Ported vs manifold vacuum advance solved!!!

[padam]

Damm,
Man Vac Adv [ MVA ] was around for years, used by GM until 1967-8. Not sure about Ferd & others. Then emissions forced the use of the USELESS PVA.
Chrysler, stupidly, chose not to use MVA during the performance years. That is why your AFB has the PVA port. MVA does not need a dedicated port [ but it often does ], it can be connected to a manifold runner or Tee-d into the brake booster line.
The paradox is that Ch was forced to use MVA in the 1970s when emissions raised engine temps. MVA would be switched on when temp reached 200+*......to cool the engine.
Folks should think about how/why MVA cools an engine....

Pretty sure it just runs the fan and water pump faster.

 

[gtxrt]

ran MV for 45 years. this last rebuilt had the dist recurved and he said to use ported with 12* initial. 1 thing i did notice no gas and exhaust smell at idle. was told years ago not use MV on a manual tranny because of off idle stumble.

 

[biomedtechguy]

ran MV for 45 years. this last rebuilt had the dist recurved and he said to use ported with 12* initial. 1 thing i did notice no gas and exhaust smell at idle. was told years ago not use MV on a manual tranny because of off idle stumble.

Funny, because Don at Four Seconds Flat insists on manifold vacuum as the source for ignition advance, and as anyone who knows him won't be surprised to hear, offers a very "forceful" explanation of the reasons why.
I switched over my ignition system from a Chrysler electronic version to the "pre-tuned" FBO setup, using their proprietary control box w/multiple spark output and an excellent rev limiter. Bypassed the ballast resistor and changed out the ignition coil to a Flamethrower III. There were other changes made, like careful, "enlightened" 6bbl carb tuning, and I had to plumb in a vacuum fitting in a pre-existing threaded hole in the rear of the Edelbrock aluminum 6bbl intake. The old vacuum advance used a ported source that had been eliminated when I had the Promax center carb metering block and other parts installed.
Anyway, everything came together and my engine no longer smelled like a refinery or made my eyes water, the idle improved greatly, and so on.
I'm 99% sure my much anticipated Progression Ignition system also uses manifold vacuum for its advance calculations.

 

[Ceedawg]

And Don didn’t tell HOW or WHY what he did helped either, most here don’t have a clue to those two things. How many here have put a vacuum gage on then watched it while driving, accelerating and decelerating?

 

[diesel_lv]

Funny, because Don at Four Seconds Flat insists on manifold vacuum as the source for ignition advance, and as anyone who knows him won't be surprised to hear, offers a very "forceful" explanation of the reasons why.
I switched over my ignition system from a Chrysler electronic version to the "pre-tuned" FBO setup, using their proprietary control box w/multiple spark output and an excellent rev limiter. Bypassed the ballast resistor and changed out the ignition coil to a Flamethrower III. There were other changes made, like careful, "enlightened" 6bbl carb tuning, and I had to plumb in a vacuum fitting in a pre-existing threaded hole in the rear of the Edelbrock aluminum 6bbl intake. The old vacuum advance used a ported source that had been eliminated when I had the Promax center carb metering block and other parts installed.
Anyway, everything came together and my engine no longer smelled like a refinery or made my eyes water, the idle improved greatly, and so on.
I'm 99% sure my much anticipated Progression Ignition system also uses manifold vacuum for its advance calculations.

Progression Ignition does, but it uses it for a different reason. To tell at idle if you are in P/N or D/R so it can bump up the timing so it doesn't stall. It uses the Ignition map based on rpm n MAP to determine advance.

 

[Ceedawg]

Progression Ignition does, but it uses it for a different reason. To tell at idle if you are in P/N or D/R so it can bump up the timing so it doesn't stall. It uses the Ignition map based on rpm n MAP to determine advance.

Is this electronic? If so I would use this over any mechanical device if going to this extent.

 

[diesel_lv]

Is this electronic? If so I would use this over any mechanical device if going to this extent.

It is internal computer controlled. Have it on my 496 stroker. Works great. It's pricey, but can be infinitely tuned via Bluetooth on the go. Go to their website. Progression Ignition

 

[biomedtechguy]

And Don didn’t tell HOW or WHY what he did helped either, most here don’t have a clue to those two things.

The way I read the following linked "blurb", I believe that it not only describes the difference between manifold vacuum and ported vacuum, but also the effects and functionality of the 2 types of vacuum source, and definitely reflects the "Don Factor" in how he "expresses his position" on any particular topic..
The way it works, and greatly improved the idle and performance (on my setup, but I think Don deals with a LOT of similar situations) is the TOTAL advance limiter that is part of the FBO system, that prevents excessive advance, more timing advance than "usual" at idle is available and implemented (when my 6bbl and ignition, and many other customers who have found great results had previously smelled like a refinery fuel rupture incident) and the greater fuel burning efficiency of a vacuum advance capable setup.
Here's what Don wrote:
Ported vacuum source, Constant manifold vacuum source explained, the difference between ported and manifold vacuum.

 

[biomedtechguy]

Progression Ignition does, but it uses it for a different reason. To tell at idle if you are in P/N or D/R so it can bump up the timing so it doesn't stall. It uses the Ignition map based on rpm n MAP to determine advance.

I believe that the vacuum signal is used for more than just that, but I don't have time to go into my thoughts that deep right now.
I will say that scenario you mention and resulting RPM "bump" from the timing bump, and the benefits to those who have automatic transmissions is going to be the same benefit I get with my manual transmission setup, but in my case, it's to compensate for the AC compressor coming on and off!

 

[Dragon Slayer]

That link has a lot of BS in it also in my humble opinion. Chrysler had engineers too and the car was a system. The motor, carb tuning, ignition. Ported was NOT an emission thing with chrysler. It was used before and during emissions. Don's wall pressure comments of equal opposite reaction is not engineering wrt to vacuum/pressure relationship. You have high vacuum under the throttle blade and as the blade opens and exposes the port that is where the air is flowing into the intake and how the vacuum in the intake is exposed to the port. The pressure gradiant moves up and down in the venturi or bore of the carb. I have run 2 vacuum gauges on the motor; one on carb and one on manifold and watched as motor run through rpm level. Might be a small difference between them but they follow each other once the port is exposed.

In one case MV, you have vacuum adv on and starting at max and it rolls off as RPM comes in and fully off at WOT. So vacuum moves off and mechanical timing comes in. So plot that curve. In the other, you have Vac adv off at idle and vacuum advance and mechanical roll on as RPM rises until vacuum advance peaks and starts coming off as you approach WOT. Plot that curve. They are different. Which one your motor likes depends on a lot of things. If it is stock chrysler gave you the setup. Problem is most cars are not. Different cam, carb, and even many distributor parts not the same. Especially the vacuum can.

Don knows what the vacuum can is doing and more importantly what his computer controlled box is doing. Does he show you the actual timing curve it will produce? Or just give you the total timing at max rpm.

 

[RemCharger]

Chevies used the shitty absolute vacuum deal so their shitty starters would work.
Oops might be a bit blunt for some.

 

[Geoff 2]

Ummm....Chebbys using MVA has nothing to do with their shitty starters. Starter operation is the same with MVA & PVA.

 

[hunt2elk]

I quit talking to Don years ago because he is so bullheaded. I do like his control boxes, but just buy them online now so I don't have to deal with him personally. Long story short, some motors run best on ported, and some run best on manifold vac. Just takes a little experimenting time to figure out which is best for your car....

 

[Dragon Slayer]

I think his sarcastic/humorous to some comment is the chevy starts can't handle high initial timing so they use the vacuum adv to get higher initial, after the car starts. By using MV.

 

[Rusty Wrench]

Put down the crack pipe and leave it on ported vacuum.

why on earth would you want to retard timing as you foot goes to the floor? Because It will stop all the pinging because your engine has too much compression.

The only time ported and manifold vacuum are different is at idle. So how is manifold vacuum going to retard the timing and ported is not?

 

[R413]

subject car has manifold vacuum to the vwc advance. I have 2 questions.

Q1) What does manifold vacuum do at idle?

A1) It is high so you set you idle timing with the vac advance pulling and advancing the timing, AT IDLE.

Q2) what does manifold vacuum do when you put your foot on the floor?

A2) the manifold vacuum goes to near zero which will retard the ignition timing From where you were.

if you have vac advance on ported vacuum it is zero at idle so at part throttle it is now advancing the timing.