Timing and vacuum advance 101

I would also like state that manifold vacuum to the distributor does not work on ALL vehicles and engine combinations as well as it does on other "in my opinion". I say this because I personally have a pretty substantial collection of cars and trucks, which includes constantly buying, selling and trading for different stock. When tuning...I have noticed some of them like ported vacuum better. While I am sure I could tune it to run on manifold vacuum...it's sort of a "why bother" kind of deal. If I were to give a percentage....I would say that around 80% of the cars I have owned benefit in drivability with straight manifold vacuum. I just wanted to put this out there in case you find your car in that 20%. Many...MANY will tell you that manifold vac is for ALL cars. But, I don't find that to be 100% accurate.

 

I just found another "Ignition timing 101" site. Pretty good stuff. I also found a pretty good discussion of vacuum advance. It's oriented towards SBC engines, but the theory applies to anything.

EDIT: Looks like this is another version of the original posting

Link to Ignition Timing 101

In depth vac advance discussion

 

I am still with you on this thread , When I was a teenager I had an old ford truck that I would take wheeling . I knew that applying vacuum to the advance canister would increase the rpm a little and I wanted just a little more throttle when idling over moderate stuff so I did some playing with pipe fittings and made a manifold vacuum source that I would hook the advance unit to when off road, At the time I did not know that I could have readjusted the idle and left it that way. All I knew was it seemed to crawl around better for me.

My 81 Cherokee Chief with a 360 AMC runs great with the advance unit hooked up to direct manifold vacuum, I have been running it that way for quite some time. I hooked up the F-Head that way just for giggles and it responded well so I plan on playing with it some more.

 

And my distributor is strictly mechanical advance, and seems to me to run very well that way.
To each his own.

 

Mechanical advance distributors runs just fine. If the advance comes in at the right time, most people will never notice the difference in driving feel between a mechanic advance distributor and mechanical.
This is another statement I do not like that the author of the article made. He called all mechanicle advance distributors "junk".
While it is true, a street car will benefit from vacuum advance over mechanical (especially during light load/cruise) a well set mechanicle distributor will work just fine and should not be referred to as "junk" by any means.
It really just comes down to trying to get closest to optimum ignition timing for give rpm and given load on the engine.
Today's computer controlled cars do that very thing. Vacuum advance was just a primitive way to try and reach optimum timing. Primitive or not, it worked pretty darn good. Mechanical advance Was/is just a bit more primitive and will not run as "optimum" as vacuum advance. Vacuum advance is primitive compared to computer controlled. So just as I would not refer to vacuum advance as being junk, I would not refer to mechanicle as being junk either. I mean...it works.
The Jeep in my avatar has no advance of any kind, I just lock it at 40 degrees and leave it, trust me...it runs "okay".
One thing about mechanicle advance only distributors is that if you do not have pretty low gears and are running an automatic trans, you will almost always fight a slight flat spot at lower rpm. You can get rid of the flat spot by cranking up the initial timing, but many times you will have to set it so high that it makes it hard to start. This is where a vacuum advance shines, because you can set your initial timing at a reasonable level and the vacuum advance will advance your timing to overcome that flat spot or stumble.
The vacuum advance will also run your engine very advanced when cruising with light load and your engine likes that. Can you feel a difference? Maybe a little. It will run cooler, it will get a little better MPG (don't expect a miracle though). I suppose the bottom line here is...if your happy with the way your Jeep runs and cannot imagine it getting any better and it's so good now that you would not even care if it was better, then don't mess with it.
If you're a borderline mental patient like me and are incapable of leaving well enough alone...then ignition timing is a good place to start.

 

:iagree: The straight mechanical advance distributors on the F-Heads are good units and run very well, Everything about the fuel and ignition systems on the early jeeps was biased towards getting as much torque from the little engine as possible. Economy was not the issue then that it is now so running as rich a mixture as possible with a mechanical advance was ( and still is ) a practical way to go on the little L and F Head engines.

If one is so inclined some custom carb tuning in the low & mid range ( running as lean as practical rather than as rich as possible ) combined with the addition of a vacuum advance ( to get the spark to the lean mixture sooner ) can yield a worthwhile economy and performance gain with a few driving habit adjustments.

Like mcgillacuddy I to am somewhat of a mental patient And find it a personal challenge to see just how efficient I can get something to run.

 

My question is with a Rochester 2G. There is no vacuum at idle. The port is clear all the way from where the distributor line connects below the front side of the float bowl to a horizontal slit below the butterfly.

 

I thought this was a thread worth bumping. I've been running my distributor on ported vacuum. It's supposed to run through the temperature controlled switch so it gets manifold vacuum when cold then ported vacuum when warmed up. Today I got a barbed 3/8 NPT fitting so I can tie into the manifold. I'm hoping it helps smooth out my idle. Even after a carb rebuild the idle isn't as smooth as I think it should be.

 

Just like our jeeps, this thread is older but still good!

 

I re-read that advance 101 article every few years.

"I used to know these things when I was young, but when I got civilized I forgot all these things."