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

This topic has always confused me. What is the difference between Manifold & Ported Vacuum Advance? Which one should I use?

I went to YouTube, and from the YouTube, I should be hooked-up to ported vacuum.

Let's start what I have for an engine, because I know that cam overlap has a lot to do with Ported or Manifold hook-ups.

After everyone reads my engine specs below, please advise what I should be using and WHY. Thank you.

1970 Buick 455 engine, that started life as a Base 350 HP engine.

The previous owner did the following engine mods. Engine rebuild is about 3,500 miles ago with the first owner.
The second owner, whom I purchased this Buick from did some engine mods I was NOT told about. Kenny Belle pistons is one we know for sure.

(1) Bored .030 over.
(2) Port match heads.
(3) installed big valve Stage 1 heads.
(4) Installed a Comp cam:
Gross valve lift: Exh. Lift. .496”
Gross Valve lift: Int. Lift. .478”
Duration @ .050: Exh. Duration .231
Duration @ .050: Int. Duration .218
Lobe separation: 112 degrees
This cam, I'm told is just a little more aggressive than the original Stage 1 cam.
(5) Old style Kenny Belle 10.5 CR pistons with valve reliefs. I do NOT know what the real CR is. I would need to pull the heads and do some measurements, but I do not want to do this at this time.
(6) Static compression is an average of 180 psi.
(7) Carburetor is a Holley 750 cfm 4150 model with mechanical secondaries.
(8) Transmission is the TH400 (BA) with a shift kit. Was installed when I purchased the Buick.
(9) Differential is a highway 2.93 posi.
(10) Ignition, MSD distributor with MSD coil. Reason why we went this way, I found that this car had a serious detonation issue at all rpms. Now we don't.
(11) Installed a new "adjustable" carburetor vacuum canister. This is adjustable vacuum canister is new to me, never saw one before.

FYI: Originally, the Buick has a 750 Qjet. Someone over tighten the carb to the manifold and warped the base of the carb. Shop purchased another Qjet, BAD right out of the box, (China junk). I didn't know I could find someone of this forum that is a Pro-Qjet builder, I'm fairly new to BuickV8.com.
Shop foreman asked me if I would convert to a Holly (American made), I said I've had Holley's before, and they worked very well. Yes, I will convert to a Holley.

That's all I can think of adding so everyone will know the specs of my engine for this discussion.

FYI: the block has NOT been decked.

Appreciate the answer to my question. VET (Navy)

 

I just changed mine from ported to manifold today and made a huge improvement. Like LARRY70GS suggested in my post it doesn't hurt to try it. I'm thinking of changing my 72 350 also.

 

Ported vs Manifold is a case of YMMV..... In other words you can have two cars that are literally the same, one prefers ported the other manifold....

 

Cars originally used manifold. Ported came about to control emissions.

Ported actually works opposite of what is required to make the most power.

You need more timing at light throttle and/or low demand/load.

If manifold, you have higher vacuum at idle, light throttle and cruise, this gives you more vacuum advance, to "assist" with adding more timing when mechanical is not yet bringing in more timing.

As soon as you get into the throttle, vacuum drops, the vacuum advance is reduced, and advance timing is pulled, as mechanical takes over. This is what you want.

Ported works opposite.
At idle, cruise and light throttle, you have little or no vacuum advance, and mechanical may not be coming in, this leads to sluggish response, lower power, more heat in the heads)exhaust valves, and higher coolant temps.


All of that helps with emissions, but costs power and efficiency.

But, you have to tune all components, not simply switching from one to the other.

As stated, some cars will run better on ported and that is directly related to carb tune, distributor, cam, et, where you may not be able to change all to facilitate manifold vacuum.

Both my nailhead and my BBB run manifold.

 

Thank you.
Have a much better understanding of this mystery.

I will check with the shop to see if they used manifold or ported, because I really don't know.

Thank you very much. VET

 

No matter if you use ported or manifold, make sure you either know your complete timing curve, or your shop does.

Over the years, times cars have changed hands, parts changed, there is no telling what mix one might find.
And more importantly. one cannot simply "set according to manual" and hit the road.

Timing curve from "starting", idle to WOT and everything in between can have detrimental and very expensive consequences. Timing, and lubrication are the two most critical things in engine longevity.

Think "Heart attack and stroke"...

 

It's only been 41 years since my last muscle car tune-up. In 1972 I had a 1972 327 chevy Corvette engine (1964 365 HP ventage) that was easy to tune.
(1) Set the points gap and dwell setting.
(2) I only set the initial timing.
(3) 30/30 solid lifter cam. Set the clearance.
(4) Holley 650 cfm Spread bore with manual choke. Never had to mess with it.
(5) Mallery ignition. Never had to mess with that.
(6) Accel ignition wires. Never had a problem.
All this in a 56 Chevy Belair, 4-speed car.

That car is as close to plug & play as I have ever owned. It was almost maintenance free.
Only 2 items I ever had to replace, clutches and rear tires.

This Buick GS 455 is a very different animal. Tuning is a bitch. It just doesn't seem to me to have the big block punch that it should have.
Maybe I was expecting more?

My 56 Chevy SB was built very strong, and HP was around 420. Maybe it's the rev's differential, the SB chevy I shifted between 6,500 and 6,700 rpms and it was a 4-speed.
Of course, the Buick is a lot less at around 5,400 to 5,600 and an automatic.
I just don't get that 510 lbs of torque RUSH.... Maybe I was expecting too much??? VET (Navy)

 

Cjay &Larry, I sent your posts to me to my shop so they can review and look into your suggestions and the vast experience you both have.

Just so you both know, I have been battling some high water temperature issues with my 455 with the A/C engaged.
With Jim Weise's advise, i'am having TA's High-performance water pump installed next week. I was fortunate to get one of TA's last units, they're inventory is running low.
Already replaced the radiator & thermostatic fan clutch.

I'm hoping this will turn the tide with the high temperature issue.
This is my first BB Buick engine.
It's very different from my SB 1964 Chevy engine.

Thank you both for your help.
This is Best forum. Everybody has been super nice and helpful. Regards VET (Navy)

 

Sorry, Trunk monkey & 69WILD.
You both are included in my post to my shop mechanics and shop owner.
Thank you both for your help.

Sometimes I don't get everybody included. VET

 

I’ve been reading a lot of posts and watching a lot of videos on ported vs manifold vacuum, and I’m starting to wonder if a lot debate comes about because maybe once you switch you should really re-tune. Just switching the vacuum hose might not be a fair comparison. I think many of Larry’s posts including his power timing thread may allude to this.

This guy on YouTube is now doing testing but re-tuning separately for each case, but his testing has now been put on hold until Spring.

https://m.youtube.com/watch?v=aynMWY9tvF4

 

Using manifold vacuum on auto cars with any sort of cam is not a great set up. As soon as you put the car in gear, rpm and vacuum drops, timing drops….motor runs rough….sort of snow balls. The ported is great to have a consistent idle. You simply run what ever initial timing gives you best idle quality, limit your total mechanical advance in your distributor so you don’t exceed 34-36 degrees centrifugal - this includes the initial ( read Larry’s power tuning) Car will idle great, because your ignition curve shouldn’t start till past your idle rpm, you will have consistent timing at idle, as well same timing when auto car is put in gear. Find a vacuum advance unit that gives you 16 degrees advance at the crank (8 degrees on the distributor) you’ll have a decent amount of timing for light throttle cruising. I have 17 initial, 36 degrees total centrifugal all in by 3,200 and vacuum advance connected to ported source on Qjet. At cruise of 2500 rpm, I’m running total of 44 degrees advance - centrifugal plus vacuum advance.
The ported vacuum provides vacuum once the throttle blades start to open. It may have been an emissions thing, but can also be a very good tuning option. Especially for auto cars with non stock cams.

 

I have an automatic trans and an any sort of cam'. Running manifold vaccum. Put it in gear and timing is solid.
Key is to pick a advance that is rated below the idle vaccum in gear. If the car is modded then many times stock parts aren't the best.

Attached is a list of gm advances for points.and hei types and the specs for them to taylor to your needs. I limit mine to about 10° by making a limiter.
Sorry about it being a pdf but it is safe to download. If someone knows how to convert it, please do.

 

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

 

The 67 430 used manifold vacuum. Spec was 2.5 deg BTDC and then 18 deg vacuum advance at idle. Made for nice smooth idle with all that timing. Mine idles at 500 in gear and with the switch pitch on high stall it doesn't move in gear with foot off brake. Put on low stall and starts moving. As others have said it all went out the window once emissions came into the picture.
Bottom line IMO, if you have mild or stock cam with plenty of vacuum, use manifold port.

Side note: since idle was heavily relying on the vacuum advance diaphragm if it leaked car ran like crap. (then only 2.5 deg initial timing) Dealers and repair shops probably loved that...sell someone a $$$ tune up or carb or even a timing chain job when all it was just a rubber diaphragm that leaked....like they all eventually will.

 

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.