Distributor Vacuum Advance

[David Dawdy]

Is their any significance to the numbers and/or letters stamped on the attaching arm of vacuum advance units, i.e., dates, amount of advance allowed, etc?

[Jack H.]

Re: Distributor Vacuum Advance

The original Delco Remy parts had three digits representing the last characters of the original part number and another 3-digit sequence representing the maximum advance the part generated. Savvy judges who know their stuff can sometimes use this stamped info to distinguish between a factory original vac advance, correct for that car's distributor and a Delco service replacement advance or a non-Delco aftermarket part...

[Joe L.]

Re: Distributor Vacuum Advance

David-----

In addition to what Jack described for original-style vacuum control units, later GM, Delco, and aftermarket replacements have a B## sequence stamped on the arm. These also identify the vacuum control and the vacuum characteristics of that control. There are no dates that I know of on any of the vacuum controls, original or later replacements.

[William C.]

Re: Distributor Vacuum Advance

There are however subtle differences in the front face where the vacuum nipple meets the vacuum hose that ID a Delco service unit from the 80's as different from the one used in the 60's. Check tour OLD Corvette restorers for the info and pictures.

[David Dawdy]

Re: Distributor Vacuum Advance

Thanks Guys, so if I have a B13 stamped on a unit, it is probably a DR replacement. How do you id the B13?

[Joe L.]

Re: Distributor Vacuum Advance

David-----

A "B13" is a later Delco replacement or an aftermarket unit----both are manufactured by the same source. A B-13 is just about equivalent to an original GM #1968861 aka Delco #1354A. This control was never originally used for a Corvette. The specs on it are as follows:

inches of vacuum to start the plunger----9-11"

16 degrees maximum crankshaft degrees @ 16-18" vacuum

This is actually very close to the specs of the original GM #1116201 or the GM #1116163. So, it should work well for any application originally using one of those controls.

[Duke W.]

Re: Distributor Vacuum Advance

The 163 is the service replacement for the 201 vacuum advance control, which nominally took 15.5" of vacuum to pull to the limit. Unfortunately, the 163 and its aftermarket equivalents can take 18" or even a little more to pull to the limit, which means a 250 or 300 HP engine may have a tough time keeping it locked at the limit at idle, and if it's not locked by manifold vacuum it can "dither", which may cause idle speed variation due to idle timing variation, which can lead to idle speed instablility.

Unfortunately, GM selected the 163 as the 201 service replacement, the aftermarket followed suit, and we are stuck with the legacy in the parts books. IMO both the B13 and B1 are boat anchors.

For 250/300 HP engines with manual transmission I recommend the NAPA VC1802 (B22) which is 16@15", but the VC1765 (B20) for 250/300HP w/PG, which is 16@12" to accomodate the lower idle speed/vacuum when idling in Drive, and the VC1765 matches the OE 1116355 specs, which was used on all '66 and '67 300 HP engines.

In fact, there are only three VACs that cover all pre-emission Corvette engines with a good match to their idle vacuum characteristics. Here's a list of NAPA part numbers, and all can be cross referenced to other brands on the Web, but I don't think the online Delco cross reference is very accurate. The '63-'64 JG does not include the VAC ID numbers for original engines, so it's a "system knowledge" call if a judge wants to take a deduct for a VAC without the correct stamped OE ID. I think some of the other year JGs are the same - no mention of the OE VAC IDs.

All SB w/ base cam/man. trans.............VC1802(B22).............0@8", 16@15"
All SB w/ base cam/PG, all 427, L-79...VC1765(B20).............0@6", 16@12"
All SB w/ Duntov, 30-30, LT-1 cam........VC1810(B28).............0@4", 16@8"

The OE L-79 VAC is 1116236, of which the VC1810 is equivalent, but the VC1765 will work fine since L-79 vacuum idle characteristic is 750-800@14-15" and a 12" VAC will reduce any propensity to detonate at part throttle or increased throttle transients.

The tolerance on the above specs is about +/- one degree and +/- 1" Hg. The basic "rule" I use is that typical idle vacuum should be not less than 2" more than what is required to pull the VAC to the limit, so if the engine idles at 17" you need a 15" VAC, 14" idle vacuum a 12" VAC, and less than 14" idle a 8" VAC.

Most OE VACs comply with this rule except the '63 L-76 and SHP 427s, some of which have ported vacuum advance, which I recommend be converted to full time vacuum advance and then a 12" VAC installed.

The Duntov cam only pulls about 12" at idle - 900 or so, and the OE 15.5" VAC dithers like crazy and gets the metering rods vibrating against the covers, and eventually the idle can destabilize and the engine can stall. I solved this problem by replacing the 201 VAC on mine with the 236 VAC circa 1965.

It's the ID number, B20 etc. that determines the specs and since all these VACs are now made by Standard Ignition Products, including those sold by Delco, you just have to make sure that the part is the box has the correct ID you want regardless of brand or part number, but also check that it meets specs (before you leave the store) as I have heard reports that that boxes have the wrong part and some parts are way out of spec. A Mighty Vac can be used to quickly check if the actuator rod starts and stops within reason of spec and VAC total advance can be checked on the engine.

Duke

[William C.]

Re: Distributor Vacuum Advance

To add to what Duke has mentioned as a word of caution, I have also seen occasional variations from the proper specs on some of the "Reproduction" vacuum advance units from the Corvette "reproduction Parts" world. Check those for proper specs befor using also.

[Joe L.]

Re: Distributor Vacuum Advance

Bill-----

My guess is that most of the folks that buy these reproduction vacuum controls are more interested in the "stamped numbers" than they are with the performance characteristics of them. Just another situation of idiotic "form over function".

[William C.]

Ok 'til they flunk a PV for poor idle! *NM*

[David Dawdy]

Re: Distributor Vacuum Advance

Thanks Bill, Joe and Duke. The explanations of poor idle is exactly what is happening with the 163 unit. I have a 201 that I'm going to try. Then it is off to the NAPA store.

Dave D.

[Hector G.]

Re: Distributor Vacuum Advance

One of the numbers on the vacuum advance unit is a part number. For instance, on a 1966 427/390 distributor, the correct part number [credit to Joe Lucia] is GM # 1115355. It appears on the unit as 355. This part has been discontinued by GM. Reproductions are still available. On a side note, there is a difference one can detect between the originals and repros.

[John D.]

Re: Distributor Vacuum Advance

David, Didn't see the other replies from Bill, Joe and Duke. So this may be a repeat and I bet it is. The 163 vacuum advance was the replacment one from GM that was listed for our 63's. One time I bought several dozen 173's from Delco Remy. When they came in most of the boxes contained C1 vacuum advances. Those I believe were made by Standard and just reboxed. Problem with the 163's and the B1 is that the spring inside the VA is just too strong. So there fore it takes a ton of vacuum to activate it properly.
My 63 has a good used 201. I learned that a lot of judges aren't aware of the fact that a 63 came from DR with a 201. Can't blame them though as Chevy sold the 163's in the parts books for an eon.
You are right about switching to NAPA> I think the number is B22 if you are running an '097 type came. If you are running a 30-30 cam then go to the B28 VA.
I also understand that not all of the repro VA's dial in that well. Notice I said not all.
Thanks, John E. DeGregory #2855

[John D.]

Re: Distributor Vacuum Advance correctuib

Daivd, Glaring typo in last post. Bought 163's. Not 173's. Never heard of 173's. Sorry. John