Ported vs manifold vacuum advance solved!!!

[diesel_lv]

I love the love/hate!!! The purpose of this was to show that the "engineers" from both Edelbrock n Holley believe in both, depending on application. There is no "Only" one way. But if anyone says "only ported" they are incorrect, every application is different.

 

[diesel_lv]

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 full mechanical advance is usually set up just below detonation, typically 32-34°. If you stomp the throttle at highway cruise, where you are at full mechanical plus vacuum, sometimes as high as 50°, the engine will detonate if you do not pull the vacuum advance out immediately.

 

[Turboomni]

https://www.moparaction.com/2020/08/16/advance-to-go/

 

[65-440]

This is always a funny debate. Unfortunately many just listen to the guy on the other end of the phone or believe what they read on the internet. Very few of us actually test and tune until our desired result is achieved....I've been through this with both of my cars, one 4 spd and one auto...BOTH run better on ported vacuum. Much cleaner off idle response.
Drive the car with a vacuum gauge connected to one source and log the readings, switch ports and do the same drive. It's good info to know. I think both my cans come in at 10"....
Full manifold may give a "cleaner" idle, but I tune for actual driving/ load not idle !

 

[Nate S]

I agree with Edelbock. A proper manifold vacuum advance system will improve idle quality, help your engine run cooler, if it’s of any importance to you - improve gas mileage and doesn’t affect street performance in the least. Ported vacuum advance was a aberration of frustrated emissions engineers staggering around in a dark world.

Ported advance existed decades before emissions.

 

[Derwud]

So here is my 2 cents.. Manifold vacuum if you over cammed or used old school tight LSA cam in your Street car.. So with it hooked to Manifold vacuum, you can run more Idle timing (because you are using manifold vacuum) and close the throttle blades, allowing more idle vacuum.. Tune that in..

For a proper built Street Engine Ported vacuum is the way to go... Sorry, no longer a fan of the Street Hemi Grind Purple Cam.. Not after installing a Stupidly HUGE Hyd Roller with a wide LSA and having 12-15 inches of vacuum..

 

[JimKueneman]

This is always a funny debate. Unfortunately many just listen to the guy on the other end of the phone or believe what they read on the internet. Very few of us actually test and tune until our desired result is achieved....I've been through this with both of my cars, one 4 spd and one auto...BOTH run better on ported vacuum. Much cleaner off idle response.
Drive the car with a vacuum gauge connected to one source and log the readings, switch ports and do the same drive. It's good info to know. I think both my cans come in at 10"....
Full manifold may give a "cleaner" idle, but I tune for actual driving/ load not idle !

I did notice that the 318 had a stumble with manifold port and not with the ported. My /6 I don't have any ported nipples plumbed so I ran off the manifold. It actually comes off idle better than the 318 with ported. I have 12/32/52 degrees on the 318 and 12/30/50 degrees on the /6.

I tend to agree with @diesel_lv either can be made to work well with the right tune.

 

[turbine68rt]

https://www.moparaction.com/2020/08/16/advance-to-go/

I found myself having to post this link in the past more times than I care to remember. Great article.

 

[#41]

I found myself having to post this link in the past more times than I care to remember. Great article.

Except that Ebooger is writing about ported vac advance vs no vac advance at all. Not really relevant to this debate.

 

[turbine68rt]

Except that Ebooger is writing about ported vac advance vs no vac advance at all. Not really relevant to this debate.

He does in fact advise against using manifold vac because of the potential for erratic idle and surging.

 

[hunt2elk]

So every always wants to get into the debate of ported vs manifold vacuum advance. I talked at length w the owner of 4 Seconds Flat. He made total sense. The ported is for cars that had some sort of computer control/smog control. Manifold is for those that do not. It is a lot to go into, but actually very simple. But for further proof, here is the definitive from a much respected source. Edelbrock!!! View attachment 1069631

 

[#41]

He does in fact advise against using manifold vac because of the potential for erratic idle and surging.

Ya I did miss that one non-committal sentence.

 

[dadsbee]

 

[Meep-Meep]

Ported vacuum for advance is absolutely brilliant because it functions somewhat like a mechanical computer. The signal is throttle position and load dependent. In short it adds timing under the right conditions and has no negative effect if your mechanical curve and initial are properly set. All it does is add timing during light loads and low throttle position. Once you mash it the signal goes away and the mech advance defaults to the timing at whatever RPM the engine is at that time.

 

[vintage chromoly]

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.

You’d want to retard it, because with vacuum advance, cruising, you’d have around 50 degrees of timing.
When you mash the gas, the vacuum drops, cylinder pressure increases and you want closer to 35 to 38 degrees on a BBM.

 

[diesel_lv]

View attachment 1069774

Hey, us retired guys gotta do something during the day.

 

[Meep-Meep]

You’d want to retard it, because with vacuum advance, cruising, you’d have around 50 degrees of timing.
When you mash the gas, the vacuum drops, cylinder pressure increases and you want closer to 35 to 38 degrees on a BBM.

This is exactly what you want - about 45-50 deg, or as much as you can tolerate. The key is to have it back off to 38 or so as you roll on the throttle. This is the beauty of a ported spark port. It takes care of itself.

 

[383man]

When you stomp on the accelerator the accelerator pump dumps significant fuel into the engine. So it goes from a lean to a rich setting. Lean mixtures burn slow (need more timing), rich mixtures burn fast (need less timing).

Actually at part throttle the eng is leaner then at WOT and thats one reason vacuum advance is used for part throttle. When the mixture is on the lean side at a light part throttle cruise its also not compressed as tight as WOT so it burns slower and the eng likes more timing for better economy and fuel milage as the normal V8 can take up to about 50 degrees timing at light part throttle. Course at WOT when the eng is richer and the mixture is compressed tighter the eng cant take 50 degrees of timing as it only needs about 35 to 40 on iron headed V8's. So actually vacuum advance is for part throttle cruising as thats where it does what its made to do. Vacuum advance was not put on engines to work at idle but it was to add timing at part throttle where the eng needs it. Thats why race dist have no vacuum advance because an eng dont need vacuum advance at WOT. I have been an auto tech since 1974 and I can say most of the stock older cars I worked with did not have any vacuum advance at idle, Some did but most I worked on did not. You can use it at idle if you set the eng and advance up to work that way but myself when I use it I like to have it come in just off idle. Now ported or full manifold vacuum will still work the same cruising at about 2500 rpm because ported vacuum is manifold vacuum as its just a port above the throttle blade. But both ported and manifold vacuum will be pretty high at 1500 to 2500 rpm at part throttle. My self I prefer to give the eng the idle timing it likes without the vacuum advance working and then I can set my mech advance for WOT and have my vacuum advance come in just off idle for more cruise timing. Ron

 

[niteshift70]

Ported means there is little to no vacuum signal at and just off idle. Its purpose is to maintain initial distributor timing at idle without any additional advance because it's not needed. Once rpm increases ported vacuum begins to advance the timing via the vacuum advance (the mechanical during wide open throttle). It also gives you more advance than mechanical when your at cruising rpm for fuel mileage. Manifold vacuum is normally high at idle which will add distributor advance at idle which you dont need.

 

[davy442]

Obviously, manifold vacuum is obtained from a port accessing the intake manifold vacuum signal, and subjecting the vacuum advance appropriately. As noted above, ported vacuum refers to a port that is located in the carburetor venturi just above the throttle plate, so at idle, the port is basically exposed/accessing atmospheric pressure. As you open up the throttle, the throttle plate edge moves upwards, passing the ported vacuum port, which now subjects the port to the manifold vacuum (pressure) which would be a lot lower than atmospheric, now creating a vacuum signal , and now advancing the timing. From my understanding, this was done as early/easy emission control, as delaying the timing at an idle delays the combustion process such that combustion is taking place a lot later, after TDC. Because of this, the "burn/expansion" is not as efficient, and the combustion gases and exhaust gases remain hotter to lower emissions/unburnt fuel. The increased temperature will also transfer more heat energy to the cooling system of the car, as both the temperature is greater, and you're burning more fuel to maintain the engine at that same idle speed. I'm guessing you'll have to open up the throttle plates to a greater degree now to maintain the same idle speed.