Technical Ported vs manifold vacuum?

Hello all yesterday I switched my street avenger holley from ported to a full manifold vacuum source on the intake. When I plugged in the manifold vacuum my idle was increased and I was able to turn my curb idle down 2 full turns its very responsive above 1000 rpm a little lug off idle but it seems my curve moved up the rpm range a little. Does anyone know if I need to tweek with the advance springs or weights or anyone opposed to using manifold instead of ported vacuum? Pros cons? Thanks guys!

 

Plenty of information here.

http://www.hotrod.com/articles/ever...now-about-vacuum-advance-and-ignition-timing/

 

This subject has been debated here many times, do a search for lot's of info (& opinions).

 

For the best power, the ignition timing should be in as quickly as possible, both the mechanical and the vacuum.
You also need to understand the way the ports work.

Full vacuum - The vacuum is at full strength at idle. It next best at "flat" land (unloaded) cruising. As the pedal is pressed (increasing speed), the full vacuum drops. as the load (going up ANY amount of incline) increases, the vacuum drops.
As you know, all these "drops", lowers the timing, dropping power.

Ported vacuum - The vacuum is at it's lowest at idle. As the pedal is pushed, the vacuum increases.
All vacuum movements are exactly the opposite from being full vacuum. So in most cases, the vacuum or ignition timing is higher, when the engine "needs" the power to satisfy the signal from the gas pedal (driver !).

Me, I run quick mechanical advance and no vacuum. I also increase the "initial" advance to help make up for the lack of any vacuum advance. I do this for a couple of reasons...one being, vacuum advance canisters fail quite regularly, and it can be many...years before you even think to check its function.

Remember, overall power is in the ignition timing, much more-so than fuel. Fuel depends on altitude, temperature, moisture in the air. Ignition timing is solely its own function, not dependent on outside influences.

Mike

 

Thanks for all the info and I will do a search on here about the subject. Also I had over 25 hg at idle at the manifold port that's the highest I've seen on any car to date it could be higher but my gauge only goes to 25 and I have zero vacuum on the ported port how can I check the function of my vacuum advance can just for future knowledge? My main motive for using full vacuum was because its supposed to run cooler?

 

The holley instructions say to plug into the timed spark port from what I understand this is a ported source?

 

This was a great read!

 

No, there should be zero vacuum at idle. That's the idea. As the "pedal is pushed", the vacuum decreases, same as always, same as manifold vacuum. It simply can't be any other way. As soon as the load levels off the vacuum comes back up, same as a manifold connection. This happens within a split second.

It helps to understand some basics before people can agree on anything. There's really no difference in vacuum signal other than at idle You can see this for yourself.

The way you do that, is connect two gauges up, one to a ported carburetor source of vacuum, and the other to a manifold source of vacuum. They will read the same going down the highway, they will read the same when accelerating, they will read the same when decelerating. When engine load increases, vacuum goes away momentarily, every time, regardless of where it is connected.

The Hot Rod article was pretty good except for this bit of inexplicable, complete nonsense:

All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.

?? Not true! Anyway..

The only difference in the advance you're going to see connected to manifold is at idle. Now - This WILL normally cause people to become VERY concerned because all they see with their $150 timing light is something like 32° BTDC on the damper and not quite understanding this ignition advance business too well to begin with the mechanic thinks that is "too much". So they back way off on the initial, mechanical advance. Oops! Normally a V8 likes 34° BTDC or even more mechanical advance timing at the far end. And this is the crux of the matter. The most important ignition timing number is where it ends up at, not where it starts out at. This vacuum advance business just confuses the novice.

Normally a manifold connection with lots of advance at idle, it will run quite a bit cooler. If you can run manifold vacuum it's certainly worth a try. It is also true that manufacturers increasingly used retarded timing strategies at idle to keep NOX emissions down. Most experts suggest a stock engine run a ported connection. A hotter street driven engine should try a manifold connection. In all cases experts agree that a street driven engine should run vacuum advance.

The big mistake people make is buggering up their mechanical timing curve because they are unclear about vacuum advance. Tuning is always performed with the vacuum advance disconnected and plugged. When someone says something like "36° BTDC total timing before 2800 RPM" this always means without vacuum advance connected.

Hot Rod article did correctly note that a V8 will happily cruise down the highway with 50° BTDC of ignition timing on level ground in high gear (low RPM) Vacuum advance is always just an "add on" top of the standard mechanical advance curve for temporary extra ignition advance or at least a very specific set of circumstances - low load, part throttle (lean) conditions. Level steady cruise mainly. The mechanical ignition curve is RPM dependent only. Vacuum advance is LOAD dependent only. They are completely independent systems, and complement each other.

As a practical matter it can sometimes be difficult to achieve a smooth steady idle using a constant source of manifold vacuum. Slight variations in engine vacuum in the manifold can setup slight variations in ignition engine timing, which start a kind of negative feedback. Surprise! This is why "ported" vacuum sources were invented in the first place, going back to at least the 1940s, long before engine emissions were ever thought of.

The best advice is "try it" and see what you think.

 

Many ways to tune a fish, and skin a cat...............Always in the camp that works best for you.....
Trial and hopefully not error...... Just because shoes say "size ten" and that's your size, doesn't mean they'll fit your feet comfortably.......

 

We fought an overheating in traffic problem for years. 350 SBC with 6-71 blower in a street rod Tried everything, new rad larger core, extra rad on the heater hoses, coolant fans, high volume pump, different pulley sizes, you name it he did it.
All that time was running a Mallory unilite with no vacuum advance. Take a guess what solved it. Yep Mallory sent out for free a unilite with 12* vacuum advance and connected to full vacuum. Many thanks to that link above. Plus it idles smoother, and fuel economy is of course better.