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A big thanks to Lars, Jebbysan and others! I was able to follow all the instructions and the '73 is running really good/has more peppiness/quicker response for sure. I think the distributor was sitting in the right spot or pretty close before I tore into it. I made a line on the distributor and intake manifold with a sharpie just in case this got outta hand and I needed to put it back where it was. When it was all said and done, that line still pretty close to where it was, so my main issue was the spring tension. I took one spring off, moved the distributor til I was "all in" by about 3k rpm. Then I took off other spring, put one silver+one black spring from the Mr Gasket kit in there/fired it back up and had to move dist almost back to where it was for the 36 degrees at 3k rpm.
The vacuum issue was a little trickier than I thought because I was really expecting good vacuum out of the sorta long pipe on right side of the carb that was capped off. Wrong, nothing there, so I found good vacuum at the right front top of carburetor.
The vac line going to the right (fwd) comes around and hits a T underneath the gizmo attached to AIR pump. From the T one goes to the AIR device, the other goes into the TCS solenoid. The other vac line (at right front of carb) makes a quick 180 and goes into choke on carb (I think that's what that is). All I did was add another T by the AIR device and then capped off the TCS valve where vacuum advance from distributor used to go.
The only other thing I'm not sure of is what model vacuum canister do I have on the distributor? At idle IIRC the advance was up to 18 with vac plugged in (12 without). The only markings I could see was a "B1" on the flat piece coming off the vac canister in bottom of distributor, underneath the part that spins. I'm not gonna lose too much sleep over it right now cause it runs good, didn't hear anything funny, and water temp oil pressure looked normal.
Ok after spending all morning dredging the interwebs I've figured out that the "B1" on the vac advance can means it is a VC24A. Do any of you know what the specs are for that can? Further dredging of the internet turned up nothing. BTW, I'm getting about 14" of vacuum measured by the mityvac. I know there is something about a 2" rule in the papers, but I'm not totally clear about that. Any help would be appreciated, thx in advance.
Last edited by Elcransonator; Mar 10, 2020 at 09:03 AM.
Reason: additional text
Technical Information Bulletin Rev C 5-22-05
Distributor Vacuum Advance Control units
Specs and facts for GM Distributors
by Lars
Lafayette, CO
I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).
As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.
Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.
Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.
The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:
How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.
How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.
Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.
Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.
Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.
With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:
Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country. For Wells part numbers (Autozone), drop the “VC” prefix and use a “DV” prefix.
ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.
Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.
Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.
Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.
The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a mild street engine. As you can see, it was selected by GM for use in many engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above). The third listing is one of my personal favorites, and you can see from the specs that it is an excellent choice for most performance engines and street engines with moderate cams, since most moderate performance engines will idle with at least 13 inches of vacuum.
After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).
From: At my Bar drinking and wrenching in Lafayette Colorado
Jebby -
That's a grossly outdated version of the paper - it should not be used, as it has incorrect and outdated information. Anyone needing the info should contact me directly to assure that they are receiving, and using, correct information.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by Elcransonator
I was really expecting good vacuum out of the sorta long pipe on right side of the carb that was capped off. Wrong, nothing there.
That long nipple is manifold vacuum on a Q-Jet. If there is no vacuum on your long nipple, someone has installed the incorrect throttle plate gasket on your carb, which will also cause other problems. I would remove the carb, remove the throttle plate, and see if the gasket is incorrect (vacuum hole in wrong place) for your carb. Either the gasket is wrong, or you have something plugging the passage (which needs to be fixed).
Jebby -
That's a grossly outdated version of the paper - it should not be used, as it has incorrect and outdated information. Anyone needing the info should contact me directly to assure that they are receiving, and using, correct information.
That long nipple is manifold vacuum on a Q-Jet. If there is no vacuum on your long nipple, someone has installed the incorrect throttle plate gasket on your carb, which will also cause other problems. I would remove the carb, remove the throttle plate, and see if the gasket is incorrect (vacuum hole in wrong place) for your carb. Either the gasket is wrong, or you have something plugging the passage (which needs to be fixed).
Lars
There is a thread you did with pics that I saw somebody linked to about the gasket/throttle plate and how much of problem this is....it was from 09’ if someone wants to search for it.
That long nipple is manifold vacuum on a Q-Jet. If there is no vacuum on your long nipple, someone has installed the incorrect throttle plate gasket on your carb, which will also cause other problems. I would remove the carb, remove the throttle plate, and see if the gasket is incorrect (vacuum hole in wrong place) for your carb. Either the gasket is wrong, or you have something plugging the passage (which needs to be fixed).
Lars
Ok thanks Lars. What about the 14" I'm getting from where I tapped in? Is that about right for vacuum at idle on this engine? Thx in advance as always.
From: At my Bar drinking and wrenching in Lafayette Colorado
14" sounds about right, and you should get that same reading from all ports connected to manifold vacuum (including the long nipple sticking out on the passenger side of the carb). Typically, all ports are manifold vacuum except 1 port on the carb that may be ported vacuum. 1 port only.
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Lars,
Does your current version have the specs for the VC24A/B1 that is on my distributor? I'm guessing that the VC24A is probably a newer number that superseded one of the older part numbers above? Do you decode the specs as follows?
Example: VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
The advance starts to work at 8-11", you get 8 total distributor degrees at 16-18" of vacuum?
If so, I guess that means with 14" at idle I would never see the 8 degrees (16 crankshaft) of distributor advance ever? It would be maybe 6 (12) ish? If I'm correct so far, I guess I would need the canister that the last two numbers would be like 12-14 instead of 16-18?
'73 L48 Timing/Vacuum ?
