Ported vs manifold vacuum for ignition advance?

[Nut124]

I seem to be getting in trouble at another forum for suggesting that the distributor vacuum advance should be connected to manifold vacuum.

I'm told that that is flat out wrong - it must be ported vacuum. They say there should be zero vacuum advance at idle. I have always used manifold vac and get the 15-20deg of vacuum advance at idle, low speed, on top of 10 static, which seems to make the engine happy. This is for mid to late 70s thru mid 80 Fiats, both carb and FI.

What is the origin and purpose of this ported vacuum? What is it supposed to do? Where and how does this ported vac come from in the engine?

I hope someone here can shed some light to this as I'm puzzled.

[BobbyB]

My short answer is “Try both ways and use what works for you”.

[HQM383]

Strap yourself in for a passionate debate

FWIW I agree with BobbyB. I have done that and found that manifold applies too much for my 11.4:1 383 sbc

[dannobee]

Traditionally, vacuum advance was connected to manifold vacuum. Ported vacuum was used for EGR valves.

On smog-dog engines of the late 70's and 80's, ported vacuum was used for both, along with thermal vacuum switches and delay valves. And the vacuum diagrams read like a road map.

[RDY4WAR]

I prefer manifold source for the vacuum advance. It's just logical to do it that way. When manifold vacuum is high, cylinder pressure is low, so more advance is desired. When manifold vacuum is low, cylinder pressure is high, so less advance is desired. Use manifold vacuum to set ideal advance for idle and cruise, then set the mechanical curve for WOT.

Also, ported vacuum is the cause of a lot of overheating problems. The ideal idle spark advance is 20-30 BTDC, not 8-12. An old friend came by my place last summer with his 80s FSB and .060" over 351W. He had a high volume water pump, 4 core radiator, and 2 big Spal fans for cooling, and it would still creep up to 215*F sitting in traffic. His engine builder told him it was because the block was .060" over. I noticed the ported vacuum advance, switching it to manifold, (timing bumped from 10* to 25*) and steady 180-185*F.

Take a look at idle tables in EFI. My '93 Camaro, '02 Tahoe, and '12 Mustang all have 20-25* advance at idle.

[JCR]

Find yourself a copy of Timing and Vacuum Advance 101 by John Hinckley.

Here is an excerpt regarding ported vacuum

“PORTED” VACUUM: Now to the widely misunderstood manifold vs. ”ported”
vacuum aberration. After 30-plus years of controlling vacuum advance systems with
full manifold vacuum, that “free” indicator of engine load and fuel mixture, along
came early emission control requirements (seven years before catalytic converter
technology was introduced), and all manner of crude band-aid systems were
introduced to try to reduce hydrocarbons and oxides of nitrogen in the exhaust
stream. One of these crude, but effective systems was GM’s Air Injection Reactor
(A.I.R.) system, which pumped fresh air into the exhaust ports to “afterburn”
pollutants in the exhaust manifolds. The key to making this system work at
maximum efficiency was retarded spark at idle; with retarded idle spark timing, the
“burn” begins late, and is not complete when the exhaust valve opens, which does
two things that were important for emissions. The incomplete burn reduced
combustion chamber temperatures, which reduced the formation of oxides of
nitrogen (NOX), and the significant increase in exhaust gas temperature ensured
rapid “light-off” and combustion of the hydrocarbons in the exhaust gas stream
when the fresh, oxygen-carrying air was introduced from the air pump.
As a result, these engines ran poorly, and an enormous amount of wasted heat
energy was transferred through the exhaust port walls into the coolant, causing
them to “run hot” at idle; cylinder pressure fell off, engine temperatures went up,
combustion efficiency went down the drain, and fuel economy went down with it.
“Ported Vacuum” was easy to implement – they just moved the distributor vacuum
port orifice in the carburetor from below the throttle plate (where it was exposed to
full manifold vacuum) to above the throttle plate, where it was exposed to manifold
vacuum only after the throttle plate opened. This meant that the vacuum advance
was inoperative at idle (retarding idle spark timing from its optimum value), and
these applications also had very low initial timing settings; they were usually set at
four degrees before TDC or less, and some even had initial timing settings as much
as two degrees after TDC. The vacuum advance still worked at highway cruise, but
not at idle, which caused all manner of problems. “Ported Vacuum” was strictly an
early pre-converter crude emissions strategy and nothing more. Don’t believe
anyone who tells you that ported vacuum is a good thing for performance and
drivability – it’s not. Anyone with 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.

[HQM383]

Also, ported vacuum is the cause of a lot of overheating problems. The ideal idle spark advance is 20-30 BTDC, not 8-12. An old friend came by my place last summer with his 80s FSB and .060" over 351W. He had a high volume water pump, 4 core radiator, and 2 big Spal fans for cooling, and it would still creep up to 215*F sitting in traffic. His engine builder told him it was because the block was .060" over. I noticed the ported vacuum advance, switching it to manifold, (timing bumped from 10* to 25*) and steady 180-185*F.

Take a look at idle tables in EFI. My '93 Camaro, '02 Tahoe, and '12 Mustang all have 20-25* advance at idle.

As I alluded to in post #3 I am on the fence as it needs to be determined what works best for the individual case. Where this applies is first bold statement above. The ideal spark advance is the advance that provides peak cylinder pressure @ 14* atdc, that is for best torque . A whole range of factors in the individual engine will culminate in that figure. Comp ratio, piston shape and material, combustion shape, fuel used and degree of atomization of that fuel, and so on.

Second in bold, a hypothetical. Would your friend even have had the overheating problem and the need for manifold vacuum if the distributor was recurved to have 25* static comp and all in at the same degrees with an appropriate slope? Would ported have sufficed and nobody know the difference or even the owner become an advocate for ported?

[Geoff2]

Nut,
You answered your own question: your engine is happy with MVA. End of story.
Unfortunately there is a whole lot of BS on this subject. Obviously if you have a lot of static initial timing, MVA might do little or nothing to improve idle.
When you reduce init timing because, say, the starter struggles with too much initial, then MVA will ALWAYS help with a cammed engine. There are no exceptions. One of the reasons MVA can get a bad rap 'Didn't work for me etc' is that folks try to use a non-adj VA unit; the result is varying timing at idle, MVA gets the blame.

[RDY4WAR]

Also, ported vacuum is the cause of a lot of overheating problems. The ideal idle spark advance is 20-30 BTDC, not 8-12. An old friend came by my place last summer with his 80s FSB and .060" over 351W. He had a high volume water pump, 4 core radiator, and 2 big Spal fans for cooling, and it would still creep up to 215*F sitting in traffic. His engine builder told him it was because the block was .060" over. I noticed the ported vacuum advance, switching it to manifold, (timing bumped from 10* to 25*) and steady 180-185*F.

Take a look at idle tables in EFI. My '93 Camaro, '02 Tahoe, and '12 Mustang all have 20-25* advance at idle.

As I alluded to in post #3 I am on the fence as it needs to be determined what works best for the individual case. Where this applies is first bold statement above. The ideal spark advance is the advance that provides peak cylinder pressure @ 14* atdc, that is for best torque . A whole range of factors in the individual engine will culminate in that figure. Comp ratio, piston shape and material, combustion shape, fuel used and degree of atomization of that fuel, and so on.

Second in bold, a hypothetical. Would your friend even have had the overheating problem and the need for manifold vacuum if the distributor was recurved to have 25* static comp and all in at the same degrees with an appropriate slope? Would ported have sufficed and nobody know the difference or even the owner become an advocate for ported?

He could have set it to have 25* initial with less curve and ported vacuum, but that likely would make the starter work harder to crank it over. For what benefit? Manifold vacuum helps a good bit there as it cranks easy with 10* and goes to 25* at idle.

[rebelrouser]

As most have said it just depends. What is right is an advance curve that the engine likes. To my knowledge most stock older engines from the factory used ported vacuum. With a big cam however manifold vacuum may make them idle a little better. I think the issue is on stock engines, when you use full manifold vacuum the idle may be high enough that it will cause them to run on when you shut it off, because even with the throttle plates closed the extra advance makes them idle a little too fast. With a big cam they are usually are trying to die all the time, and the extras advance helps, might be some advantage during overlap to fire the mixture early to help keep the plugs clean as well.

[RDY4WAR]

Advancing the idle spark timing helps tremendously with a big cam. Having a complete burn means faster blowdown and less residual exhaust gas and temperature lingering when the intake valve opens. This means less reversion diluting the intake charge, improving intake vacuum. It also gives the exhaust a deeper, throatier sound.

The overheating issue can't be understated though is often overlooked. A prime recipe for overheating is a big cam with low compression and late spark timing.

[Dan Timberlake]

Debate ? for sure.

I only feel it is proper to add to the others that that said the use of ported vacuum existed, and was even used by choice, by some manufacturers way before even the toothless first Clean Air Act was the LAW in 1963.
Despite what GM (manufacturing) Engineer, auto enthusiast and 2009 Corvette Hall of Fame Inductee John Hinckley said in "that" article.

https://www.jalopyjournal.com/forum/thr ... t-11530522

[chimpvalet]

If applying this to a Fiat with full IR intake set-up some care will be needed in creating a steady source of vacuum. The solution needs to be one which steadies the pulsing at idle, this done by way of a small reserve canister. To take from all 4 cylinders it is advised to choke each vacuum line near its runner with a small idle jet, this to minimize cyl-cyl interference, then combine all at the canister. If this engine is also that which has been cammed somewhat then idle vac will be limited, and will fall off quite rapidly when the IR throttles are opening in any case, correct?

The FI set-up will be plenum type if stock, quite straightforward for establishing manifold vacuum. No way either will facilitate ported vacuum as I see it.

Cheers
Steve

[HQM383]

Debate ? for sure.

I only feel it is proper to add to the others that that said the use of ported vacuum existed, and was even used by choice, by some manufacturers way before even the toothless first Clean Air Act was the LAW in 1963.
Despite what GM (manufacturing) Engineer, auto enthusiast and 2009 Corvette Hall of Fame Inductee John Hinckley said in "that" article.

https://www.jalopyjournal.com/forum/thr ... t-11530522

I like the statement on the bottom pic above that backs up post two on this thread:

“The point at which vacuum is taken from the manifold depends upon the condition to be satisfied or results to be obtained.”

If this is coming from an era not concerned with emissions then it is purely from a standpoint of engine operation. John Hinckley described the effects of one way ported can be applied - to create a condition to manipulate burnt gasses. Maybe this use has skewed its functionality?

[HQM383]

Also, ported vacuum is the cause of a lot of overheating problems. The ideal idle spark advance is 20-30 BTDC, not 8-12. An old friend came by my place last summer with his 80s FSB and .060" over 351W. He had a high volume water pump, 4 core radiator, and 2 big Spal fans for cooling, and it would still creep up to 215*F sitting in traffic. His engine builder told him it was because the block was .060" over. I noticed the ported vacuum advance, switching it to manifold, (timing bumped from 10* to 25*) and steady 180-185*F.

Take a look at idle tables in EFI. My '93 Camaro, '02 Tahoe, and '12 Mustang all have 20-25* advance at idle.

As I alluded to in post #3 I am on the fence as it needs to be determined what works best for the individual case. Where this applies is first bold statement above. The ideal spark advance is the advance that provides peak cylinder pressure @ 14* atdc, that is for best torque . A whole range of factors in the individual engine will culminate in that figure. Comp ratio, piston shape and material, combustion shape, fuel used and degree of atomization of that fuel, and so on.

Second in bold, a hypothetical. Would your friend even have had the overheating problem and the need for manifold vacuum if the distributor was recurved to have 25* static comp and all in at the same degrees with an appropriate slope? Would ported have sufficed and nobody know the difference or even the owner become an advocate for ported?

He could have set it to have 25* initial with less curve and ported vacuum, but that likely would make the starter work harder to crank it over. For what benefit? Manifold vacuum helps a good bit there as it cranks easy with 10* and goes to 25* at idle.

Like I said earlier I’m on the fence because I see where both can be employed. The hypothetical was to illustrate to a degree how we come to choose what we believe is best. Your reply gives a good example of using manifold vacuum. If an engine has high cranking compression and high initial advance is tough on the starter then manifold does well as a starter saver. A good gear reduction starter and lower cranking compression due to big cam and the complexity of adding another system to introduce spark advance may not be required.

I have trialed both on my 383. First it must be noted that the ported signal is not as strong as manifold, not in my case anyway. I have 11.4:1 static compression and a cam 244* @.050” intake on 110* lobe sep 106* ICL. Manifold vacuum gets as high as 20” when cruising at highway speeds @ 3250rpm. Ignition is set with 20* initial and 34* total mechanical. No problem starting with a mini reduction starter and just a flick of the key when warmed up to restart. Doesn’t get hot in traffic and throttle response can’t be questioned. I tried manifold vacuum to see if there were any gains to be had as we are all always in search of gains. I went through the process of rotating the distributor at idle to find how much advance it would like. After I got past ~28* BTDC idle rpm did not increase any further. I went as far as 80* BTDC. Rpm went up by 150rpm. Adjustable vac advance canister was already at its minimum degrees applied setting and when plugged into MVA took initial to 36*. So adding 16*. This 16* was always being added as that 20* vacuum was coming in as low as 60kp/h (~2100rpm). This noticeably dulled throttle response at steady state cruising where ignition advance would have been maybe 45-50* (mech + vac). Must have been too much negative pressure on the piston and/or complete combustion too early. Ported source when measured with vac gauge applies vac gradually. That 2100rpm applies 5” vac up to 15” vac at 3250rpm. Throttle response was sharper. Exactly how much * of advance ported is applying at each rpm point I don’t know but I would be getting the benefits of some vac advance without going too far. So for me it’s ported. I could dick around re-curving the distributor and whatever else to employ MVA but due to no issues now with starting, overheating or performance for what benefit?

I’ll refer to BobbyB in post two again.