm?

https://www.v8buick.com/index.php?t...specs-1965-66-66-67-68-69-70-71-72-74.329332/

BQUICK · Dec 23, 2023

I've see ones with as little as 10 and as much as 22deg.

Houndogforever · Dec 23, 2023

I found this video showing the different reactions of vacuum. He has a gauge for both ported and manifold.

Schurkey · Dec 24, 2023

BQUICK said: ↑

I've see ones with as little as 10 and as much as 22deg.
Click to expand...

Photo of one I modified to limit the amount of advance. Started off at 25 degrees.

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What is the difference between Manifold & Ported Vacuum Advance? Which one should I use?

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69WILD Ron

Emanuel Bader 67` GS 400

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