can you school me about vacuum dist advance ?

[Paul G]

http://www.gofastforless.com/ignition/advance.htm

According to this article, to find the correct total timing you can use your MPH at the end of the quarter mile. He says to increase your timing until your trap speed drops off. Then go back to the point you achieved your highest MPH. That is good information.

But that's only WOT timing, which has little to do with daily street driving. (not saying it isn't good info)

I agree. The way I read it, WOT timing, total timing, is for maximum acceleration, maximum power potential, at high RPM. We all want that for that rare occasion we have to tromp the go peddle and run the engine to red line through all the gears.   Once we have the timing curve set up for the most power, we can then tune the vacuum advance system for the most economical street driving. I have discovered we can use both mechanical advance for max power, and vacuum advance for economy at low RPM and cruising. It is working out so far in my car. For lack of a better definition, mine is a high performance street car, not a race car.

After installing my overdrive trans I had to re adjust my timing curve. Not the base timing or the total timing, those stayed the same. I adjusted the rate at which the timing increased, the curve. It is that chart Nacho has a link to. Once overdrive came in, at a slow speed steady cruise, my RPM dropped just as engine load increased. The timing curve was wrong for that, had terrible detonation at and below 2000RPM in overdrive. Had to use a heavier "light" spring to hold out some timing below 2000 RPM. Base timing and total timing stayed the same. I can only assume that below 2000 RPM I lost a little power with the heavier spring.

If I have learned nothing else, I have learned that no matter how much you think you know, you don't know chit.

[flyinlow]

On a stock MOPAR it works when the throttle plates are open above idle and there is still enough vacuum in the intake to operate the advance unit. Above 10-15 inches of vacuum. Light acceleration,cruise, not at idle,WOT or decellerating.

Some  GM cars in the '60s ran manifold vacuum to the distributor. Mopar always ran ported vacuum, except a few years that would run manifold vacuum thru a temperature controlled switch on A/C  cars to speed up the idle when they got hot.

I tried manifold vacuum advance back in the 70's on relatively stock 440 Magnums and noticed a slightly rougher idle. Not sure why, but with an exhaust analyzer hooked up the unburnt hydrocarbon level would go up alot with the engine set up this way. You would have to back the idle screw off to set the idle speed back to normal after making the change to manifold vacuum. This might have caused issues with the carb.    When running this way I did notice pinging on rapid throttle opening for a fraction of a second until the vacuum advance unit returned to unadvanced position as the manifold vacuum dropped to below the level at which it would work.

I use ported vacuum on both street driven Chargers now. Set up the mechanical side first, then add what ever vacuum advance you can add without detonation rearing its ugly head. Both engines make enough vacuum for the distributor and power brakes to work. I limit the vacuum advance to about 8* of crankshaft advance.

So Nacho, thats all the schooling on vacuum advance I can offer.

[AKcharger]

OK, new direction on vacuum advance (V/A)...what happens when it goes bad?

Here's my story; just finished a 1600 mile trip in my '70 and car ran fine till we hit the Fl border then it had an occasional "miss" at highway speeds. It progressed to a constant miss and rapidly becoming almost undrivable.  Well I thought "ECU!" so I pulled over and swapped out the ECU (I carry 2 spares) and it did the same thing. I drove for a few more miles and thought "Runs OK at idle, but misses at Higher RPM...Vacuum advance?" so pulled over again and pulled and capped the V/A hose and it ran like a champ!

I didn't do any other trouble shooting but where should I look for the problem?

[Nacho-RT74]

Damaged dist vacuum due broken diaphragm will get you on just a vacuum leak problem.

[AKcharger]

Was thinking that. Wierd how rebuilt and "new" parts last only a fraction of what factory original parts last. I mean that distributor is 10 years old and replaced a perfectly good 30 year old factory dist (points) that is so frustrating!

[flyinlow]

OK, new direction on vacuum advance (V/A)...what happens when it goes bad?

Here's my story; just finished a 1600 mile trip in my '70 and car ran fine till we hit the Fl border then it had an occasional "miss" at highway speeds. It progressed to a constant miss and rapidly becoming almost undrivable.  Well I thought "ECU!" so I pulled over and swapped out the ECU (I carry 2 spares) and it did the same thing. I drove for a few more miles and thought "Runs OK at idle, but misses at Higher RPM...Vacuum advance?" so pulled over again and pulled and capped the V/A hose and it ran like a champ!

I didn't do any other trouble shooting but where should I look for the problem?

Most likely thing to go bad on a vacuum unit is the diaphragm. With this condition there is no vacuum advance. This causes a slight decrease in the part throttle power output and fuel millage on a stock engine. Also a small vacuum leak above idle. The spring could break or linkage bind and cause it to stay in the advanced position. In this condition ,pulling the vacuum line off would probably not effect the engine however.

Pull the cap off and make sure the vacuum advance plate is in it's base position (not advanced) Recheck your timing. Has the distributor moved? Check the timing change with vacuum applied to the advance unit. 

[AKcharger]

Thanks flyin

- pulling off V/A hose DEFIANTLY made it run better
- only thing that may have contributed was I'd pull the rotor every night when we stayed at a hotel for theft prevention...maybe I hit something?
- will check out timing and other items when down there next

Thx