I've been meaning to try manifold vacuum advance with my Y block. Somebody at H.A.M.B. has already compared ported and manifold vacuum readings at highway cruise. Not surprisingly they are the same.

Maybe plagiarism is a bit harsh of a term, this isn't a writing class or Ph.D. Program. Still, copy and paste without source or attribution can lead to some issues, the OP didn't claim the words were his, but no comment otherwise, either.

Unless you're a drag racer you want vacuum advance. Maybe more people would understand it if it was called vacuum retardation? It makes for better fuel economy and better off idle part throttle acceleration and all around driveability. The thing to remember is that vacuum advance is completely unrelated to mechanical advance. The former is load dependent, the latter on engine RPM. They complement each other. Very ingenious system, primitive by today's standards but worked well for decades.

 

Actually, "vacuum advance" is the best term - for two reasons. First, because it does actually advance the timing with vacuum. Second, because there were/are cans that actually retard the timing with vacuum. However, from what I've seen they are always dual-ported cans, meaning that they advance the timing with vacuum on one port and retard the timing with vacuum on another port. But they are serious smog-era cans, and the vacuum system on engines that used them was usually pretty complex, with temp-controlled valves, delay valves, check valves, etc.

As for the adjustable cans, the adjustment screw, which is accessed via the vacuum nipple, puts pre-load on a spring. The vacuum overcomes the spring to move the arm and thereby advance the timing. So the adjustment doesn't truly change the amount of advance you get, just at what level of vacuum you get it. But, that can make quite a difference in operating results. We recently tuned the vacuum advance on a 300 six and found that before adjusting it that the arm started moving at 12" of vacuum and the advance was fully in by 16". So we turned the screw 2 turns and found that the arm started moving at 10" of vacuum and was fully in at 14". In other words, each turn shifted the advance curve downward by 1" of vacuum.

We tuned the advance by continuing to turn the screw and then driving the truck until we got to a point where giving the engine more throttle at low RPM in a high gear caused pinging. That meant we had gone too far and had too much timing. So we backed it off two turns, verified that it didn't ping, and called it good. In doing so we were able to back off the initial lead to make starting easier, but still have all the timing the engine would stand w/o pinging, which maximized economy.

 

Well, OK - maybe it really should be called vacuum advance, tho clearly it's often misunderstood badly.

And it sounds as if I misunderstood the mechanism of vacuum adjustment - "when" is adjustable "how much" not so much?

 

Yes. We didn't change the overall amount of advance the can gave, just when it gave it. And we wanted to get full advance when cruising, but have it such that much more throttle would decrease the advance to prevent pinging.

 

Are the factory diaphragms adjustable too?

 

Through removal or addition of shims (calibrated washers), yes.

 

Some are. Takes a really small Allen wrench

 

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[ originally posted by winginit
"Good read on this topic" /]

. As soon as I read the above I knew winginit didn't write the article, although should have included some guess or mention where it came from...


. I also wasn't buying cold, wet rich mixtures burning faster than lean mixtures...


. I was talking marine engines which usually doesn't even have a vacuum advance... because while cars may cruise at 25 HP and take good advantage of a vacuum advance... mid size V8 boats tend to cruise at 100 - 300 HP and so vacuum advance isn't advantageous and may even cause overheating/pinging at that continuous power output level...


. Engines often don't want to drop below 1200-1500 RPMs with a lot of idle ignition advance... some people don't seem to mind that fast of an idle... or have to for emissions testing reasons... they don't test here, so I go with low RPM lopey idle...


. With direct manifold vacuum advance at idle, as soon as you floor it, you loose vacuum/vacuum advance/ignition advance and the engine is sluggish/dying taking off...


. I build my street and marine engines with 10.5 - 13:1 compression ratio and usually use octane booster so that they can run like they did back in the 1960's on Blue Sunoco '260'... I hate the sound of a soggy 1970's 7:1 (called nominal "8.5:1") compression ratio engines... I like snappy engines... with a lot of throttle response... and power past 4000 RPMs...

 

Two thoughts:

From my MSD distributor instructions:

"Vacuum Advance: The vacuum advance will advance the timing up to 10° during partial throttle driving (with 15 lbs. of vacuum). The vacuum line should be routed to a ported vacuum outlet above the throttle plates."

Second thought:

The reason you connect to ported vacuum is so that you do not get any vacuum advance unless your accelerator is depressed (butterflies in carb are open). Why would you want vac advance if your foot is off the pedal? I don't think you do..I think you want the engine to idle at whatever initial timing you set...typically 10-15 degrees. My MSD can give me 10 degrees of vac advance if I hit 15 degrees of vacuum. So...if I used manifold vac...I'd go from 10 degrees initial timing at idle to 25 at idle...I've never tried it, but I wonder how the car would run?

 

I haven't tried it on my 64, but I bet it runs great. Nice and cool in hot weather, especially.

But more modern engines have vacuum operated power brakes.. air conditioning compressor, automatic transmission.. smog pumps, and golly knows what else, kicking on and off, and that would mean a varying engine idle.

 

That's the real problem - any variation in idle RPM changes the vacuum since there is more/less air going through the same restriction. Any change in vacuum changes the timing, which changes the RPM, which changes the vacuum, which…. It is a vicious circle that has always caused problems for me so I've moved to ported vacuum and am happy.

 

Is it possible that manifold vacuum connection has a "stronger" vacuum signal and so more advance than ported? Not "more" necessarily, though faster? I think my truck had a 2100 as stock carb for 1964, and probably only a ported vacuum connection anyway.

The engine is supposedly original to the truck, but it has no PCV system - just a road draft tube. Pretty sure that's from an earlier system anyway.

The big problem for many is ya can't just give the distributor a big ole twist clockwise and call it good. Engine timing is a package deal, mainly dialing in the mechanical weights and springs for a specific RPM range and selecting the desired advance slot on the breaker plate. Buying a curved dizzy is one way around this. They've done all this for you.

 

351Cleveland C4 is going to test that theory for us with dual vacuum gauges - one on ported and one on manifold vacuum. However, that test is really only good for that series of carb as different carbs have different spots from which they port the vacuum. But, I think we will find that the two vacuum sources normally are the same once you get much above idle.

Having said that, there are some oddball carbs out there that have "different" ported vacuum. I know of a Carter YF that gives something like 5" of vacuum at idle and only gets to about 10" of vacuum when cruising - when manifold is at 15+. Apparently there was some special need for that vacuum signal back around 1970 so Carter provided it. Had to hook that advance up to manifold vacuum as there wasn't a good ported source.

 

There can't be any difference in the total vacuum, at least not enough to measure with a gauge, or matter. That's well established.

Though maybe it's a more positive signal or "snappier" in the effect on the advance mechanism. I don't understand air pressure and velocity or that kind of thing very well.

Our GM engineer friend seems to think there's a difference worth utilizing though.

 

Whomever he is, I've seen a lot of people disagree with several of his statements. Enough so that I'm questioning his push for using only manifold vacuum. It doesn't fit with my experience, but perhaps that's due to using emissions-era vacuum advances? However, I've used aftermarket ones, like Crane's, and have gotten the same results.