Willyrd
Well-Known Member
1 like, 1 agree, 1 disagree, 1 thanks, perfectly balanced as all things should
To Vacuum Advance or Not to Vacuum Advance
- Thread starter cudajames
- Start date
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You have got to be kidding. I disagree !
Willyrd
Well-Known Member
Come on let the kid dream
Not kidding, I do not use a vacuum advance on most of my classics. Pure stock, yes. I havn't found a benefit yet, doubt I ever do....
Willyrd
Well-Known Member
not even mpg? as futile as it isNot kidding, I do not use a vacuum advance on most of my classics. Pure stock, yes. I havn't found a benefit yet, doubt I ever do....
Not anything that I could figure with a calculator. Remember, from the factory a 318 might have been set at 2* btdc initial. When you touched the gas it would pull what, 22* with the VA? My 318 is currently at 20* initial.
512Stroker
We are all here because we are not all there.
Good info
The distributor I currently have was setup By Don at FBO specifically for my engine. I just started playing with it, yes it is adjusted using an allen wrench.
I am going to leave it as is for the summer, will take another stab at it this fall when time allows. The engine is very driveable at the current settings and pulls like a freight train when your on it. Last tank of fuel (91 Octane) averaged 13.8 mpg, mostly highway 3000rpm @ 70 mph
I dont know how much better I can make it without screwing it up LOL
yellow rose
Overnight Sensation
Not kidding, I do not use a vacuum advance on most of my classics. Pure stock, yes. I havn't found a benefit yet, doubt I ever do....
That's because you generally don't run much cam. As the cam gets bigger, the benefits of vacuum advance become bigger. Especially if you hook it to manifold vacuum and let it pull extra timing at idle.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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get you an A833/GVoverdrive,gear it for 65@1450,install a 224 or less SFT cam@185psi,, and double your fuel-mileage.
Never found a benefit...
Not going to disagree.
I never disagree when somone says that's what they found. Heck I wasn't there!
As long as they don't say that's what the rest of us will find.
To be exact we'll have to pick a year, transmission, carb as to what the factory did. Dodge tended spec more initial than Plymouth, at least in the 60s. Don't know why. Same distributor.
So looking at strictly stock here:
For '72, initial timing TDC, (-2.5 to 2.5)
'69 Dodge (CAP only) TDC
'69 Plymouth (CAP), 5 ATC
'69 Plymouth, 5 BTC
Where I'm going with this is open the throttle and timing advances quickly with rpm.
Non-CAP '68 Plymouth 318 Timing.
The stock 318 was relatively efficient even at low rpm, and so it didn't need a lot of lead time for the burns if you floored it from idle.
But most of the time we are looking at distributors for emissions equiped vehicles.
Take a '68 CAP equiped 318 with manual transmission as an extreme example. A super quick and relatively long primary advance gets the timing where it needs to be by the rpms where the engine is fully loaded.
On a more gentle throttle application, vacuum advance for a '68 318 starts as soon as the port is exposed.
So to use the example, if vacuum is at 10"Hg, could pull in as much as 11*.
If the engine is at 1000 rpm, timing should be around10*, for a total of 21* BTC.
As soon as we change compression, combustion chamber, cam, smog controls, or anything else that effects mixture, pressure, load, then timing conditions change.
Here's a comparison with factory 4 bbl 273 timing.
We can see the hi-po 273 neads more time at lower rpm. But around 1500 rpm, combustion efficiencies improve with rpm.
Not going to disagree.
I never disagree when somone says that's what they found. Heck I wasn't there!
As long as they don't say that's what the rest of us will find.
Pretty much; if gentle on the throttle.
To be exact we'll have to pick a year, transmission, carb as to what the factory did. Dodge tended spec more initial than Plymouth, at least in the 60s. Don't know why. Same distributor.
So looking at strictly stock here:
For '72, initial timing TDC, (-2.5 to 2.5)
'69 Dodge (CAP only) TDC
'69 Plymouth (CAP), 5 ATC
'69 Plymouth, 5 BTC
Where I'm going with this is open the throttle and timing advances quickly with rpm.
Non-CAP '68 Plymouth 318 Timing.
The stock 318 was relatively efficient even at low rpm, and so it didn't need a lot of lead time for the burns if you floored it from idle.
But most of the time we are looking at distributors for emissions equiped vehicles.
Take a '68 CAP equiped 318 with manual transmission as an extreme example. A super quick and relatively long primary advance gets the timing where it needs to be by the rpms where the engine is fully loaded.
On a more gentle throttle application, vacuum advance for a '68 318 starts as soon as the port is exposed.
So to use the example, if vacuum is at 10"Hg, could pull in as much as 11*.
If the engine is at 1000 rpm, timing should be around10*, for a total of 21* BTC.
As soon as we change compression, combustion chamber, cam, smog controls, or anything else that effects mixture, pressure, load, then timing conditions change.
Here's a comparison with factory 4 bbl 273 timing.
We can see the hi-po 273 neads more time at lower rpm. But around 1500 rpm, combustion efficiencies improve with rpm.
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Exactly.
Amen to the words I quoted. Well put!
Got me curious as to what Plymouth's vacuum advance was for that '67 273 Power pack.
Wow! Big difference compared to the '68 2bbl 318. It adds in more advance at lower vacuum.
We can also guess from this that it ran lean and mean at even 3/4 throttle;
with power enrichment on this engine probably at or below 6"Hg.
Wow! Big difference compared to the '68 2bbl 318. It adds in more advance at lower vacuum.
We can also guess from this that it ran lean and mean at even 3/4 throttle;
with power enrichment on this engine probably at or below 6"Hg.
Adding more timing at part throttle will reduce exhaust temp by a bunch and you'll improve fuel economy. You said you didn't care about gas mileage but I bet you care about excess fuel washing down your cylinder walls. You might also care about a 300 or 400 degree difference in exhaust gas temp.
Then again, "maybe he doesn't".
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Well; the headers will last longer if not approaching cherry red some/most of the time. Not to mention the underhood temps.And anything near the headers.......oh like, say the wires and such.
To the streeter that says the Vcan is not needed, any guy, no one in particular ;
The expanding exhaust gases need to transfer their energy to the crank to make locomotion possible, and there is a theoretical best time to do this, to maximize power output. If the pressure peak comes too late, the engine will not make it's optimum power. If the peak comes to early, the pressure gets stalled by the crank being semi-immoveable. When you start the fire,,in large part, determines when the pressure peak happens.
The mechanical advance system, from stall speed on, determines this peak pressure transfer point..... at WOT.
But honestly, how much time does your street engine spend at WOT, compared to Part-Throttle? Would you guess 3 or 4 seconds per hour? Or maybe you are a wild man, and spend 15 seconds per hour at WOT and 59:45 at some other throttle setting. Ok wait, you say you are really wild and spend 2 minutes per hour at WOT; which being around 30 x 4 second WOT runs to the speed limit in that hour, or once every 2 minutes.. That's really really wild.
Ok 2 minutes per hour maths out to 96.7% of the time is spent at not-WOT. So 96.7% of the time spent above idle, without a Vcan, your engine is operating with the wrong timing. No wonder the fuel-consumption is thru the roof. No wonder your oil needs changing every week. No wonder your rings wear out early. No wonder...... a lotta things.
There are times, at part throttle, when the street-engine is wanting timing perhaps in the 50* plus range,maybe pushing 60* and if you have only 20*, or even approaching 30* at that time, then the fire may not even be finished burning inside the chamber, and so finishes in the header............ which destroys the overlap cycle, and destroys the headers ability to do it's job, and of course makes a lovely red glow.
But ,you say, you don't need a Vcan. Hey; it's your money, you can burn it if you want to. Me; I can't afford that.
Of course if your car is a racecar, and you trailer it to the track, this does obviously not apply, cuz your engine operates mostly in two modes; idle and WOT,lol. Still, I can see times the engine could benefit from a Vcan, just used in a different way.
End rant.
skep419
No show, all go!
At WOT the v-can shouldn't be doing anything?
Correct.
And I don't think it is when foot braking - loading the converter for launch - but the times I was doing that I wasn't looking at the vacuum gage.
But when leaving the line from idle or close to idle, then maybe there's some vac advance for a fraction of a second. But that would be the same fraction when the engine isn't fully loaded. Never paid attention to the vacuum when leaving that way either. LOL.
Some of the guys who do more racing can chime in on this.
There is no vacuum when you first mash it .. that is why the carb has an accelerator pump. But vacuum starts to build after that.
If it helps for 1/2 sec in the 60 ft it could make a .10 difference in your 60 ft and overall times. Just pick ing a ramdom
If it helps for 1/2 sec in the 60 ft it could make a .10 difference in your 60 ft and overall times. Just pick ing a ramdom
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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In a hi-performance street engine;
the sparkport is far enough up the side of the throttle bore that in neutral, with the transfer slot synced, it will take something like 1500-1700 rpm to to put vacuum to it... in neutral, with no load; and of course the secondaries are closed. Of course as soon as you put it into gear, this changes.
If you continue reving it up still in neutral, the vacuum will plateau at some number that is highly cam-specific, somewhere between say 1800 and 2200, and I'll guess in the range of 20-22 inches. After this plateau, the vacuum will begin a slow downwards march with rpm/ throttle opening. Load changes everything. Idle-timing changes the vacuum onset.
The tuners job is to match the Vcan's operation to the application, in; the amount of the timing assist, the onset point, and the rate of advance.
Say you had a 360 cuber with a 220*@.050 cam. and the compression maxed for your combo. This will make a stinking torque monster.
Say it's an automatic with a 2200 TC . And say your T-port sync landed you at 14* of idle-timing, with a two stage timing curve that starts advancing at 800 and stage one stops at 28*@2800rpm. So the mechanical advance is 28 less 14= 14*, and it takes 2800 less 800=2000rpm for it to all come in. Ok, that maths out to 14/ (2000 divided by 100)= .7degree per 100 rpm. Now you can calculate the advance anywhere between idle and 2800rpm.
Ok So, at 2200 you will have 24* mechanical timing. Since your brakes will not hold the car at WOT, say you opened the throttle to just before break-away and the vacuum jumps to 22inches, and 22 appears at the spark-port..... and your Vcan is set to bring in 22 degrees more timing at 22" of vacuum, so you add those 22, to the 24 mechanical... for a total of 46*, see how that works? Badaboom, you have a torque monster. Now; as soon as the tires break traction, you floor it and the vacuum decays , and the advance drops out, and so as the engine climbs up the torque curve, it is dropping timing to keep itself out of detonation.
But say the 46* gets you into detonation with your iron heads. Well you just get a different Vcan that brings in only 9*say, and your total is now 24+9=33*, and so on it goes. If you think you can use more, you mod the Vcan, by cutting the stops, to get it....Most Vcans I have seen,can be modded up to 22 even 24 degrees.
To match that with mechanical only, you would have to set the mechanical plus base(idle) timing to 33* at 2200, and see how your iron-headed, 220-cammed 360, likes all the timing it takes below 2200 to achieve the 33* at 2200.
To figure out exactly how much timing your combo wants at stall, first you need to figure out what signal you are getting at the sparkport, and then you need a dash-mounted, dial-back, timing delay box, to start playing with it. Sooner or later, if you start logging data, you will get a picture of what she likes. Then you go make it happen,as best as you can.
Or you can just keep throwing stuff at her to see what sticks, but I can tell you that pulling out the D three times in 5 miles is a PITA.
The above is a theoretical situation. Yeah I ran that setup once but with a clutch and aluminum heads,lol and 185psi cylinder pressure.
Now; with 3.55s and 27" tires; 65=2870 rpm, and the timing is 28 mechanical (with 14idle), and the Vcan is maxed out at 22* so the engine is seeing 50* for cruising; this may be too much, but probably still not enough,. But if you add 22* to 36*, (if you had curved your D to all in by 2800), then 58* might be too much;
only the dial-back timing device will sort it out for you.
Or endless trial and error.
Happy HotRodding
the sparkport is far enough up the side of the throttle bore that in neutral, with the transfer slot synced, it will take something like 1500-1700 rpm to to put vacuum to it... in neutral, with no load; and of course the secondaries are closed. Of course as soon as you put it into gear, this changes.
If you continue reving it up still in neutral, the vacuum will plateau at some number that is highly cam-specific, somewhere between say 1800 and 2200, and I'll guess in the range of 20-22 inches. After this plateau, the vacuum will begin a slow downwards march with rpm/ throttle opening. Load changes everything. Idle-timing changes the vacuum onset.
The tuners job is to match the Vcan's operation to the application, in; the amount of the timing assist, the onset point, and the rate of advance.
Say you had a 360 cuber with a 220*@.050 cam. and the compression maxed for your combo. This will make a stinking torque monster.
Say it's an automatic with a 2200 TC . And say your T-port sync landed you at 14* of idle-timing, with a two stage timing curve that starts advancing at 800 and stage one stops at 28*@2800rpm. So the mechanical advance is 28 less 14= 14*, and it takes 2800 less 800=2000rpm for it to all come in. Ok, that maths out to 14/ (2000 divided by 100)= .7degree per 100 rpm. Now you can calculate the advance anywhere between idle and 2800rpm.
Ok So, at 2200 you will have 24* mechanical timing. Since your brakes will not hold the car at WOT, say you opened the throttle to just before break-away and the vacuum jumps to 22inches, and 22 appears at the spark-port..... and your Vcan is set to bring in 22 degrees more timing at 22" of vacuum, so you add those 22, to the 24 mechanical... for a total of 46*, see how that works? Badaboom, you have a torque monster. Now; as soon as the tires break traction, you floor it and the vacuum decays , and the advance drops out, and so as the engine climbs up the torque curve, it is dropping timing to keep itself out of detonation.
But say the 46* gets you into detonation with your iron heads. Well you just get a different Vcan that brings in only 9*say, and your total is now 24+9=33*, and so on it goes. If you think you can use more, you mod the Vcan, by cutting the stops, to get it....Most Vcans I have seen,can be modded up to 22 even 24 degrees.
To match that with mechanical only, you would have to set the mechanical plus base(idle) timing to 33* at 2200, and see how your iron-headed, 220-cammed 360, likes all the timing it takes below 2200 to achieve the 33* at 2200.
To figure out exactly how much timing your combo wants at stall, first you need to figure out what signal you are getting at the sparkport, and then you need a dash-mounted, dial-back, timing delay box, to start playing with it. Sooner or later, if you start logging data, you will get a picture of what she likes. Then you go make it happen,as best as you can.
Or you can just keep throwing stuff at her to see what sticks, but I can tell you that pulling out the D three times in 5 miles is a PITA.
The above is a theoretical situation. Yeah I ran that setup once but with a clutch and aluminum heads,lol and 185psi cylinder pressure.
Now; with 3.55s and 27" tires; 65=2870 rpm, and the timing is 28 mechanical (with 14idle), and the Vcan is maxed out at 22* so the engine is seeing 50* for cruising; this may be too much, but probably still not enough,. But if you add 22* to 36*, (if you had curved your D to all in by 2800), then 58* might be too much;
only the dial-back timing device will sort it out for you.
Or endless trial and error.
Happy HotRodding
But at WOT its way below most vacuum advances.
I don't have any data logs with a drag race start.
But it really doesn't matter. Manifold vacuum increases only a little with rpm.
(The logger is reading MAP and I've converted it vacuum. Its about an 1"Hg different than I see on the dash gage (mechanical).)
Here's a short WOT onto an interstate from a rolling start.
Dark blue is vacuum. Orange is acceleration, Green is rpm.
Next is one to give some idea of what happens when running a restricteed carb.
This is on a dyno, the dashed lines show a run with just two barrels of "750" on a 340.
Rolling into WOT in 3rd gear (torqueflite).
First marker: 3970 rpm, 2.2" Hg
Second Marker: 5740 rpm, 3.5" Hg
Pretty unusual for a vacuum advance to add anything down around 3"Hg.
The one on my FSJ does. AMC 360, smogged, with a 2 bbl (Motorcraft). Believe me - I've never spun it to 5500 rpm!
At the top, I think I'd feel Ok with glancing at it.
Its at the starting line I know I wouldn't.
If I had a safe place to launch it like at the track, with lots of room, it would be interesting to watch the mechanical gage in the dash.
I think the MAP response is slow. Maybe because its electronic device, maybe because mine is on a long small plastic line through the firewall to the logger.
I agree about vac dropping. It seems to me vac drops pretty immediately with throttle opening, but maybe that's just my perceptions.
Its at the starting line I know I wouldn't.
If I had a safe place to launch it like at the track, with lots of room, it would be interesting to watch the mechanical gage in the dash.
I think the MAP response is slow. Maybe because its electronic device, maybe because mine is on a long small plastic line through the firewall to the logger.
I agree about vac dropping. It seems to me vac drops pretty immediately with throttle opening, but maybe that's just my perceptions.
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