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I've got a pretty much stock 400 other than 2 1/4" exhaust on stock manifolds, a 4 barrel intake and Holley Sniper, and timing chain installed straight up rather than original. It runs about 18" of mercury vacuum at idle.
I just put a new vacuum advance can (O'reilly brand BWD) on the distributor since the old one was toast. I set the base timing to about 12* BTDC and then hooked the vacuum advance up to manifold vacuum (yes, I've read many debates on ported vs manifold) and when I put my timing light on it, I was surprised to see about 32* advance at idle! Adjusting the screw on the vacuum advance can didn't make any difference in the amount of advance at idle. However, I got it adjusted so it didn't ping under about half-throttle acceleration. The engine idles a little rougher with the advance can connected than without. My question is: Is 20* advance normal at idle for a vacuum advance can? It seems excessive to me.....
Yes, a vacuum can (can) pull in a whole bunch of advance. There's no engine load at idle, so the manifold vacuum is at its highest. So whatever the can is rated for, you get it. Now out on the highway or level cruise there won't be quite as much manifold vacuum. Maybe 14" or 15" or so? Depends on truck weight, gearing, engine condition etc.
Different applications had different cans, each having a slightly different total amount of vacuum advance or, the vacuum # inches where it starts to pull in. A lot of guys will put together a stop limiter on the arm to limit the total amount of vacuum advance. It seems like there's no adjustment for "how much" as there is "when". As a practical matter crank it to full advance, test drive it, and back off till it don't rattle no mo' on part throttle accelerations and cruise. A short momentary light rattle every now and then once in a while on deceleration is OK.
Under certain very specific conditions - level ground, high gear, the ignition timing spec for later V8 (non-flathead) may be 50° or 52° BTDC, something like that, maybe more. Depends on the compression ratio. The only way to do that, to get that, is with lots of vacuum advance. 36° Mechanical is only seen wide open throttle in low gear at high RPM, while at a steady cruise in high gear the ignition timing is severely retarded, so without the vacuum advance will tend to run very hot and inefficient.
What you want to avoid is buggering or retarding the initial or mechanical advance curve once it is set, to try and compensate for excessive vacuum advance. Most V8 should happily idle at 30° to 35° though.
As an experiment connect the distributor back up to the carburetor port. Then set the idle at 600 to 650 RPM or so where you like it. OEM spec is probably 500 or 550 or so. Anyway what you'll find if you put a timing light on it, anything above a fairly slow idle the vacuum advance starts to pull in anyway and is right in there at 30° or thereabouts. It starts to pull in early and quickly wherever it is connected to. I think this is why people like a faster idle in the first place. All that extra timing advance at idle smooths things out.
There's no load on the engine at idle, so ping or engine knock isn't a concern. If it runs good, leave it. Usually an unsteady idle is the problem. Slight changes in the idle RPM changes the engine vacuum slightly, starting a kind of feedback loop. That's why "ported" distributor connection was invented in the first place, to achieve a steady idle.
Thanks Tedster9!
Yes, overall the truck seems to perform just fine the way its set. I performed the adjust, drive, rattle, repeat as you described. It just idles a little rough at the 650 RPM where I have it set. It actually idles smoother with vacuum advance unhooked and plugged, though the coolant temp gets warmer as one might expect. But..... if you forget about it, and try driving it without any vacuum advance.. it will rattle like coffee can full of marbles!
I'll stick it back on the ported vacuum port just see if there's a big difference in drive-ability or performance.
Thanks again!
It actually idles smoother with vacuum advance unhooked and plugged ... But.... try driving it without any vacuum advance.. it will rattle like coffee can full of marbles! !
That's strange. It should run OK without vacuum advance connected.
Part of the issue may be that I haven't ever checked how much mechanical advance I have....... and....thinking back, I may have had the initial timing set at 15* on the run when I forgot to hook up the vacuum advance... my memory isn't as good as it once was.
I'll play with a few things Saturday when I get some time and see what I can make happen....depending on how much snow we get.
I'll try not to get into long division on this. First adjusting the vacuum can has no effect on how much advance it pulls in. It only determines when it comes in. All you are doing is changing the tension on the diaphragm. Ford cans come in all flavors and are interchangeable within the distributor type ie. Duraspark or points as long as it is for a V8. The cans are stamped on the arm with a number that number represents the maximum amount of advance that can is capable of producing. Pictured are 4, 10, an 12 degree cans, that is cam degrees, double that number for crank degrees.
Here is a page from a 71 shop manual for a non-smog 400 (D0OF-U). That would be a 12 degree can set at 3 turns CCW.
You need to check your mechanical advance using a dial back timing light. Without a distributor machine, "that covers pretty much all of us" tuning the distributor on the engine can be a long process.
I looked at my old invoice and found that the vacuum advance can that I had received was a Standard Motor Products part number VC221 which has a "10" stamped into the arm which would make sense that I would get 20 crank degrees with it.
From the looks/sounds of things, I may need a VC230 which looks like it has a "6" stamped into it which would be 12* at crank.
I'll have to check it out tomorrow while it snows...
Twenty degrees of vacuum advance is not too much, in fact that is a good place to be. If you are getting a rough idle when the vacuum advance is connected and it improves when you disconnect it you may have gotten a bad can that is creating a vacuum leak. I always set and check the operation of those things with a handheld vacuum pump with the engine off. If you don't have a vacuum pump O'Reilly's may have a loaner. The vacuum can should reach full travel at around 15-20 inches of vacuum and hold that without leaking down. Just pull the distributor cap off and you can watch everything move as you add vacuum.
Also the 32 degrees you have at idle is just about right. You may just need to readjust your idle mixture now that you have a working vacuum advance. I run my Cleveland with 16 initial and 20 degrees vacuum and it's happy with that.
If you are getting a rough idle when the vacuum advance is connected and it improves when you disconnect it you may have gotten a bad can that is creating a vacuum leak. I always set and check the operation of those things with a handheld vacuum pump with the engine off.
I thought about that at the time.... I went low-tech and just put a new piece of hose on it and sucked on it like a straw..... it would actuate, but I probably didn't wait around long enough to tell if it had a leak (vacuum hose doesn't taste that great) ;-) I'll try my vacuum pump with a gauge and see what happens.
Originally Posted by Crop Duster
You may just need to readjust your idle mixture now that you have a working vacuum advance. I run my Cleveland with 16 initial and 20 degrees vacuum and it's happy with that.
I'll play with the idle mixture when I get a chance then..... with the Sniper that's pretty simple to do.
When I went to the parts store to get the vacuum advance can, the kid behind the counter looks at me and goes: "A what?? I don't even know how to look for that!" and I was a little annoyed at first....... but then I realized, the kid was probably 18-20-ish and the major automakers haven't even built a vehicle with vacuum advance in over 30 years!.....Then I just felt dinosaur-ish.
I believe the adjustments on vacuum advance cans do limit advance. I could be wrong here but I've experimented with lots of advance curves. The allen adjustment limits the travel of the diaphram which limits the advance. Not all cans have the adjustment. I have modified several of them to reduce from 20 to 10
For a mild to medium 400 Ford This is what I have found to work well.
Get rid of ported vacuum, emmision controls (they don't really work anyway)
Use full manifold vacuum
Initial of about 10/14
Vacuum of about 10 ( this will yield 20/24 at idle)
Centrifigal around 25
For a "total" advance number of 35/39
Lean to the higher number if you live at high altitude
I believe the adjustments on vacuum advance cans do limit advance. I could be wrong here but I've experimented with lots of advance curves. The allen adjustment limits the travel of the diaphram which limits the advance. Not all cans have the adjustment. I have modified several of them to reduce from 20 to 10
For a mild to medium 400 Ford This is what I have found to work well.
Get rid of ported vacuum, emmision controls (they don't really work anyway)
Use full manifold vacuum
Initial of about 10/14
Vacuum of about 10 ( this will yield 20/24 at idle)
Centrifigal around 25
For a "total" advance number of 35/39
Lean to the higher number if you live at high altitude
You are correct I forgot about a type that is somewhat adjustable in the amount of advance it pulls in. The one at the top in the picture that looks like a dual diaphragm but is actually a single has a different type of limit stop in it than the type in the picture below it. It is a 10 degree can but can be adjusted a little. The one on the bottom which is probably the most common single diaphragm type has a slot in the arm that can't be adjusted. You can make that out in the second picture. The length of that slot determines the amount of advance the can is capable of. I prefer this type because once you determine the amount of advance you want they are much more adjustable as to the amount of vacuum it requires to move the arm.
For a mild to medium 400 Ford This is what I have found to work well.
Get rid of ported vacuum, emmision controls (they don't really work anyway)
Use full manifold vacuum
Initial of about 10/14
Vacuum of about 10 ( this will yield 20/24 at idle)
Centrifigal around 25
For a "total" advance number of 35/39
Most all OEM V8 run 36° to 38° BTDC, this would be without vacuum advance. It depends on compression, camshaft, etc, but that's right in there for a stock engine.
Vacuum advance adds on top of that, mostly during steady cruise on level ground in high gear, although it is constantly working depending on engine load. 50° BTDC (or more) timing advance is typical OEM curve under those specific conditions, sounds crazy but it's true.
Cropduster….can you say which particular application would call for the vacuum advance marked with a 4 ? I would love to find one of those. The lowest I have found is a 6.
That is a good deal for a new one. It will fit 351C,400. The gear looks like a bronze gear but it just has a coating on it, it is cast iron. They have a 13 an 18 degree advance plate in them set in the 18 degree slot with some real heavy springs so it will have to be recurved to use in anything except a big truck. If you just need the can your best bet would be your local Ford dealer.
