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Adjustable Vacuum Advance

39K views 15 replies 8 participants last post by  Yellomalibu 
#1 ·
Of the adjustable vacuum advance cans I've seen for sale, all of them can be adjusted by sticking a 3/32" allen wrench in through the front, but only about half of them come with limiter plates that go on the end inside the distributor.

Is the limiter plate necessary? If so, why do some kits leave them out?
 
#2 ·
Depends on what your engine needs. Most of the adjustable canisters in my opinion have too much advance in them, which I believe is around 20 degrees, thus the need for the limiter. Post your engine specs (cam, compression etc) and some folks here might be able to give you a good starting point. Most of the time the degrees get cut in half or pretty close to it.
 
#3 ·
The engine is a 1984 454 from a GMC C3500. It has 8:1 compression, an Isky 270 cam, Edelbrock 650 AVS carb, high-rise intake, headers, TH 400 w/2000 stall, and 2.73's in back.

The vacuum can that was in it before is an AR10. The arm of the new vacuum can (Summit HEI upgrade) sticks out about 3/16" of an inch more than the AR10 can. Through trial and error, I set the new can with about 8 full turns back from all-the-way-in clockwise. The engine seems to really like it there as part-throttle response is crisper and it doesn't require near as much foot pressure on the accelerator to cruise at 65 mph.

I am concerned about detonation. There isn't any blatant rocks-in-a-coffee-can rattle, but with the 3" exhaust its pretty hard to be sure. I'm under the impression that without the limiter plate, the vacuum can is giving full vacuum (too much), while the adjustment with the allen wrench merely controls the rate in which it comes in.
 
#4 ·
You should set the mechanical advance with vacuum disconnected and engine RPM at 2500 or 3000 RPM, what ever it takes to get total advance. At a guess I would set at 30-32 degrees total. Someone else step in and correct this.

Then check the total advance with vacuum connected and engine RPM at 3000 RPM. Adjust the advance to for the total cruise advance you need.
 
#7 ·
I finally got the adjustable vacuum advance can dialed in. My engine likes about 8 complete turns back from full-in clockwise. It took a bit of trial-and-error going back one turn at a time until the part-throttle rattle went away.

First thing I noticed was improved part-throttle snap. Much less pressure on the pedal to get things going. Second thing I noticed was much less foot pressure needed to cruise on the highway. The car now cruises at 70 mph with about the same foot pressure that was previously needed at 60 mph.

So, less pressure from the foot mean less gas going into the engine at cruise. When I checked at the last fill-up, the car went from 11 mpg overall to 12.5 mpg overall. Nice improvement. With the old can, I was getting 14-15 mpg on long cruises. I'm looking forward to what this new can will do.

Before this new can, the distributor had an non-adjustable AR10 can. The engine then, according to my perception, ran beautifully. I only switched out to the new adjustable can as it was part of an HEI upgrade kit. I know that this latest improvement was due solely to the new vacuum can as it was the last to go in -- about 6 weeks after the cap, coil, module, etc -- due to my unfamiliarity with adjustable cans and the need to look things up.

It's nice bonus when you can get something that already works really well to work even better.
 
#10 ·
I was looking into adding a limiter plate, but as most kits don't include them, I tried the can as it was.

I don't hear any pinging at the present adjustment (there was at a looser spring adjustment), and the throttle response, mileage, and driveability have all improved. I think I just got lucky and my engine is happy with how much timing the present vacuum can gives it.
 
#11 ·
Was playing around with timing this afternoon. I've had this adjustable vacuum advance for awhile, but I only just started using a light.

When I turned the allen wrench all the way clockwise (about 12 full turns), the can was giving about 20 deg of advance. When I turned it all the way counter-clockwise, the amount of advance was zero. Halfway between the two was about twelve. I finally settled with the can giving 16 deg advance.

This seems to go against everything I've read. I thought that the allen wrench screw merely adjusted the rate of the advance from the can, but not the amount. But that's not true on this can. The allen wrench screw definitely affects the amount, from zero to 20 deg. And neither does it seem to affect the rate, as the amount of vacuum advance was the same both at idle and at WOT. Anyone else had this experience?

So why does Crane make a limiter plate? Is theirs' different? I even made a limiter plate that works from the opposite end of Crane's, but it seems that I won't be needing it now. I know that a lot of guys make a limiter plate for stock cans, but I assume that's because stock cans don't have the allen wrench screw.

Another question: I was under the impression that at WOT, there is no vacuum advance. Then why do I still get 50+ degrees when I'm checking the timing at WOT? I am, of course, not checking it in drive. Does the engine need to be under load to have the vacuum advance drop away, or I am misunderstanding something?
 
#13 ·
When I turned the allen wrench all the way clockwise (about 12 full turns), the can was giving about 20 deg of advance. When I turned it all the way counter-clockwise, the amount of advance was zero. Halfway between the two was about twelve. I finally settled with the can giving 16 deg advance.

This seems to go against everything I've read. I thought that the allen wrench screw merely adjusted the rate of the advance from the can, but not the amount. But that's not true on this can. The allen wrench screw definitely affects the amount, from zero to 20 deg. And neither does it seem to affect the rate
Never seen one where the amount of advance was adjustable via the Allen screw. Apparently, there's at least two distinct designs of adjustable vacuum advance.

With no way to adjust the rate, the kind you have would be less desirable.

the amount of vacuum advance was the same both at idle and at WOT. Anyone else had this experience?
Define WOT. The WOT I'm aware of will scatter the connecting rods if you try to measure the advance in neutral in your driveway.

So why does Crane make a limiter plate? Is theirs' different?
Clearly.

I even made a limiter plate that works from the opposite end of Crane's, but it seems that I won't be needing it now. I know that a lot of guys make a limiter plate for stock cans, but I assume that's because stock cans don't have the allen wrench screw.
Yup.

Another question: I was under the impression that at WOT, there is no vacuum advance. Then why do I still get 50+ degrees when I'm checking the timing at WOT? I am, of course, not checking it in drive. Does the engine need to be under load to have the vacuum advance drop away, or I am misunderstanding something?
Far as I can tell, you must be doing something wrong. WOT in park/neutral will result in about ten thousand RPM just long enough to put holes in the oil pan.

The adjustable vacuum advance cans I have had changed only the amount of advance.
I have not heard of one that could change rate of advance.
Crane brand.

Rate is controlled by the mechanical weights and springs.
Weights and springs (and the center piece) change the rate and curve of centrifugal advance. Has nothing at all to do with vacuum advance.

Vacuum advance contributes to driveability and fuel economy during normal driving. It is zero at full throttle (under load) unless you are under-carburetored.
Since it takes 5--6--7 or more inches of vacuum to actuate the vacuum advance, it would require a SEVERELY under-carbed engine to pull enough vacuum at WOT to activate the vacuum advance. I don't think it's possible.
 
#12 ·
The adjustable vacuum advance cans I have had changed only the amount of advance.
I have not heard of one that could change rate of advance.

Rate is controlled by the mechanical weights and springs.

Set your total advance with vacuum disconnected and plugged with the engine running fast enough to have full mechanical advance.
The amount of total advance depends on your engine build and design.

The amount of mechanical advance is controlled by the weights and sleeve on the advance stop pin.

Vacuum advance contributes to driveability and fuel economy during normal driving. It is zero at full throttle (under load) unless you are under-carburetored.
 
#15 ·
no, it depends on manifold vacuum, not RPM. Your 50 or so all in is about perfect if your engine will stand it without pinging. Big advance like this is the key to fuel economy, it allows to lean out the mix because lean mix burns slower.

You can't hold your engine at WOT long enough to see anything useful, the RPM will go through the roof and pieces of the engine will fall out the bottom.

Sounds to me like your is pretty well right. If there's no pinging you could try advancing it 2° more.
 
#16 ·
Just to clarify what Tom and Schurkey(?) are saying:
WOT = Wide Open Throttle.
Bringing the rpm's up to 4 or 5 grand can be done in neutral with nowhere near wot.
I would not advise having your head (or any other body part) under the hood if you plan to hold the throttle at wot in neutral for more than half a second.

...and yes, load does effect vacuum. In my DD (an 84 C10) I can hear the "max A/C" mixture door opening and closing (squeaky spring). It requires vacuum to open it, the spring pulls it closed when there is little or no vacuum. I can hear the door closing when start going up a hill (pulling a trailer) long before I reach wot.
A more common example: The vacuum modulator on your transmission will hold out the shift longer when the engine is under load - because there is less vacuum.

Sorry for bringing a thread up from the past - I was just doing some research on adjustable vacuum advance and found this thread... replied before I remembered how I got here and didn't check the dates. :D
 
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