Vacuum advance information and myths
>As to springs; I had collected about a dozen old distributors from the 60s>80s, and just robbed what I needed. Same with cams, and same with VA cans.
But
>as to Vcans, I soon found out 2 things. 1) that as pirated, none of them were compatible to run with the power-timing curves required to be used with 87E10 and 11.3 Scr, and 2) any/all of mine were suitable to be modified, up to about 22>24 degrees, which thus made them compatible.
> as to how this is done, I simply filed off the stops until the arm stopped pulling in any further. And I set the vacuum delay screw, inside the hose-nipple, to the minimum preload.
>as to ported or not, here's where it gets tricky.
For an automatic transmission equipped car;
Firstly; you have to know that if your Auto equipped engine has a high stall, then it almost doesn't matter whatever your timing is below stall,
and if your stall is 3500 or more, well then almost anything goes.
But
> if you have a manual-trans car, AND even a modest cam, AND are a streeter like me; you have a problem.
Secondly, you have to understand why you might need the extra advance at idle and or at low rpm. I challenge you to start your car and bring it up to operating temperature and check the timing.
> then idle it down to just before it stalls, then add fuel via the mixture screws until she gets happy. Then check the timing.
Next, grab your distributor, and arbitrarily pull in some advance. Then reduce the idle speed until just before it stalls, and add mixture screw fuel as may be required. Then check the timing.
Repeat as many times as necessary, until adding advance no longer causes the rpm to rise, or the mixture screws are maxed out.
Then idle it up to 750, and see if she'll maybe take a lil more timing. and check the timing one last time.
If at any time, adding timing causes the Rpm to drop, then back up! that was too much.
Now; compare the timing between the start of this test, to at the end.
What you got at the end, is the timing that the engine needs, to make max IDLE-power, in neutral/park.. Don't be surprised to read timing in excess of 25>30 degrees.
Now, if you mod your Vcan to pull in 22* like I did, and hook it to manifold vacuum, then you can only run about 3* of mechanical timing...... Which means that when you floor it, that vacuum Advance will drop out and yur stuck with turtle-acceleration due to not having enough power-timing. So in this case, I wouldn't want full-time Manifold-controlled VA.
but why in the world did I mod my Vcan for 22*?
Well, I have an overdrive manual trans. and I can gear the back to run 65mph down to 1600, and my, at-that-time 11.3/1 high-compression 367, will pull that gear no problem, but it does it best if I give her the timing that she needs and wants. What I discovered is that no matter how hard I tried, with factory distributor parts, I just could not give her that, and so, my hiway fuel-economy suffered terribly. My car was a DD and, MOST of the mileage on the odometer is accumulated while on the hiway; so I had to do something!
So I bought/installed a dash-mounted, dial-back, timing module that had a range of 15 degrees, and began experimenting.
What I found was that, running 65mph at 1600, at optimum timing, and optimum fueling, the 367 got very little better fuel-economy, than at 2200, similarly optimized. And, the starter-gear( ~11/1) while running 2200 was dynomite, whereas the one to run at 1600(~8/1) , was pretty lazy.
So it was a no-brainer to run the 65=2200.
But
in order to run there with optimized timing, she needed about 56* of cruise timing, which means 56 less 22=34 degrees of mechanical timing. But the 367 did not like 34* of power timing below 3000. So I had to make a compromise to save the engine.
With more experimentation, I found that the 367 had no problem and liked 28*@2800, and was fine with most of that, even at idle. So I built a timing curve to start at 14*@1000rpm, and end about 28*@2800. this is a range of 14*, at slope of 1.29 degrees per 100 rpm. So at 2200 this amounted to 15*. and adding the 14* initial, plus the 22* of Vcan timing it now totals 51*at 2200; just a little shy of the 56* optimum.
To get the 56* I just crank in the timing module.
So that is how I ended up with the 22* Vcan. ..... which is still 5* short, to run without the module.
>another thing that happens with a manual-trans running a lot of idle-timing on the manifold VA, is that the engine has just too much idle power to drive slowly, without slipping the clutch.
I found out that my 367, would idle down to 500, in gear, with an 11/1 starter gear, and would thus pull itself at 3.66 mph; that was slow enough for me. But, it required the Idle timing to be about 5*advanced, or less, which meant that I could not run the VA unless the base timing was 22 less 5=17* retarded. Any more than 5* advance, and she would start to get jumpy; and if I didn't clutch it, she would get very jumpy, and it did not matter what her idle AFR was.
So in my case, Manifold-controlled VA was impossible.
Automatics get jumpy too, but generally, the convertor sucks it up. and street-guys tend to throw a lot of low-speed timing at their engines anyway, to try and get some torque back, that was lost in the combo, due to it not having enough cylinder pressure caused by the combination of too low an Scr, and a late-closing intake valve. And so their engines idle sounding like crap; but that's what they like; and it ain't my money paying for their gas.
Now
>as to automatics and low-stalls. Pretty much everything I said for a manual-trans, also applies to these; and here's why.
Most Normally-Aspirated SBMs, no matter how they are built, are gonna accept about 28>32* of power-timing @2800. How you get there is a crap-shoot.
Suppose your Vcan is hooked to manifold vacuum, and the Vcan makes 14* . So then cruising at 2800 that would be 28 mechanical plus 14 in the can equals 42 degrees, which could be between, 14* and up to 18* short of optimum. For example, my engine liked close to 60* to cruise at 2800 with IIRC 3.55s.
So then, right off the line, with only 42*, the engine is running very retarded at cruising speed.
>Why is this important?
Well, when your peak cylinder pressure occurs at a certain place in the piston's travel from top to bottom, there will be exactly one place, where the crank will be in the optimum position to transfer that pressure peak.
Too soon and the pressure will drive the conrod onto the crank, hammering the crap outta the soft bearings; not to mention what happens if the engine gets into detonation.
Too late, and peak pressure never occurs, and almost worse, is that the heat in the chamber chases after the piston, ever lower down the cylinder wall, and instead of that heat pushing on your driveshaft, it heats up the cooling system. And the possibility exists that at after a certain rpm, the combustion gasses have not yet finished burning when the exhaust valve opens, and now that heat goes into the head-port and heats the cooling system from there. All of this is bad news for your engine and for power, and for fuel economy. So you never want to run too far retarded, and 14 to 18 degrees is extremely retarded, and that's gonna cost you fuel-economy big time..
Now then, still working on the low-stall automatic.
Suppose it stalls at 2000 rpm, and suppose the engine wants not more than 24*, but she also wants 28* at 2800 and 32* at 3200. So that is a nice straight slope of 4* per 400 rpm = 1* per 100. That means at @1000 rpm she will Get not want, 14 degrees, and at 800 it would get 12*.
So then, maybe you get the idea that 12>14 is not nearly enough, and you want to hook the 14*Vcan to the manifold, but not to exceed 24*@2000 rpm@WOT
So then, at 800 idle, the total Idle-timing will be 12initial plus 14 in the Vcan =26
Sounds good right? she idles great the Transfer slots are about right and to get the Idle-Afr on the money, you have leaned the mixture screws right out. Great.
Right up until you step on the gas, and the engine stumbles because of the lean mixture screws. So you richen that up and Good to go right?
But now, whenever you are cruising along at a low throttle opening and below stall, the engine is continuously rich, and you blame the lousy fuel economy on anything but the manifold-plumbed Vcan. And yur scared to install a higher stall because you may think that your fuel-economy will be worse.
Here's a secret, the Vcan on ported vacuum usually begins coming in around 1600 to 1800 in first gear, depending on the base timing, and the actual manifold vacuum. This is a function of where the throttles are, relative to the signal port, which is a function of how much power is being called for. So if you have a 4000 pound boat, 2.76 gears, and a 2000 stall, even modest acceleration will call the Vcan into play. But with a 3000 pound car, 3.91s, and a 2800 stall, a similar throttle opening may call for more VA, because the vacuum is staying higher. In other words, I cannot tell you exactly when your particular combo will begin pulling vacuum advance, so 1600 to 1800 is just a guess.. That's where mine began @3650 pounds with a starter gear of 10.97, and the throttle set to "barely accelerating."
The point is this, the Vcan on the Spark port only begins operating at very low throttle openings, after that, it operates pretty much the same as when directly plumbed to the intake.
BTW
I'm no expert on this.
But I am an expert when it comes to my combo cuz I tried just about everything you can imagine ..... on it.
